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The understanding of tropical phenomena and their representation in numerical models still raise important scientific and technical questions, particularly in the coupling between the dynamics and diabatic processes. Among these phenomena, tropical cyclones (TC) are of critical interest because of their societal impacts and because of uncertainties in how their characteristics (cyclogenesis processes, occurrence, intensity, latitudinal extension, translation speed) will change in the framework of global climate change. The monitoring of TCs, their forecasts at short to medium ranges, and the prediction of TC activity at extended range (15-30 days) and seasonal range are also of great societal interest.

The aim of the session is to promote discussions between scientists focusing on the physics and dynamics of tropical phenomena. This session is thus open to contributions on all aspects of tropical meteorology between the convective and planetary scale, such as:

- Tropical cyclones,
- Convective organisation,
- Diurnal variations,
- Local circulations (i.e. island, see-breeze, etc.),
- Monsoon depressions,
- Equatorial waves and other synoptic waves (African easterly waves, etc.),
- The Madden-Julian oscillation,
- etc.

We especially encourage contributions of observational analyses and modelling studies of tropical cyclones and other synoptic-scale tropical disturbances including the physics and dynamics of their formation, structure, and intensity, and mechanisms of variability of these disturbances on intraseasonal to interannual and climate time scales.

Findings from recent field campaigns such as YMC and PISTON are also encouraged.

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Co-organized by CL2/NH1
Convener: Jean Philippe Duvel | Co-conveners: Eric Maloney, Kevin ReedECSECS, Enrico Scoccimarro, Allison WingECSECS
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| Attendance Wed, 06 May, 14:00–18:00 (CEST)

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Chat time: Wednesday, 6 May 2020, 14:00–15:45

D2977 |
EGU2020-12614<span style="font-size: .8em!important; font-weight: bold; vertical-align: super; color: green!important;"></span>
| solicited
Kazuhisa Tsuboki, Hiroyuki Yamada, Tadayasu Ohigashi, Taro Shinoda, Kosuke Ito, Munehiko Yamaguchi, Tetsuo Nakazawa, Hisayuki Kubota, Yukihiro Takahashi, Nobuhiro Takahashi, Norio Nagahama, and Kensaku Shimizu

Typhoon is a tropical cyclone in the western North Pacific and the South China Sea. It is the most devastating weather system in East Asia. Strong winds and heavy rainfalls associated with a typhoon often cause severe disasters in these regions. There are many cases of typhoon disasters even in the recent decades in these regions. Furthermore, future projections of typhoon activity in the western North Pacific show that its maximum intensity will increase with the climate change. However, the historical data of typhoon (best track data) include large uncertainty after the US aircraft reconnaissance of typhoon was terminated in 1987. Another problem is that prediction of typhoon intensity has not been improved for the last few decades. To improve these problems, in situ observations of typhoon using an aircraft are indispensable. The T-PARCII (Tropical cyclone-Pacific Asian Research Campaign for Improvement of Intensity estimations/forecasts) project is aiming to improve estimations and forecasts of typhoon intensity as well as storm track forecasts.

In 2017, the T-PARCII team performed dropsonde observations of intense Typhoon Lan in collaboration with Taiwan DOTSTAR, which was the most intense typhoon in 2017 and caused huge disaster over the central Japan. It was categorized as a supertyphoon by JTWC and as a very intense and huge typhoon by JMA. Typhoon Lan moved northeastward to the east of the Okinawa main island and it was located around 23 N on 21 and 28 N on 22 October. In these two days, we made dropsonde observations at the center of the eye and in the surrounding area of the eyewall. The observations showed that the central pressure of Lan slightly increases from 926 hPa on 21 to 928 hPa on 22 October with the northward movement. On the other hand, The JMA best track data indicate that the central pressure decreases from 935 hPa on 21 to 915 hPa on 22 October. The observations also showed a significant double warm core structure in the eye and the maximum wind speed along the eyewall. The dropsonde data were used for forecast experiments. The result shows an improvement of typhoon track prediction.

The T-PARCII team also made aircraft observations of Typhoon Trami during the period from 25 to 28 September 2018 in collaboration with the SATREPS ULAT group and DOTSTAR. Trami was almost stationary during the period to the south of the Okinawa main island. Then, it moved northward and finally made a landfall over the central part of Japan. This also caused a big disaster and electricity was shut down for several days in the central part of Japan. Typhoon Trami showed a drastic change of intensity from 25 to 26 September with a large change of eye size from about a diameter of 60 km to 200 km. Dropsonde observations showed the change of central pressure and maximum wind speed as well as the thermodynamic structure of the eye.

How to cite: Tsuboki, K., Yamada, H., Ohigashi, T., Shinoda, T., Ito, K., Yamaguchi, M., Nakazawa, T., Kubota, H., Takahashi, Y., Takahashi, N., Nagahama, N., and Shimizu, K.: Dropsonde Observations of Intense Typhoons in 2017 and 2018 in the T-PARCII Project, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-12614, https://doi.org/10.5194/egusphere-egu2020-12614, 2020

D2978 |
EGU2020-3164<span style="font-size: .8em!important; font-weight: bold; vertical-align: super; color: green!important;"></span>
| solicited
| Highlight
Suzana Camargo, Chia-Ying Lee, Adam Sobel, and Michael Tippett

Here I will describe recent results on the influence of climate change on tropical cyclones (TC) using the Columbia Hazard (CHAZ) model. Using environmental conditions from reanalysis and climate models and a statistical-dynamical downscaling methodology (Lee et al. 2018), CHAZ generates synthetic TCs that can be used to analyze TC risk.  I will first discuss the current knowledge and uncertainties in TC frequency projections. Then I will present our recent projections on TC frequency using CHAZ. Focusing on the North Atlantic, I will finish by discussing how we can use a combination of observations, high-resolution models and CHAZ synthetic TCs in the historical period to inform the reliability of the models' TC frequency projections. 

Reference:

Lee, C.-Y., M.K. Tippett, A.H. Sobel, and S.J. Camargo, 2018. An environmentally forced tropical cyclone hazard model. J. Adv. Model. Earth Sys., 10, doi: 10.1002/2017MS001186.

How to cite: Camargo, S., Lee, C.-Y., Sobel, A., and Tippett, M.: Tropical cyclones and climate change: Recent results and uncertainties , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-3164, https://doi.org/10.5194/egusphere-egu2020-3164, 2020

D2979 |
EGU2020-18644<span style="font-size: .8em!important; font-weight: bold; vertical-align: super; color: green!important;"><span title="Early career scientist: an ECS is an undergraduate or postgraduate (Masters/PhD) student or a scientist who has received their highest degree (BSc, MSc, or PhD) within the past seven years. Provided parental leave fell into that period, up to one year of parental leave time may be added per child, where appropriate.">ECS</span></span>
| Highlight
Kieran Bhatia, Alex Baker, Gabriel Vecchi, Hiroyuki Murakami, James Kossin, Pier Luigi Vidale, Kevin Hodges, and Thomas Knutson

Tropical cyclone (TC) rapid intensification events are responsible for intensity forecasts with the highest errors, and hurricanes that rapidly intensify cause a disproportionate amount of the fatalities and damage from TCs. According to a recent study by Bhatia et al. (2019), natural variability cannot account for the recent (1982-2009), observed increase in the highest TC intensification rates in the Atlantic Basin. These results agree well with the main conclusions of Bhatia et al. (2018), which demonstrated climate change could significantly increase TC intensification rates worldwide by the end of 21st century.

Expanding on the work of Bhatia et al. (2018, 2019), TC intensification trends are analyzed for the period 1982-2017 using two observational datasets, the International Best-Track Archive for Climate Stewardship (IBTrACS) and the Advanced Dvorak Technique-HurricaneSatellite-B1 (ADT-HURSAT). The extended observational datasets confirm significant upward trends in intensifications metrics. To explore a physical explanation for the climate change response of TC intensification, we use ERA5 reanalysis data to calculate trends in the favorability of storm environments. When evaluating environmental data, we use 6-hour increments at specific annuli around already-formed storms in order to focus on synoptic conditions unique to storm evolution and not genesis. The robust trends in a 36-year times series and corresponding evolution of storm environments corroborates a climate change fingerprint on TC intensification.

How to cite: Bhatia, K., Baker, A., Vecchi, G., Murakami, H., Kossin, J., Vidale, P. L., Hodges, K., and Knutson, T.: An Environmental Explanation for the Recent Increase in Tropical Cyclone Intensification, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-18644, https://doi.org/10.5194/egusphere-egu2020-18644, 2020

D2980 |
EGU2020-1885<span style="font-size: .8em!important; font-weight: bold; vertical-align: super; color: green!important;"><span title="Early career scientist: an ECS is an undergraduate or postgraduate (Masters/PhD) student or a scientist who has received their highest degree (BSc, MSc, or PhD) within the past seven years. Provided parental leave fell into that period, up to one year of parental leave time may be added per child, where appropriate.">ECS</span></span>
Jonathan Lin, Kerry Emanuel, and Jonathan Vigh

This paper describes the development of a model framework for Forecasts of Hurricanes using Large-ensemble Outputs (FHLO). Computationally inexpensive, FHLO quantifies the forecast uncertainty of a particular tropical cyclone (TC) through O(1000) ensemble members. The model framework consists of three components: (1) a track model that generates synthetic tracks from the TC tracks of an ensemble numerical weather prediction (NWP) model, (2) an intensity model that predicts the intensity along each synthetic track, and (3) a TC wind field model that estimates the time-varying twodimensional surface wind field. In this framework, we consider the evolution of a TC’s intensity and wind field as though it were embedded in a timeevolving environmental field. The environmental fields are derived from the forecast fields of ensemble NWP models, leading to probabilistic forecasts of track, intensity, and wind speed that incorporate the flow-dependent uncertainty. Each component of the model is evaluated using four years (2015- 2018) of TC forecasts in the Atlantic and Eastern Pacific basins. We show that the synthetic track algorithm can generate tracks that are statistically similar to those of the underlying global ensemble models. We show that FHLO produces competitive intensity forecasts, especially when considering probabilistic verification statistics. We also demonstrate the reliability and accuracy of the probabilistic wind forecasts. Limitations of the model framework are also discussed.

How to cite: Lin, J., Emanuel, K., and Vigh, J.: Forecasting Hurricanes using Large-Ensemble Output, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-1885, https://doi.org/10.5194/egusphere-egu2020-1885, 2020

D2981 |
EGU2020-2921<span style="font-size: .8em!important; font-weight: bold; vertical-align: super; color: green!important;"></span>
Remi Tailleux, Bethan Harris, Christopher Holloway, and Pier-Luigi Vidale

While it is well accepted that tropical cyclones (TCs) derive their energy from surface enthalpy fluxes over the ocean, there is still little understanding of the precise causes and effects by which the latter ends up as TC vortex kinetic energy. For example, Potential Intensity (PI) theory, which has been so far the main framework for predicting TC intensities, assumes a balance between the Carnot power input and the kinetic energy dissipated by surface friction, but says nothing of the detailed physical processes linking the two. A similar criticism pertains to the WISHE (Wind Induced Surface Heat Exchange) theory. To achieve a causal theory of TC intensification, the main difficulty is in linking the power input to kinetic energy production, rather than kinetic energy dissipation. Because kinetic energy is produced at the expense of available potential energy (APE), APE theory is arguably the most promising candidate framework for achieving a causal theory of TC intensification. However, in its current form, the usefulness of APE theory appears to be limited in a number of ways because of its reliance on a notional reference state of rest. First, APE production associated with standard reference states (i.e., horizontally averaged density field, density field of initial sounding, adiabatically sorted states, ...) is usually found to systematically overestimate the kinetic energy actually produced in ideal TC simulations, similarly as the Carnot theory of heat engines; moreover, the standard APE is only connected to vertical buoyancy forces, but says nothing of the radial forces required to drive the secondary circulation. To address these shortcomings, this work presents a new theory of available energy (AE) that is based on the use of an axisymmetric vortex reference state in gradient wind balance. This theory possesses the following advantages over previous frameworks:

 

  • The available energy (AE) thus constructed possesses both a mechanical and thermodynamic component. The thermodynamic component is analogous to the well-known Slantwise Convective Available Potential Energy (SCAPE), whereas the mechanical component is proportional to the anomalous azimuthal kinetic energy;
  • The rate of AE production by surface enthalpy fluxes is found to be a very accurate predictor of the amount of potential energy actually converted into kinetic energy in idealised TC simulations based on the Rotunno and Emanuel (1986) axisymmetric model, although a few exceptions are found for cold SSTs;
  • In addition to the expected thermodynamic efficiencies, the production term for AE also involves mechanical efficiencies predicting the fraction of the sinks/sources of angular momentum creating/destroying AE;
  • The AE is related to a generalised buoyancy/inertial force that has both vertical and horizontal components; at low levels, such a generalised force has radially inward and vertically upward components, as required to drive the expected secondary circulation.

The new theory, therefore, appears to possess all the ingredients to form the basis for a causal theory of TC intensification.

How to cite: Tailleux, R., Harris, B., Holloway, C., and Vidale, P.-L.: Linking surface enthalpy fluxes to the forces driving the secondary circulation: towards a causal theory of tropical cyclone intensification, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-2921, https://doi.org/10.5194/egusphere-egu2020-2921, 2020

D2982 |
EGU2020-19270<span style="font-size: .8em!important; font-weight: bold; vertical-align: super; color: green!important;"></span>
Benoit Vanniere, Malcolm Roberts, Pier Luigi Vidale, Kevin Hodges, and Marie-Estelle Demory

Previous studies have shown that, the number, intensity and structure of simulated tropical cyclones (TC) in climate models get closer to the observations as the horizontal resolution is increased. However, the sensitivity of tropical cyclone precipitation and moisture budget to changes in resolution has received less attention. In this study, we use the five-model ensemble from project PRIMAVERA/HighResMIP to investigate the systematic changes associated with the water budget of tropical cyclones in a range of horizontal resolutions from 1º to 0.25º. Our results show that despite a large change in the distribution of TC intensity with resolution, the distribution of precipitation per TC does not change significantly. This result is explained by the large scale balance which characterises the moisture budget of TCs, i.e. radii of ~15º a scale that low and high resolution models represent equally well. The wind profile is found to converge between low and high resolutions for radii > 5º, resulting in a moisture flux convergence into the TC with similar magnitude at low and high resolutions. In contrast to precipitation per TC, the larger TC intensity at higher resolution is explained by the larger surface latent heat flux near the center of the storm, which leads to an increase in equivalent potential temperature and warmer core anomalies, despite representing a negligible contribution to the moisture budget. We discuss the complication arising from the choice of the tracking algorithm when assessing the impact of model resolution and the implications of such a constraint on the TC moisture budget in the context of climate change.

How to cite: Vanniere, B., Roberts, M., Vidale, P. L., Hodges, K., and Demory, M.-E.: The moisture budget of tropical cyclones: large scale environmental constraints and sensitivity to model horizontal resolution, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-19270, https://doi.org/10.5194/egusphere-egu2020-19270, 2020

D2983 |
EGU2020-12563<span style="font-size: .8em!important; font-weight: bold; vertical-align: super; color: green!important;"></span>
Neetu Suresh, Lengaigne Matthieu, Vialard Jerome, Mangeas Morgan, Menkes Christophe, Suresh Iyyappan, Leloup Julie, and Knaff John

Tropical cyclones (hereafter TC) are amongst the most devastating natural phenomena for coastal regions worldwide. While there has been tremendous progress in forecasting TC tracks, intensity forecasts have been trailing behind. Most operational statistical-dynamical forecasts of TC intensity use linear regression techniques to relate the initial TC characteristics and most relevant large-scale environmental parameters along the TC track to the TC intensification rate. Historically, different operational prediction schemes have been developed independently for each TC-prone basin, making it difficult to compare skills between different TC basins. We have thus developed global TC intensity hindcasts using consistent predictors derived from a single atmospheric dataset over the same period. Linear hindcast schemes were built separately for each TC basin, based on multiple linear regression. They display comparable skill to previously-described similar hindcast schemes, and beat persistence by 20–40% in most basins, except in the North Atlantic and northern Indian Ocean, where the skill gain is only 10–25%. Most (60–80%) of the skill gain arises from the TC characteristics (intensity and its rate of change) at the beginning of the forecast, with a relative contribution from each environmental parameter that is strongly basin-dependent. Hindcast models built from climatological environmental predictors perform almost as well as using real-time values, which may allow to considerably simplify operational implementation in such models. Our results finally reveal that these models have 2 to 4 times less skill in hindcasting moderate (Category 2 and weaker) than in hindcasting strong TCs.

This last result suggests that linear models may not be sufficient for TC intensity hindcasts. Many physical processes involved in TCs intensification are indeed non-linear. We hence further investigated the benefits of non-linear statistical prediction schemes, using the same set of input parameters as for the linear models above. These schemes are based on either support vector machine (SVM) or artificial neural network algorithms. Contrary to linear schemes, which perform slightly better when trained individually over each TC basin, non-linear methods perform best when trained globally. Non-linear schemes improve TC intensity hindcasts relative to linear schemes in all TC-prone basins, especially SVM for which this improvement reaches ~10% globally, partly because they better use the non-seasonal variations of environmental predictors. The SVM scheme, in particular, partially corrects the tendency of the linear scheme to underperform for Category 2 and weaker TCs. Although the TC intensity hindcast skill improvements described above are an upper limit of what could be achieved operationally, it is comparable to that achieved by operational forecasts over the last 20 years. This improvement is sufficiently large to motivate more testing of non-linear methods for statistical TC intensity prediction at operational centres.

How to cite: Suresh, N., Matthieu, L., Jerome, V., Morgan, M., Christophe, M., Iyyappan, S., Julie, L., and John, K.: Global assessment of linear and non-linear statistical-dynamical hindcast models of Tropical Cyclones intensity, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-12563, https://doi.org/10.5194/egusphere-egu2020-12563, 2020

D2984 |
EGU2020-3136<span style="font-size: .8em!important; font-weight: bold; vertical-align: super; color: green!important;"></span>
Chidong Zhang and Ji-Eun Kim

This presentation introduces a theory in which the dynamic core of the MJO is described in terms of a harmonic oscillator that can be excited by stochastic forcing. The mechanism for selecting MJO scales comes from momentum damping. The resonant solution to the equation for a damped harmonic oscillator on an equatorial beta plane represents the equatorial Kelvin wave for small damping and large zonal wavenumbers and the MJO for large (3 – 5 days) damping and zonal wavenumber one. This theory demonstrates the distinction between the Kelvin wave and MJO and their continuous transition. In contrast to most other MJO theories that compete against each other, this theory embraces most other theories in that they provide possible sources of energy (forcing) to the dynamic core of the MJO.

How to cite: Zhang, C. and Kim, J.-E.: Dynamic Core of the MJO, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-3136, https://doi.org/10.5194/egusphere-egu2020-3136, 2020

D2985 |
EGU2020-6394<span style="font-size: .8em!important; font-weight: bold; vertical-align: super; color: green!important;"></span>
Guosen Chen, Bin Wang, and Fei Liu

Madden-Julian Oscillation (MJO) is the dominant mode of atmospheric intraseasonal variability and the cornerstone for subseasonal prediction of extreme weather events. Climate modeling and prediction of MJO remain a big challenge, partially due to lack of understanding the MJO diversity. Here, we delineate observed MJO diversity by cluster analysis of propagation patterns of MJO events, which reveals four archetypes: standing, jumping, slow eastward propagation, and fast eastward propagation. Each type of MJO exhibits distinctive east-west asymmetric circulation and thermodynamic structures. Tight coupling between the Kelvin wave response and major convection is unique for the propagating events (slow and fast propagations), while the strength and length of Kelvin wave response distinguish slow and fast propagations. The Pacific sea surface temperature anomalies can affect MJO diversity by modifying the Kelvin wave response and its coupling to MJO convection. An El Niño state tends to increase the zonal scale of Kelvin wave response, to amplify it, and to enhance its coupling to the convection, while a La Niña state tends to decrease the zonal scale of Kelvin wave response, to suppress it, and to weaken its coupling to the major convection. This effect of background sea surface temperature on the MJO diversity has been verified by using a theoretical model. The results shed light on the mechanisms responsible for MJO diversity and provide potential precursors for foreseeing MJO propagation.

How to cite: Chen, G., Wang, B., and Liu, F.: Diversity of the Madden-Julian Oscillation, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-6394, https://doi.org/10.5194/egusphere-egu2020-6394, 2020

D2986 |
EGU2020-19432<span style="font-size: .8em!important; font-weight: bold; vertical-align: super; color: green!important;"><span title="Early career scientist: an ECS is an undergraduate or postgraduate (Masters/PhD) student or a scientist who has received their highest degree (BSc, MSc, or PhD) within the past seven years. Provided parental leave fell into that period, up to one year of parental leave time may be added per child, where appropriate.">ECS</span></span>
Ajda Savarin and Shuyi Chen

Large-scale convection associated with the Madden-Julian Oscillation (MJO) initiates over the Indian Ocean and propagates eastward across the Maritime Continent (MC) into the western Pacific. As an MJO enters the MC, it often weakens or completely dissipates due to complex interactions between the large-scale MJO and the MC landmass and its topography. This is referred to as the MC barrier effect, and it is responsible for the dissipation of 40-50% of observed MJO events. One of the main reasons for the MJO’s weakening and dissipation over the MC is the diurnal cycle (DC), one of the strongest modes of variability in the region. Due to the complex nature of the MJO and the MC’s complicated topography, the interaction between the DC and the MJO is not well understood.

In this study, we examine the MJO-induced variability of the DC of precipitation over the MC. We use gridded satellite precipitation products (TRMM 3B42 and GPM IMERG) to: (1) track the MJO convective envelope using the Large-scale Precipitation Tracking algorithm (LPT), (2) analyze the changes in the DC of precipitation over the MC relative to the passage of the MJO. We find that the presence of an MJO not only increases the amount of precipitation over the MC, but that the increase is more pronounced over water than over land. The results from observations are compared to those from two reanalysis datasets (ERA5, MERRA-2). The reanalysis datasets are then used to examine the dynamic and thermodynamic changes that drive the variability in the DC of precipitation relative to the MJO. In addition, we separate MJO events into two groups based on whether they cross the MC, and independently examine their influences on the evolution of the DC of precipitation.

How to cite: Savarin, A. and Chen, S.: MJO-Induced Variability of the Diurnal Cycle of Precipitation over the Maritime Continent , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-19432, https://doi.org/10.5194/egusphere-egu2020-19432, 2020

D2987 |
EGU2020-144<span style="font-size: .8em!important; font-weight: bold; vertical-align: super; color: green!important;"><span title="Early career scientist: an ECS is an undergraduate or postgraduate (Masters/PhD) student or a scientist who has received their highest degree (BSc, MSc, or PhD) within the past seven years. Provided parental leave fell into that period, up to one year of parental leave time may be added per child, where appropriate.">ECS</span></span>
Rosa Vargas Martes and Angel Adames Corraliza

Easterly Waves (EW) in the Pacific Ocean (PEW) and over Africa (AEW) account for a large fraction of rainfall variability in their respective regions. Although multiple studies have been conducted to better understand EWs, many questions remain regarding their structure, development, and coupling to deep convection. Recent studies have highlighted the relationship between water vapor and precipitation in tropical motion systems. However, EW have not been studied within this context. On the basis of Empirical Orthogonal Functions (EOFs) and a novel plume-buoyancy framework, the thermodynamic processes associated with EW-related convection are elucidated. A linear regression analysis reveals the relationship between temperature, moisture, and precipitation in EW. Temperature anomalies are found to be highly correlated in space and time with anomalies in specific humidity. However, this coupling between temperature and moisture is more robust in AEWs than PEWs. In PEWs moisture accounts for a larger fraction of precipitation variability. Results suggest that the convective coupling mechanism in AEW may differ from the coupling mechanism of PEWs.

How to cite: Vargas Martes, R. and Adames Corraliza, A.: Understanding the role of water vapor and temperature in easterly wave-related convection, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-144, https://doi.org/10.5194/egusphere-egu2020-144, 2019

D2988 |
EGU2020-5230<span style="font-size: .8em!important; font-weight: bold; vertical-align: super; color: green!important;"><span title="Early career scientist: an ECS is an undergraduate or postgraduate (Masters/PhD) student or a scientist who has received their highest degree (BSc, MSc, or PhD) within the past seven years. Provided parental leave fell into that period, up to one year of parental leave time may be added per child, where appropriate.">ECS</span></span>
Ambrogio Volonté, Andrew G. Turner, and Arathy Menon

The Indian summer monsoon is a vital source of water and a cause of severe impacts for more than a billion people in the Indian subcontinent. The INCOMPASS project investigates the mechanisms driving its onset and progression through an observational field campaign supplemented by high-resolution numerical simulations for the 2016 season using UK Met Office models. A 4.4 km resolution convection-permitting limited-area model simulation (driven at its boundaries by a daily-initialised global model) is used in this study, and verified against observations, along with short-lead-time operational global forecasts.

These data show that the monsoon progression towards northwest India in June 2016 is a non-steady process, modulated by the interaction between moist low-level southwesterly flow from the Arabian Sea and a northwesterly incursion of descending dry air from western and central Asia. The location and extent of these two flows are closely linked to mid-latitude dynamics, through the southward propagation of potential vorticity streamers and the associated formation of cyclonic circulations in the region where the two air masses interact. Particular focus is devoted to the use of Lagrangian trajectories to characterise the evolution of the airstreams and complement the Eulerian monsoon progression analysis. The trajectories confirm that the interaction of the two airstreams is a primary driver of the general moistening of the troposphere associated with monsoon progression. They also indicate the occurrence of local diabatic processes along the airstreams, such as turbulent mixing and local evaporation from the Arabian Sea, in addition to moisture transport from remote sources.

In summary, this combined Eulerian-Lagrangian analysis reveals the non-steady nature of monsoon progression towards northwest India. This process is driven by the interaction of different air masses and influenced by a synergy of factors on a variety of scales, such as mid-latitude dynamics, transient weather systems and local diabatic processes.

This work has recently been accepted for publication on the Quarterly Journal of the Royal Meteorological Society.

How to cite: Volonté, A., Turner, A. G., and Menon, A.: Airmass analysis of the processes driving the progression of the Indian summer monsoon, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-5230, https://doi.org/10.5194/egusphere-egu2020-5230, 2020

D2989 |
EGU2020-21506<span style="font-size: .8em!important; font-weight: bold; vertical-align: super; color: green!important;"><span title="Early career scientist: an ECS is an undergraduate or postgraduate (Masters/PhD) student or a scientist who has received their highest degree (BSc, MSc, or PhD) within the past seven years. Provided parental leave fell into that period, up to one year of parental leave time may be added per child, where appropriate.">ECS</span></span>
Kathleen Schiro, Sylvia Sullivan, Jiabo Yin, and Pierre Gentine

In the tropics, the majority of high-intensity precipitation comes from the organization of multiple convective cells into mesoscale convective systems (MCS). Here, we use a synthesis of multi-decade satellite and reanalysis data to investigate relationships between the column water vapor content (CWVC), instability (CAPE), and precipitation from MCS. We find a linear relationship between MCS maximum precipitation intensity and CAPE and a sharp, nonlinear relationship between this maximum precipitation intensity and CWVC. The latter suggests that a deep layer of inflow to the MCS dominates buoyancy and precipitation production. From these multidecade data, we can also illustrate robust shifts in the probability distributions of precipitation intensity with the El Niño Southern Oscillation. El Niño-La Niña relative gains and losses in precipitation intensity can be understood with a vertical momentum budget and the role of environmental relative humidity and large-scale circulation therein. Understanding the associated vertical moisture structure and instability is essential to better predict future variability in tropical precipitation.

How to cite: Schiro, K., Sullivan, S., Yin, J., and Gentine, P.: The vertical moisture structure and precipitation intensity distributions associated with tropical convective systems, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-21506, https://doi.org/10.5194/egusphere-egu2020-21506, 2020

D2990 |
EGU2020-19042<span style="font-size: .8em!important; font-weight: bold; vertical-align: super; color: green!important;"><span title="Early career scientist: an ECS is an undergraduate or postgraduate (Masters/PhD) student or a scientist who has received their highest degree (BSc, MSc, or PhD) within the past seven years. Provided parental leave fell into that period, up to one year of parental leave time may be added per child, where appropriate.">ECS</span></span>
Silas Boye Nissen and Jan O. Haerter

In radiative-convective equilibrium (RCE) simulations, self-aggregation is the spontaneous emergence of one or several long-lasting convective clusters from an apparently homogenous atmosphere (Wing, 2019). This phenomenon may implicate the formation of tropical cyclones (Wing et al., 2016; Muller et al., 2018) and large-scale events such as the Madden-Julian Oscillation (Arnold et al., 2015; Satoh et al., 2016; Khairoutdinov et al., 2018). However, it remains poorly understood how cold pools (CPs) contribute to self-aggregation. Using a suite of cloud-resolving numerical simulations, we link the life-cycle and the spatial organization of CPs to the evolution of self-aggregation. By tracking CPs, we determine the maximal CP radius Rmax ≈ 20 km and show that cloud-free regions exceeding such radii always grow indefinitely. Besides, we identify a minimum CP radius Rmin ≈ 8 km below which CPs are too cold, hence negatively buoyant, to initialize new convective cells. Finally, we suggest a simple mathematical framework that describes a mechanism, where cloud-free areas are likely to form when CPs have small Rmax, whereas large Rmax hampers cavity formation. Our findings imply that interactions between CPs crucially control the dynamics of self-aggregation, and known feedbacks may only be required in stabilizing the final, fully-aggregated state.

How to cite: Nissen, S. B. and Haerter, J. O.: Self-aggregation conceptualized by cold pool organization, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-19042, https://doi.org/10.5194/egusphere-egu2020-19042, 2020

D2991 |
EGU2020-1436<span style="font-size: .8em!important; font-weight: bold; vertical-align: super; color: green!important;"></span>
Yudi Liu, Dawei Li, and Lang Huang

To explore the characteristics of the concentric eyewall of a typhoon during its formation and replacement processes, with Super Typhoon Muifa in 2011 as the example case, the Weather Research and Forecast (WRF) mode was used to carry out a numerical simulation to reproduce the entire formation and replacement processes of the concentric eyewall. The physical quantities such as the tangential wind speed, radar echo, radial wind speed, vertical wind speed, and potential vortex were diagnosed and analyzed. The results of the analysis show that the outward expansion of the isovelocity in the lower troposphere was the early signal of the formation of the outer eyewall. After the outer eyewall formed, there was a center of second-highest tangential wind speed in the corresponding area. The second-highest wind speed increased as the strength of the outer eyewall increased, and the position of the second-highest wind speed center was retracted with the retraction of the outer eyewall. The tangential wind speed of the moat area was smaller than that corresponding to the concentric eyewall and this feature gradually disappeared with the increase of the height. The echo in the moat area was weak, and this characteristic was particularly evident when the moat area was relatively wide and the outer eyewall was relatively strong. With the formation and development of the outer eyewall, the intensity of the inflow in the boundary layer corresponding to the inner eyewall was reduced, the intensity of the outflow in the upper layers declined, and the intensities of the inflow and outflow corresponding to the outer eyewall were enhanced. After the second outer eyewall matured, there was a significant inflow in the upper layer of the moat area. Once the outer eyewall formed, a large amount of hydrometeors appeared in the corresponding area, and there was a strong ascending motion inside that area. The strength of the ascending motion and the content of hydrometeors increased as the outer eyewall increased. When the moat area was relatively wide, the divergent airflow generated by the developed outer eyewall in the upper layer would produce a significant descending motion in the moat area.

How to cite: Liu, Y., Li, D., and Huang, L.: Characteristics of the Concentric Eyewall Structure of Super Typhoon Muifa during Its Formation and Replacement Processes, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-1436, https://doi.org/10.5194/egusphere-egu2020-1436, 2019

D2992 |
EGU2020-2527<span style="font-size: .8em!important; font-weight: bold; vertical-align: super; color: green!important;"></span>
Chi-Cherng Hong, Chih-Hua Tsou, Kuan-Chieh Chen, and Chi-Chun Chang

This study addressed the abrupt increase in the development speed of Tropical storms (TSs) to severe TSs ( Category 3, referred to as STS) in the western North Pacific (WNP) during the late 1990s. Although the annual mean number of TSs in the WNP exhibited an abrupt decrease in the late 1990s, the annual mean number of STs did not exhibit significant change. This caused the ratio of annual mean number of STSs to the total TSs number displayed an abrupt increase in the late 1990s. Our observations indicate that the mean lifetime of a STS during the post 1990s period was approximately 70 hours shorter than that in the pre-1990s period because of the northwestward shift of the mean TS genesis location in response to the mega-La Niña-like mean state change in the late 1990s. This indicated that the TSs have developed into STSs with a faster speed since the late 1990s. The eddy kinetic energy budget of synoptic-scale eddy (SSE) indicated that the enhancement of scale interaction of intraseasonal oscillation (ISO)−SSE played a critical role in accelerating the TS–to–STS development. A further diagnosis revealed that the increase of ISO–SSE interaction was attributed to the mega–La Niña–like mean state change. The mega–La Niña–like associated anticyclone anomaly and warm oceanic condition in the WNP substantially modified the mean TS genesis location (northwestward shift), enhanced the ISO magnitude, and shifted the ISO propagation northward, thereby amplifying the ISO–SSE interaction in the WNP in the late 1990s.

Key words: Tropical storm, abrupt increase,  Intraseasonal oscillation (ISO), ISO–SSE interaction

How to cite: Hong, C.-C., Tsou, C.-H., Chen, K.-C., and Chang, C.-C.: Effect of ISO-SSE Interaction on the Rapid intensification of TS in the WNP Since the Late 1990s , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-2527, https://doi.org/10.5194/egusphere-egu2020-2527, 2020

D2993 |
EGU2020-2552<span style="font-size: .8em!important; font-weight: bold; vertical-align: super; color: green!important;"></span>
Kevin Hodges, Daniel Befort, and Antje Weisheimer

This study assesses the representation of Tropical Cyclones (TC) in an ensemble of seasonal forecast models from five different centres (ECMWF, UK Met Office, DWD, CMCC, Météo-France). Northern Hemispheric Tropical Cyclones are identified using a widely applied objective Tropical Cyclone tracking algorithm based on relative vorticity fields. Analyses for three different aspects are carried out: 1) assessment of the skill of the ensemble to predict  the TC frequencies over different ocean basins, 2) analyse the dependency between the model's ability to represent TCs and large-scale biases and 3) assess the impact of stochastic physics and horizontal resolution on TC frequency.

For the July to October season all seasonal forecast models initialized in June are skilful in predicting the observed inter-annual variability of TC frequency over the North Atlantic (NA). Similarly, the models initialized in May show significant skill over the Western North Pacific (WNP) for the season from June to October. Further to these significant positive correlations over the NA, it is found that most models are also able to discriminate between inactive and active seasons over this region. However, despite these encouraging results, especially  for skill over the NA, most models suffer from large biases. These biases are not only related to biases in the large-scale circulation but also to the representation of intrinsic model uncertainties and the relatively coarse resolution of current seasonal forecasts. At ECMWF model uncertainty is accounted for by the use of stochastic physics, which has been shown to improve forecasts on seasonal time-scales in previous studies. Using a set of simulations conducted with the ECMWF SEAS5 model, the effects of stochastic physics and resolution on the representation of Tropical Cyclones on seasonal time-scales are assessed. Including stochastic physics increases the number of TCs over all ocean basins, but especially over the North Atlantic and Western North Pacific.

How to cite: Hodges, K., Befort, D., and Weisheimer, A.: Tropical Cyclones in European Seasonal Forecast Models, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-2552, https://doi.org/10.5194/egusphere-egu2020-2552, 2020

D2994 |
EGU2020-3137<span style="font-size: .8em!important; font-weight: bold; vertical-align: super; color: green!important;"></span>
Lidia Nikitina, Paul Prikryl, and Shun-Rong Zhang

Convective bursts have been linked to intensification of tropical cyclones [1]. We consider a possibility of convective bursts being triggered by aurorally-generated atmospheric gravity waves (AGWs) that may play a role in the intensification process of tropical cyclones [2]. A two-dimensional barotropic approximation is used to obtain asymptotic solutions representing propagation of vortex waves [3] launched in tropical cyclones by quasi-periodic convective bursts. The absorption of vortex waves by the mean flow and formation of the secondary eyewall lead to a process of an eyewall replacement cycle that is known to cause changes in tropical cyclone intensity [4]. Rapid intensification of hurricanes and typhoons from 1995-2018 is examined in the context of solar wind coupling to the magnetosphere-ionosphere-atmosphere (MIA) system. In support of recently published results [2] it is shown that rapid intensification of TCs tend to follow arrival of high-speed solar wind when the MIA coupling is strongest. The coupling generates internal gravity waves in the atmosphere that propagate from the high-latitude lower thermosphere both upward and downward. In the lower atmosphere, they can be ducted [5] and reach tropical troposphere. Despite their significantly reduced amplitude, but subject to amplification upon over-reflection in the upper troposphere, these AGWs can trigger/release moist instabilities leading to convection and latent heat release. A possibility of initiation of convective bursts by aurorally generated AGWs is investigated. Cases of rapid intensification of recent tropical cyclones provide further evidence to support the published results [2].

References

[1] Steranka et al., Mon. Weather Rev., 114, 1539-1546, 1986.

[2] Prikryl et al., J. Atmos. Sol.-Terr. Phys., 2019.

[3] Nikitina L.V., Campbell L.J., Stud. Appl. Math., 135, 377–446, 2015.

[4] Willoughby H.E., et al., J. Atmos. Sci., 39, 395–411, 1982.

[5] Mayr H.G., et al., J. Geophys. Res., 89, 10929–10959, 1984.

How to cite: Nikitina, L., Prikryl, P., and Zhang, S.-R.: Rapid intensification of tropical cyclones: Vortex waves seeded by aurorally-generated atmospheric gravity waves?, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-3137, https://doi.org/10.5194/egusphere-egu2020-3137, 2020

D2995 |
EGU2020-3731<span style="font-size: .8em!important; font-weight: bold; vertical-align: super; color: green!important;"></span>
Wei Zhang, Gabriele Villarini, Enrico Scoccimarro, and Malcolm Roberts

This study examines the climatology and structure of precipitation associated with tropical cyclones based on the Atmospheric Model Intercomparison Project (AMIP) runs of the Process-based climate simulation: advances in high resolution modelling and European climate risk assessment (Primavera) Project during 1979-2014. We assess the role of spatial resolution in shaping tropical cyclone precipitation along with inter-model variability by evaluating climate models with a variety of dynamic cores and parameterization schemes. AMIP runs that prescribe historical sea surface temperatures and radiative forcings can well reproduce the observed spatial pattern of tropical cyclone precipitation climatology, with high-resolution performing better than low-resolution ones in the first order. Overall, the AMIP runs can also reproduce the fractional contribution of tropical cyclone precipitation to total precipitation in observations. Similar to tropical cyclone precipitation climatology, the factional contrition is better simulated by high-resolution models. All the models in the AMIP runs underestimate the observed composite tropical cyclone rainfall structure over both land and ocean, and we identify differences in this factor between high-resolution and low-resolution models. The underestimation of rainfall composites by the AMIP runs are also supported by the radial profile of tropical cyclone precipitation. This study shows that the high-resolution climate models can reproduce well the spatial pattern of tropical cyclone climatology and underestimate the composite rainfall structure, with increased spatial resolution that overall improves the performance of simulation.

How to cite: Zhang, W., Villarini, G., Scoccimarro, E., and Roberts, M.: Tropical Cyclone Precipitation in the AMIP Experiments of the Primavera Project, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-3731, https://doi.org/10.5194/egusphere-egu2020-3731, 2020

D2996 |
EGU2020-3788<span style="font-size: .8em!important; font-weight: bold; vertical-align: super; color: green!important;"></span>
xiaodian shen, qimin cao, baolin jiang, wenshi lin, and lan zhang

This study simulated the evolution of Typhoon Hato (2017) with the Weather Research and Forecasting model using three bulk schemes and one bin scheme. It was found that the track of the typhoon was insensitive to the microphysics scheme, whereas the degree of correspondence between the simulated precipitation and cloud structure of the typhoon was closest to the observations when using the bin scheme. The different microphysical structure of the bin and three bulk schemes was reflected mainly in the cloud water and snow content. The three bulk schemes were found to produce more cloud water because the application of saturation adjustment condensed all the water vapor at the end of each time step. The production of more snow by the bin scheme could be attributed to several causes: (1) the calculations of cloud condensation nuclei size distributions and supersaturation at every grid point that cause small droplets to form at high levels, (2) different fall velocities of different sizes of particles that mean small particles remain at a significant height, (3) sufficient water vapor at high levels, and (4) smaller amounts of cloud water that reduce the rates of riming and conversion of snow to graupel. The distribution of hydrometeors affects the thermal and dynamical structure of the typhoon. The saturation adjustment hypothesis in the bulk schemes overestimates the condensate mass. Thus, the additional latent heat makes the typhoon structure warmer, which increases vertical velocity and enhances convective precipitation in the eyewall region.

How to cite: shen, X., cao, Q., jiang, B., lin, W., and zhang, L.: Sensitivity of precipitation and structure of Typhoon Hato to bulk and explicit spectral bin microphysics schemes, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-3788, https://doi.org/10.5194/egusphere-egu2020-3788, 2020

D2997 |
EGU2020-738<span style="font-size: .8em!important; font-weight: bold; vertical-align: super; color: green!important;"></span>
Daniele Peano, Enrico Scoccimarro, Alessio Bellucci, Malcolm Roberts, Annalisa Cherchi, Alessandro D'Anca, Fabrizio Antonio, Sandro Fiore, and Silvio Gualdi

Tropical cyclones (TCs) transport energy and moisture along their pathways interacting with the climate system and TCs activities are expected to extend further poleward during the 21st century.

For this reason, it is important to assess the ability of state-of-the-art climate models in reproducing an accurate meridional distribution of TCs as well as a reasonable meridional portrait of moisture transport associated with TCs.

Since high resolutions are required to reconstruct observed TCs activity, the present work is based on the simulations performed as part of HighResMIP in the framework of the community CMIP6 effort. To inspect this feature, two horizontal resolutions for each climate model are considered. Besides, the impact of boundary conditions, i.e. observed ocean surface state, is examined by considering both coupled and atmosphere-only configurations.

In the present work, the north Atlantic region is analyzed as a sample region, while the same approach is applied on a multi-basin basis. In the sample area, climate models present a good ability in reproducing the TCs distribution, with a general underestimation at lower latitudes and a slight overestimation at high-latitudes compared to observed TCs tracks (e.g. IBTRACK).

The meridional distribution of moisture transport associated with TCs is evaluated by considering the radial average of the integrated water vapor transport along the TC tracks. When compared to observation (IBTRACS and JRA-55 reanalysis), the simulated moisture transport associated with TCs displays reasonably good performance in atmosphere-only high-resolution models configuration. The interannual variability of water vapor associated with TCs, instead, is poorly represented in climate models.

Climate models in high-resolution configuration can then be used in estimating future TCs meridional distribution and changes in meridional moisture transport associated with TCs.

This effort is part of HighResMIP and it is developed in the framework of the EU-funded PRIMAVERA project.   

How to cite: Peano, D., Scoccimarro, E., Bellucci, A., Roberts, M., Cherchi, A., D'Anca, A., Antonio, F., Fiore, S., and Gualdi, S.: Meridional distribution of moisture transport associated to Tropical Cyclones, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-738, https://doi.org/10.5194/egusphere-egu2020-738, 2019

D2998 |
EGU2020-3982<span style="font-size: .8em!important; font-weight: bold; vertical-align: super; color: green!important;"><span title="Early career scientist: an ECS is an undergraduate or postgraduate (Masters/PhD) student or a scientist who has received their highest degree (BSc, MSc, or PhD) within the past seven years. Provided parental leave fell into that period, up to one year of parental leave time may be added per child, where appropriate.">ECS</span></span>
Jufen Lai, Chaofan Li, and Riyu Lu

Interannual variation of tropical cyclone (TC) landfall frequency is not consistent along the coast of East Asia, with large contrast of north and south East Asia coast regions in boreal summer. This study examines interannual variations of TC landfall frequency over north and south East Asia and identifies roles of the western North Pacific subtropical high (WNPSH) and TC genesis frequency associated with these variations. Although the total number of landing TC of north and south East Asia is similar, interannual variations of TC landfall frequency are relatively independent to each other, with the corresponding correlation coefficient north and south of 25°N is only –0.024 from 1979 to 2017. TC landfall over north East Asia is largely modulated by the circulation related to the WNPSH, while TC landfall in the south has no significant relationship with the WNPSH or other remote large-scale circulations. The WNPSH effectively regulates TC landfall in the north by modulating TC genesis east of the Philippines and steering flows. Nonetheless, the two factors have weak contradictory effects on TC landing in the south region. The frequency of TC genesis around the South China Sea directly connects to the TC landfall over south East Asia, which is modulated by the surrounding genesis environment, including relative humidity and relative vorticity. This work favors for a better understanding of the seasonal forecasts of TC landfall frequency and the subsequent climate service over East Asia.

How to cite: Lai, J., Li, C., and Lu, R.: Considerable differences of the interannual variations for the tropical cyclone landfall over north and south East Asia in summer, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-3982, https://doi.org/10.5194/egusphere-egu2020-3982, 2020

D2999 |
EGU2020-4286<span style="font-size: .8em!important; font-weight: bold; vertical-align: super; color: green!important;"><span title="Early career scientist: an ECS is an undergraduate or postgraduate (Masters/PhD) student or a scientist who has received their highest degree (BSc, MSc, or PhD) within the past seven years. Provided parental leave fell into that period, up to one year of parental leave time may be added per child, where appropriate.">ECS</span></span>
Chengyao Ye, Liping Deng, Wan-Ru Huang, and Jinghua Chen

This paper explores the modulation by Madden–Julian Oscillation (MJO) on tropical-cyclone (TC; hereafter, MJO TC) genesis over the Western North Pacific (WNP) and the South China Sea (SCS) under different El Niño Southern Oscillation (ENSO) conditions. Analyses used Joint Typhoon Warning Center (JTWC) Best Track data, the Real-Time Multivariate MJO (RMM) index, and European Center for Medium-Range Weather Forecasts (ECMWF) Interim (ERA-Interim) reanalysis data. Results showed that MJO has significant modulation on both SCS and WNP TC genesis in neutral years, with more (fewer) TCs forming during active (inactive) MJO phases. However, during El Niño and La Niña years, modulation over the two regions differs. Over the SCS, the modulation of TC genesis is strong in La Niña years, while it becomes weak in El Niño years. Over the WNP, MJO has stronger influence on TC genesis in El Niño years compared to that in La Niña years. Related Genesis Potential Index (GPI) analysis suggests that midlevel moisture is the primary factor for MJO modulation on SCS TC genesis in La Niña years, and vorticity is the secondary factor. Over the WNP, midlevel moisture is the dominant factor for MJO TC genesis modulation during El Niño years. The main reason is increased water-vapor transport from the Bay of Bengal associated with the active MJO phase related westerly wind anomalies; these features are a significant presence over the SCS during La Niña years, and over the WNP during El Niño years.

How to cite: Ye, C., Deng, L., Huang, W.-R., and Chen, J.: Comparison of Madden–Julian Oscillation Modulation on tropical-cyclone genesis over the South China Sea and Western North Pacific under different El Niño Southern Oscillation conditions, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-4286, https://doi.org/10.5194/egusphere-egu2020-4286, 2020

D3000 |
EGU2020-4372<span style="font-size: .8em!important; font-weight: bold; vertical-align: super; color: green!important;"><span title="Early career scientist: an ECS is an undergraduate or postgraduate (Masters/PhD) student or a scientist who has received their highest degree (BSc, MSc, or PhD) within the past seven years. Provided parental leave fell into that period, up to one year of parental leave time may be added per child, where appropriate.">ECS</span></span>
Thi Ngoc Huyen Ho, S.-Y. Simon Wang, Karthik Balaguru, Kyo-Sun Lim, and Jin-Ho Yoon

Tropical cyclones (TCs) are the most dangerous climatic events in Vietnam. Recently, most of the studies have focused on TCs frequency and intensity, yet the rainfall events caused by them have not been got adequate attention. We show here the long-term change of TCs activity developed in both the South China Sea and the Philippines Sea and estimated its potential impacts during the period of 1977 – 2016. The trend analysis reveals that TCs have not shown obvious variability in numbers and destructiveness ability, whereas the TCs-induced rainfall events and its spatial distribution exhibit more complex patterns in different parts of Vietnam. For example, increasing rainfall amounts in the northern part is likely caused by TCs despite the fact that the TCs frequency did not exhibit much of significant changes. Evaluating rainfall caused by TCs activity is of great practical significance for Vietnam. Our findings suggest that in addition to the TCs frequency and intensity, TCs-induced rainfall events should be considered and included in future preparedness and response plans both on regional and national scale.

How to cite: Ho, T. N. H., Wang, S.-Y. S., Balaguru, K., Lim, K.-S., and Yoon, J.-H.: Long-term change of tropical cyclones activity and its potential impacts on Vietnam, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-4372, https://doi.org/10.5194/egusphere-egu2020-4372, 2020

Chat time: Wednesday, 6 May 2020, 16:15–18:00

D3001 |
EGU2020-7521<span style="font-size: .8em!important; font-weight: bold; vertical-align: super; color: green!important;"></span>
Liang Wu

Two high-resolution climate models (the HiRAM and MRI-AGCM3.2) are used to simulate present-day western North Pacific (WNP) tropical cyclone (TC) activity and investigate the projected changes for the late 21st century. Compared toobservations, the models are able to realistically simulate many basic features of the WNP TC activity climatology. Future projections with the coupled model inter-comparison project phase 5 (CMIP5) under Representative Concentration Pathway (RCP) 8.5 scenario show a tendency for decreases in the number of WNP TCs, and of increases in the more intense TCs. It is unknown to what cause this inverse variation with number and intensity should be generally linked to similar large-scale environmental conditions. To examine the WNP TC genesis and intensity with environmental variables, we show that most of the current trend of decreasing genesis of TCs can be attributed to weakened dynamic environments and the current trend of increasing intensity of TCs might be linked to increased thermodynamic environments. Thus, the future climate warms under RCP 8.5 will likely lead to strong reductions in TC genesis frequency over the WNP, with project decreases of 36-63% by the end of the twenty-first century, but lead to greater TC intensities with rapid development of thermodynamic environments.

How to cite: Wu, L.: Changes in the Past and Projected Western North Pacific Tropical Cyclone Activity in a Warmed Climate, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-7521, https://doi.org/10.5194/egusphere-egu2020-7521, 2020

D3002 |
EGU2020-10844<span style="font-size: .8em!important; font-weight: bold; vertical-align: super; color: green!important;"></span>
Jean Philippe Duvel

Numerous low and mid-level vortices are initiated respectively north and south of 15°N in West Africa and enter the North Atlantic where they may trigger cyclogenesis. Applying an objective vortex tracking algorithm on 38 years of meteorological re-analysis, this work investigates the vortex origin and their role in cyclogenesis with an emphasis on: (i) orography, (ii) seasonal variations and, (iii) merge between low and mid-level vortex tracks. North path vortices are mostly initiated downstream of Hoggar Mountains (5°E, 24°N) and south path vortices are mostly initiated downstream of Fouta Djallon Mountains (15°W, 10°N). About 55% of cyclogeneses in the Main Development Region (MDR: east of 60°W; 5 to 20°N) is associated with vortices initiated on the continent east of 10°W. MDR cyclonic activity is governed by seasonal and interannual variations of the local Genesis Potential Index (maximal in August-September) and not by the number of vortices entering the Ocean. North path vortices, which are more numerous in July, are thus less cyclogenetic compared to south path vortices that are more numerous in August-September. Considering together vortices initiated on the continent and near the coast, about 20% of the cyclogeneses are associated with merge of north and south path vortices and about 14% with north path vortices only. The remaining part is mostly associated with south path vortices. In addition, south path vortices with greater intensity and vertical development between Greenwich and the coast are more cyclogenetic.

How to cite: Duvel, J. P.: On vortices initiated over West Africa and their impact on North Atlantic tropical cyclones, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-10844, https://doi.org/10.5194/egusphere-egu2020-10844, 2020

D3003 |
EGU2020-12173<span style="font-size: .8em!important; font-weight: bold; vertical-align: super; color: green!important;"></span>
Mincheol Moon and Kyung-Ja Ha

The Weather Research and Forecast (WRF) models have been used to investigate the sensitivity of simulations of Typhoon Tembin (1214) to changes in three horizontal grid spacings of 12km, 8km, and 6km and the effect of the cold wake generated by the previous Typhoon Bolaven (1215). It was observed that Bolaven-generated cold wakes cooled up to 7 °C in the sea around the Korean Peninsula. There are many previous studies on track dynamics influenced by sea surface temperature (SST) gradient due to the cold wakes. However, the intensity and precipitation of the following tropical cyclone (TC) is not yet certain. Here we show that the effect of SST gradient on the following TC are examined with WRF models of varying resolutions using modified SST from observed real-time data of the Ieodo Ocean Research Station and the Yellow Sea buoy in Korea. In the track of TC, a higher resolution showed the faster and more eastward movement of TCs in all SST conditions. TC tends to move more eastward at all resolutions particularly when the cold wake is generated in the western region of TC. When there is no cold wake, the intensity of TC is very sensitive to the resolution of the experiment. If a cold wake is maintained on the western (eastern) sides, the intensity of TC is weaker(stronger) than no cold wake experiment and is less sensitive to differences in resolution. The precipitation rate of TCs in the cold wake of the eastern (western) region is lower (higher) than when there is no wake. As the aspect of horizontal resolutions, the precipitation rate of TC in higher resolution shows stronger than lower resolution. The TC-generated cold wake significantly affects intensity and movement in cold wake cases in the western region, regardless of the horizontal grid, for various reasons.

How to cite: Moon, M. and Ha, K.-J.: Effect of SST gradient caused by Typhoon-Generated Cold Wake on the Subsequent Typhoon Tembin in models of varying resolutions, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-12173, https://doi.org/10.5194/egusphere-egu2020-12173, 2020

D3004 |
EGU2020-12834<span style="font-size: .8em!important; font-weight: bold; vertical-align: super; color: green!important;"><span title="Early career scientist: an ECS is an undergraduate or postgraduate (Masters/PhD) student or a scientist who has received their highest degree (BSc, MSc, or PhD) within the past seven years. Provided parental leave fell into that period, up to one year of parental leave time may be added per child, where appropriate.">ECS</span></span>
Han-Kyoung Kim, Sang-Wook Yeh, Nam-Young Kang, and Byung-Kwon Moon

Tropical cyclone (TC) genesis frequency over the western North Pacific (WNP) is reduced significantly since the late 1990s, coinciding with a Pacific decadal oscillation (PDO) phase change from positive to negative. In this study, the underlying mechanism for this reduction is investigated through analysis of asymmetric central Pacific (CP) El Niño-Southern Oscillation (ENSO) properties induced by the negative PDO phase. Results suggest that the significant reduction is caused by asymmetric CP ENSO properties, in which the CP La Niña is more frequent than the CP El Niño during negative PDO phases; furthermore, stronger CP La Niña occurs during a negative PDO phase than during a positive PDO phase. CP La Niña (El Niño) events generate an anticyclonic (cyclonic) Rossby wave response over the eastern WNP, leading to a significant decrease (increase) in eastern WNP TC genesis. Therefore, more frequent CP La Niña events and the less frequent CP El Niño events reduce the eastern WNP mean TC genesis frequency during a negative PDO phase. In addition, stronger CP La Niña events during a negative PDO phase reinforce the reduction in eastern WNP TC genesis. The dependency of CP ENSO properties on the PDO phase is confirmed using a long-term climate model simulation, which supports our observational results. 

Acknowledgements: This work was supported by a National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIT; No. 2019R1A2C1008549).

How to cite: Kim, H.-K., Yeh, S.-W., Kang, N.-Y., and Moon, B.-K.: Asymmetric impact of CP ENSO on the significant reduction of tropical cyclone genesis frequency over the WNP since the late 1990s, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-12834, https://doi.org/10.5194/egusphere-egu2020-12834, 2020

D3005 |
EGU2020-16658<span style="font-size: .8em!important; font-weight: bold; vertical-align: super; color: green!important;"><span title="Early career scientist: an ECS is an undergraduate or postgraduate (Masters/PhD) student or a scientist who has received their highest degree (BSc, MSc, or PhD) within the past seven years. Provided parental leave fell into that period, up to one year of parental leave time may be added per child, where appropriate.">ECS</span></span>
Arathy Menon, Ambrogio Volonté, Andrew Turner, and Kieran Hunt

Monsoon depressions (MD) are synoptic-scale cyclonic vortices that form over the Bay of Bengal and propagate northwestward through the monsoon trough onto the Indian subcontinent, bringing substantial amounts of rainfall to central and northern India. Despite their importance, key questions on the mechanisms driving their generation and development are still open. In this study we inspect the structure and dynamics of a MD case study (1-10 July 2016) using a set of high-resolution simulations performed within the INCOMPASS project. The simulations are performed at a grid spacing of 17 km, 4.4 km and 1.5 km (with parametrised convection for the former experiment and explicit convection for the latter two). Initial results of this study show that the two higher-resolution simulations are more effective in resolving intense rainfall caused by deep convection, convergence lines and orographic enhancement. The evolution of the case-study MD can be divided into two stages: initially the MD is completely embedded in a close-to-saturated environment up to mid-troposphere, whilst in the following stage the intrusion of low-potential-temperature dry air at low- and mid-levels starts interacting with the MD. During this latter stage, the dry-air intrusion brings in low PV-air towards the centre of the depression. Further analysis of the case study takes advantage of a system-relative framework to look into detail at the time evolution of dynamic and thermodynamic parameters around the storm centre and at its small- and meso-scale structure. For example, the 1.5 km-spacing simulation enables us to highlight the presence of individual vorticity towers embedded within the MD. In summary, using a suite of high-resolution numerical simulations of a case-study MD, we are able to achieve a detailed understanding of its structure and dynamics, highlighting the processes driving its evolution.

How to cite: Menon, A., Volonté, A., Turner, A., and Hunt, K.: Structure and dynamics of a case-study monsoon depression in high-resolution numerical simulations using the Met Office Unified Model, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-16658, https://doi.org/10.5194/egusphere-egu2020-16658, 2020

D3006 |
EGU2020-17026<span style="font-size: .8em!important; font-weight: bold; vertical-align: super; color: green!important;"></span>
Corinna Hoose, Hyunju Jung, Peter Knippertz, Tijana Janjic, Yvonne Ruckstuhl, and Robert Redl

Tropical weather prediction remains one of the main challenges in atmospheric science due to a combination of insufficient observations, data assimilation algorithms optimized for midlatitudes and large model errors. Due to a strong dependency of many people in the tropics on rainfall variability, combined with a high vulnerability, improved precipitation forecasts have the potential to create substantial benefits in areas such as agriculture, water management, energy production and disease prevention.

Recent studies found that the coupling of equatorial waves to convection is key to improving weather forecasts in the tropics on the synoptic to subseasonal timescale but many models struggle to realistically represent this coupling. Here we use aquaplanet simulations with the ICOsahedral Nonhydrostatic (ICON) model with a 13 km horizontal grid spacing to study the underlying mechanisms of convectively coupled equatorial waves in an idealized framework. We filter the divergence at 200 hPa using a standard wave filtering tool tapering to zero that allows us to identify dynamical characteristics of convectively coupled waves in our simulations. To diagnose thermodynamical aspects of wave-convection couplings, we compare the obtained waves to the total precipitable water and analyze the spatial variance of the budget analysis for column-integrated moist static energy. The same filtering tool and diagnostics are carried out on a realistic ICON simulation with a 2.5 km horizontal grid spacing from the DYnamics of the Atmospheric general circulation Modeled On Non-hydrostatic Domains (DYAMOND) project.

In the future we plan to run and analyze idealized tropical channel simulations with 2.5 km horizontal resolution, i.e. using the same grid spacing as in the DYAMOND simulation. The comparison between the idealized and the realistic simulations identifies mechanisms of wave-convection coupling. In addition, we will apply this set of diagnostics to forecast experiments using different approaches of data assimilation.

 

How to cite: Hoose, C., Jung, H., Knippertz, P., Janjic, T., Ruckstuhl, Y., and Redl, R.: Disentangling the mechanisms of wave-convection coupling in the tropics, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-17026, https://doi.org/10.5194/egusphere-egu2020-17026, 2020

How to cite: Hoose, C., Jung, H., Knippertz, P., Janjic, T., Ruckstuhl, Y., and Redl, R.: Disentangling the mechanisms of wave-convection coupling in the tropics, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-17026, https://doi.org/10.5194/egusphere-egu2020-17026, 2020

How to cite: Hoose, C., Jung, H., Knippertz, P., Janjic, T., Ruckstuhl, Y., and Redl, R.: Disentangling the mechanisms of wave-convection coupling in the tropics, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-17026, https://doi.org/10.5194/egusphere-egu2020-17026, 2020

D3007 |
EGU2020-18988<span style="font-size: .8em!important; font-weight: bold; vertical-align: super; color: green!important;"><span title="Early career scientist: an ECS is an undergraduate or postgraduate (Masters/PhD) student or a scientist who has received their highest degree (BSc, MSc, or PhD) within the past seven years. Provided parental leave fell into that period, up to one year of parental leave time may be added per child, where appropriate.">ECS</span></span>
Julia Windmiller, David Leutwyler, and Tom Beucler
A peculiar feature of the tropical atmosphere is its tendency to be either rather moist or very dry. It has
been shown that the two states are separated by a sharp margin, well identified by the anti-mode of
the bimodal tropical column water vapor distribution (CWV). Despite recent progress in understanding
physical processes governing the spatio-temporal variability of CWV near the Equator, the behavior of
this meander remains elusive, and we lack a simple framework to understand the bimodality of tropical
CWV in observations. A tendency of the atmosphere to be either moist or dry can also be found in
idealized simulations of radiative-convective equilibrium (RCE). A number of studies assessing self-
aggregation of convection in RCE simulations have indicated that positive feedbacks between
radiation and moisture lead to spatially concentrated moisture in a small area of the computational
domain. As a result, RCE simulations also exhibit a bimodal distribution of CVW and a distinct moist
margin.
Motivated by theory originating from statistical physics, we argue that, in steady-state RCE, the length
of the margin separating moist and dry regions should minimize. Based on these ideas, we design a
diagnostic relating the moist margin length to the circumference of a defined equilibrium shape. Using
that Moist Margin Length Index (MMLi), we assess the evolution of self-aggregation in idealized
convection-resolving RCE simulations and contrast it to the oscillations of the Atlantic Inter-Tropical
Convergence Zone in the ERA5 reanalysis product.
We find that MMLi successfully describes aspects of convective organization not captured by more
traditional metrics of convective organization (e.g., the CVW's interquartile range) while offering a new
way to characterize the seasonal cycle of convective organization in the Atlantic ITCZ. Overall, our
new framework uses the moist meander margin to connect the observationally-motivated, object-
oriented view of convective organization with the processes driving the spatio-temporal evolution of
the CWV field. It can be broadly deployed across models and observations to lead further insight into
the bimodal nature of CWV while offering an efficient and visual way to quantify convective
organization.

How to cite: Windmiller, J., Leutwyler, D., and Beucler, T.: Quantifying Convective Aggregation using the Moist Tropical Margin’s Length, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-18988, https://doi.org/10.5194/egusphere-egu2020-18988, 2020

D3008 |
EGU2020-19806<span style="font-size: .8em!important; font-weight: bold; vertical-align: super; color: green!important;"></span>
José M. Castanheira and Carlos A. F. Marques

The Madden-Julian oscillation (MJO) is a major intraseasonal tropical atmospheric mode which modulates the precipitation in the Tropical Indian and Pacific  oceans. It is a large atmospheric convective system, dominated the zonal wave number one scale, that moves eastward from the east coast of Africa to eastern Pacific in a time scale of  30-70 days.

The MJO can have impact in global weather but is yet poorly simulated in most atmospheric circulation models. Therefore, it is important to understand the convective-dynamical nature of the MJO to understand the reasons for its poor representation in models.

Here we present a diagnostic study of the MJO by decomposing the circulation associated with a multivariate MJO index onto 3-Dimensional inertio-gravitic and Rossby modes, based on the ERA-I reanalysis. Results show that the main dynamical features of MJO are represented by  a combination of  Kelvin and the first (lr = 1) equatorial Rossby modes with zonal wavenumbers 1 to 4. The vertical structures of the waves correspond to a first baroclinic mode in the troposphere. Moreover, a space–time spectral analysis confirmed the existence of an eastward moving MJO signal in the equatorial Rossby modes.

Nonlinear interactions between the westward moving equatorial Rossby waves and eastward moving Kelvin waves may be the cause for the slow eastward progression of the MJO. 

How to cite: Castanheira, J. M. and Marques, C. A. F.: The dynamical composition of the Madden-Julian oscillation, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-19806, https://doi.org/10.5194/egusphere-egu2020-19806, 2020

D3009 |
EGU2020-20548<span style="font-size: .8em!important; font-weight: bold; vertical-align: super; color: green!important;"><span title="Early career scientist: an ECS is an undergraduate or postgraduate (Masters/PhD) student or a scientist who has received their highest degree (BSc, MSc, or PhD) within the past seven years. Provided parental leave fell into that period, up to one year of parental leave time may be added per child, where appropriate.">ECS</span></span>
Lisa Maillard, Julien Boucharel, Lionel Renault, and Thomas Arsouze

Tropical Cyclones (TCs) are among the most destructive natural phenomena on Earth and severely impact nearly a billion people. Coupled models have become a necessary tool to improve our knowledge on those natural hazards. Improving their ability to statistically represent TCs is of prior importance. In the present study, we investigate the impact of the mechanical interaction between the surface oceanic current and the atmosphere (i.e., the Current FeedBack, CFB) on the statistic of TCs in different basins. We perform sensitivity experiments using the EC-Earth model in its High-Resolution version (1/12˚), by switching on and off CFB. As CFB has been shown to strongly improve the realism of the oceanic circulation at both large scale and mesoscale, we expect an improvement, i.e., a better realism, of the statistical TCs representation when CFB is taken into account in the model. Improving coupled models will help design forecast schemes with lead times longer than those currently provided by operational forecasts centers.

How to cite: Maillard, L., Boucharel, J., Renault, L., and Arsouze, T.: Impact of the current feedback on the representation of tropical cyclones in coupled models, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-20548, https://doi.org/10.5194/egusphere-egu2020-20548, 2020

D3010 |
EGU2020-12456<span style="font-size: .8em!important; font-weight: bold; vertical-align: super; color: green!important;"></span>
Jainn Shi, Scott Braun, Zhining Tao, and Jason Sippel

This presentation will focus on simulations of the early stages of Hurricane Nadine (2012), which interacted with the SAL and never intensified beyond a minimal hurricane. Given the complexity of aerosol effects on cloud microphysics and radiation and their subsequent effects on deep convective clouds, there is a need to assess the combined microphysical and radiative effects of aerosols. We use the Goddard Space Flight Center version of the Weather Research and Forecasting model with interactive aerosol-cloud-radiation physics to study the influence of the SAL and other aerosols (sea salt and black/organic carbon) on Nadine via a series of model sensitivity runs. The results from the control experiment with all aerosols will be compared to the dropsonde and CPL aerosol lidar backscatter data collected during the NASA Hurricane and Severe Storm Sentinel (HS3) field campaign. Comparison of model results and dropsonde data shows evidence of the intrusion of Saharan air into the storm core. Simulation results also show the possible intrusion of biomass-burning aerosols that originated from forest fires in the Northwestern United States a few days before Nadine reached hurricane strength. In addition, we will also present results from three sets of 30-member ensemble simulations: 1) without aerosol coupling, 2) with all aerosols, and 3) with only dust aerosol, to study the aerosol impact on Nadine.

How to cite: Shi, J., Braun, S., Tao, Z., and Sippel, J.: Influence of Saharan Dust and Other Aerosols on Hurricane Nadine (2012) as Revealed by the Comparison of Ensemble Model Results and NASA HS3 Data, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-12456, https://doi.org/10.5194/egusphere-egu2020-12456, 2020

D3011 |
EGU2020-1036<span style="font-size: .8em!important; font-weight: bold; vertical-align: super; color: green!important;"><span title="Early career scientist: an ECS is an undergraduate or postgraduate (Masters/PhD) student or a scientist who has received their highest degree (BSc, MSc, or PhD) within the past seven years. Provided parental leave fell into that period, up to one year of parental leave time may be added per child, where appropriate.">ECS</span></span>
Sambrita Ghatak and Jai Sukhatme
The Quasi-Biweekly Oscillation (QBWO), with a period between the synoptic scale and the Madden Julian Oscillation (MJO), is an important component of tropical intraseasonal variability (ISV). While this mode has received a lot of attention over India (during the summer monsoon) and South China sea region, less attention has been paid on South-western Indian Ocean (SWIO). Apart from improving our understanding of ISV, this mode is important in the SWIO during boreal winter as this is an active basin for tropical cyclones (TC), and the QBWO significantly influences TCs. Here we study details of the genesis of the QBWO, its propagation, vertical structure and evolution. The data used comprises of NCEP-NCAR and ERA-interim reanalysis and NOAA outgoing longwave radiation (OLR) from 2000-2010. A composite analysis based on 10-30 day filtered data during the boreal winter reveals a well-organized convectively-coupled wave-train pattern, namely the QBWO, over the SWIO. It emerges from south of the equator (5S), between 50-80E, and then propagates south-westward. The horizontal structure exhibits a slight southwest-northeast tilt, but mainly longitudinal elongation. After arriving Madagascar, the system shows more pronounced southward migration. Further, negative (positive) OLR anomalies are tightly coupled with a cyclonic  (anticyclonic) circulation at 850 hPa. In the end, the QBWO with associated anomalous convection dies down near 40S, between 40-70E. Overall, over this oceanic basin, the QBWO has a period of approximately 18 days, wavelength of about 5000-6000 km, a southward (westward) phase speed of 1.9 (2.7)°/day and a near zero group velocity. Near equator, the system emerges with an equivalent barotropic structure. When mature with strongest convection around 10-20S, the system becomes weakly baroclinic, with relative vorticity anomaly changing sign near 400 hPa, and in the dying phase after 30S, the QBWO becomes equivalent barotropic again. Finally, the centres of relative vorticity and vertical motion near the equator are consistent with the characteristics of equatorial Rossby waves, whereas the cyclonic circulation is tightly coupled with anomalous convection as the wave moves away from the equator.

How to cite: Ghatak, S. and Sukhatme, J.: South-westward Propagating Quasi-biweekly Oscillation over South-western Indian Ocean during Boreal Winter, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-1036, https://doi.org/10.5194/egusphere-egu2020-1036, 2019

D3012 |
EGU2020-1215<span style="font-size: .8em!important; font-weight: bold; vertical-align: super; color: green!important;"><span title="Early career scientist: an ECS is an undergraduate or postgraduate (Masters/PhD) student or a scientist who has received their highest degree (BSc, MSc, or PhD) within the past seven years. Provided parental leave fell into that period, up to one year of parental leave time may be added per child, where appropriate.">ECS</span></span>
Haoyan Liu

This study investigates the intensity change of binary tropical cyclones (TCs) under the influence of their mutual interaction in an idealized three-dimensional full-physics numerical model with a finest horizontal resolution of 3 km. The two identical TCs merge within the initial separation distance of 600 km.

Due to the interaction between binary TCs, the intensity evolution presents two weakening stages and an unchanged stage between them. Such intensity change of each one in binary TCs is correlated to the upper-layer vertical wind shear (VWS) caused by the other TC. During the first stage, the upper-layer anticyclone (ULA) of one TC results in the upper-layer VWS and ventilates the warm core of the other TC above the outflow layer, which causes the intensity of the binary TCs decreasing. During the second stage, as the ULA stretches downward and outward, the upper-layer VWS changes to the opposite direction, along with the intensity decreasing first and then increasing. Meanwhile, the intensity of the binary TCs stays unchanged. In the last stage, the binary TCs weaken again as the upper-layer VWS increases to some extent except the merging cases. When the two TCs approach each other before merging, the upper-layer VWS in one TC is almost caused by the upper-layer cyclone and outflow of the other, which induces highly asymmetric structure and weakens the vortex. In addition, the horizontal size of the ULA quantified by the Rossby radius of deformation seems to be a critical separation distance of binary TCs, exceeding which the VWS is small enough to influence the intensity.

How to cite: Liu, H.: Intensity Change of Binary Tropical Cyclones in an Idealized Three-Dimensional Model, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-1215, https://doi.org/10.5194/egusphere-egu2020-1215, 2019

D3013 |
EGU2020-1704<span style="font-size: .8em!important; font-weight: bold; vertical-align: super; color: green!important;"><span title="Early career scientist: an ECS is an undergraduate or postgraduate (Masters/PhD) student or a scientist who has received their highest degree (BSc, MSc, or PhD) within the past seven years. Provided parental leave fell into that period, up to one year of parental leave time may be added per child, where appropriate.">ECS</span></span>
Nannan Qin, Da-Lin Zhang, William Miller, and Chanh Kieu

Recent studies show that some hurricanes may undergo rapid intensification (RI) without contracting the radius of maximum wind (RMW). A cloud-resolving WRF-prediction of Hurricane Wilma (2005) is used herein to examine what controls the RMW contraction and how a hurricane could undergo RI without contraction. Results show that the processes controlling the RMW contraction are different within and above the planetary boundary layer (PBL). In the PBL, radial inflows contribute to contraction, with frictional dissipation acting as an inhibiting factor. Above the PBL, radial outflows and vertical motion govern the RMW contraction, with the former inhibiting it. A budget analysis of absolute angular momentum (AAM) shows that the radial AAM flux convergence is the major process accounting for the spinup of the maximum rotation, while the vertical flux divergence of AAM and the frictional sink in the PBL oppose the spinup. During the RI stage with no RMW contraction, the local AAM tendencies in the eyewall are smaller in magnitude and narrower in width than those during the contracting RI stage. In addition, the AAM following the time-dependent RMW decreases with time in the PBL and remains nearly constant aloft during the contracting stage, whereas it increases during the non-contracting stage. These results reveal different constraints for the RMW contraction and RI, and help explain why a hurricane vortex can still intensify after the RMW ceases contraction

How to cite: Qin, N., Zhang, D.-L., Miller, W., and Kieu, C.: Inner-core dynamics during the rapid intensification of Hurricane Wilma (2005) with a steady radius of the maximum wind, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-1704, https://doi.org/10.5194/egusphere-egu2020-1704, 2019

D3014 |
EGU2020-2934<span style="font-size: .8em!important; font-weight: bold; vertical-align: super; color: green!important;"><span title="Early career scientist: an ECS is an undergraduate or postgraduate (Masters/PhD) student or a scientist who has received their highest degree (BSc, MSc, or PhD) within the past seven years. Provided parental leave fell into that period, up to one year of parental leave time may be added per child, where appropriate.">ECS</span></span>
Rui Xiong and Mengqian Lu

The western North Pacific (WNP) is one of the most active tropical cyclone (TC) regions, which can inflict enormous death and massive property damage to surrounding areas. Although many studies about tropical cyclone activities on multi-timescales have been done, most of them focus on the entire basin, variations within the basin deserve more investigations. Besides TC characteristics on different timescales, to investigate the impacts of environment variables on TC and provide informative factors for prediction is another concern in the research community. In this study, we adopt several data science techniques, including Gaussian kernel estimator, wavelet, cross-wavelet coherence and regression analyses, to explore the spatiotemporal variations of TC genesis and associated environmental conditions. Significant semiannual and annual variations of TC genesis have been found in the northern South China Sea (NSCS) and oceanic areas east of the Philippines (OAEP). In the southeast part of WNP (SEWNP), TC genesis shows prominent variations on ENSO time scale. With reconstructed TC series on those frequencies, we further quantify the influences of environmental variables on the primary TC signals over WNP. About 40% of the identified TC variance over NSCS and OAEP can be explained by variability in vertical shear of zonal wind and relative humidity. In the SEWNP, TC genesis reveals strong nonlinear and non-stationary relationships with vertical shear of zonal wind and absolute vorticity. Besides, A probabilistic clustering algorithm is used to describe the TC tracks in the WNP. The best track dataset from JMA is decomposed into three clusters based on genesis location and curvature. For each cluster, we analyze the relationships between TC properties, such as genesis location, trajectory and intensity, and associated environmental conditions using the self-organizing map. The spatial patterns of sea surface temperature have huge impacts on TC genesis location, while the trajectory is largely influenced by geopotential height.

How to cite: Xiong, R. and Lu, M.: Tropical cyclone genesis and trajectory characteristics in the western north Pacific, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-2934, https://doi.org/10.5194/egusphere-egu2020-2934, 2020

D3015 |
EGU2020-3277<span style="font-size: .8em!important; font-weight: bold; vertical-align: super; color: green!important;"></span>
Yunfei Zhang, Xiang Li, Tiejun Ling, Chenqi Wang, and Hongyu Qu

Tropical cyclone (TC) activity has significant seasonal, interannual and interdecadal variations. Accurate prediction of TC seasonal activities before the onset of the coming TC season (June-November) can provide sufficient time for the government and the public to prepare for tropical cyclone disasters and minimize risks and life losses.
Based on COAWST model, we developed a new regional coupled seasonal forecasting system for the Northwest Pacific Ocean including a series of technology improvements. The results of multi-year hindcast experiments show that the coupled seasonal forecasting system can effectively improve the tropical cyclone frequency and intensity forecast compared to the CFSv2 real-time seasonal forecast, especially the tropical cyclone frequency forecast of the TC exceeding the typhoon level, but there is still a certain gap between the results in the forecasting system and the observed TC frequency and intensity, which is mainly reflected in the fact that the forecasting season has a higher frequency of TCs and the peak of strong TCs is relatively weaker. This gap may be caused by the forecasting bias of the sea surface temperature.

How to cite: Zhang, Y., Li, X., Ling, T., Wang, C., and Qu, H.: Coupled tropical cyclone seasonal forecasting system over the Northwest Pacific Ocean in NMEFC, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-3277, https://doi.org/10.5194/egusphere-egu2020-3277, 2020

D3016 |
EGU2020-3772<span style="font-size: .8em!important; font-weight: bold; vertical-align: super; color: green!important;"></span>
Karthik Balaguru, Gregory Foltz, Ruby Leung, John Kaplan, Wenwei Xu, Nicolas Reul, and Bertrand Chapron

Rapid Intensification (RI) of hurricanes is difficult to predict and poses a formidable threat to coastal populations. While a warm upper-ocean is well-known to favor RI, the role of salinity is less clear. In this study, using a suite of observations, we demonstrate that the subsurface oceans' influence on Atlantic hurricane RI exhibits two regimes. In the western region, which includes the Gulf of Mexico and the western Caribbean Sea, temperature stratification plays an important role in hurricane RI with little impact from salinity. On the other hand, in the eastern region dominated by the Amazon-Orinoco plume, salinity stratification prominently impacts RI. While a weak temperature stratification aids cold wake reduction for hurricanes in the western region, a strong salinity stratification causes less hurricane-induced mixing and surface cooling in the eastern region. Finally, in both regions, the relevance of the cold wake, and consequently the ocean sub-surface, is enhanced during RI compared to weaker intensification.

How to cite: Balaguru, K., Foltz, G., Leung, R., Kaplan, J., Xu, W., Reul, N., and Chapron, B.: Prominent influence of salinity on Atlantic hurricane rapid intensification, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-3772, https://doi.org/10.5194/egusphere-egu2020-3772, 2020

D3017 |
EGU2020-6149<span style="font-size: .8em!important; font-weight: bold; vertical-align: super; color: green!important;"><span title="Early career scientist: an ECS is an undergraduate or postgraduate (Masters/PhD) student or a scientist who has received their highest degree (BSc, MSc, or PhD) within the past seven years. Provided parental leave fell into that period, up to one year of parental leave time may be added per child, where appropriate.">ECS</span></span>
Dariusz Baranowski

The island of Sumatra is characterized by extremely high average precipitation accumulation dominated by a strong diurnal cycle of convection, which develops over steep its topography. As a result, the island often suffers from precipitation driven natural hazards, which account for more than 50% of all natural disasters. Such hazardous events inflict great socio-economic loss and damage.

This study addresses this topic via a synergistic methodology employing analysis of meteorological data (precipitation, surface winds) and three independent datasets of floods, in order to consistently analyze spatio-temporal variability in flooding events and the environmental conditions leading to them. Two flood databases were derived from crowd-sourcing: Twitter and local papers. Additionally, the database from Indonesian agency BNPB was used. All three datasets were analyzed independently for the 2014-2018 period. While not all floods are identified in every data base, the results obtained from our analyses agree for all key elements of this study, providing cross calibration and increasing confidence in our findings.

On a subseasonal time scale, the amount of rainfall over the island is variable as well and strongly modulated by eastward propagating modes of organized convection: the Madden-Julian Oscillations (MJO) and convectively coupled Kelvin waves (CCKW), both of which affect the local diurnal cycle through multi-scale processes. This study investigates the relationship between those two modes of organized convection and flooding in Sumatra using several data forms of flood validation. It is shown that CCKWs constitute a critical dynamical predictor for flood onset.

Although our results agree with importance of MJO, indicated by previous studies, we find that only about 27% of floods in Sumatra were immediately preceded by favorable MJO conditions and all of them were in fact also associated with a CCKW embedded within an envelope of enhanced MJO convection. This was the case during the flood in Padang on 31 May, 2017, when the MJO was active over the Indian Ocean but its enhanced precipitation had not yet reached Sumatra. Instead, a strong CCKW, which initiated over eastern Africa, brought anomalous precipitation exceeding 10 mm/day to Padang and triggered a flood.

Comprehensive analysis shows that nearly half of robust CCKW events, which propagated between 80E and 110E, were associated with floods in Sumatra. From a different perspective, nearly all of floods in Sumatra were preceded by anomalous precipitation associated with a CCKW and about 60% of floods in Sumatra were immediately preceded by a strong CCKW event. This percentage is substantially higher than for favorable MJO conditions, indicating stronger interaction of local convection with CCKWs. Even though CCKW activity is modulated by the MJO itself and more CCKWs are found when the MJO is active in the Indian Ocean, during analyzed 2014-2018 period nearly 30% were associated with CCKW alone. Therefore, this study shows that CCKWs are important contributor to extreme weather in Sumatra and constitute a potential source of predictability of such hazardous events.

How to cite: Baranowski, D.: Convectively coupled Kelvin waves contribution to hazardous weather in Sumatra., EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-6149, https://doi.org/10.5194/egusphere-egu2020-6149, 2020

D3018 |
EGU2020-7306<span style="font-size: .8em!important; font-weight: bold; vertical-align: super; color: green!important;"><span title="Early career scientist: an ECS is an undergraduate or postgraduate (Masters/PhD) student or a scientist who has received their highest degree (BSc, MSc, or PhD) within the past seven years. Provided parental leave fell into that period, up to one year of parental leave time may be added per child, where appropriate.">ECS</span></span>
Peter Pfleiderer and Carl-Friedrich Schleußner

Hurricanes are the most damaging natural disasters in the Caribbean and global warming is expected to increase their impacts. It is well understood that tropical cyclones (TC) will intensify as sea surface temperatures warm and that the amount of precipitation brought by these TCs is going to increase with the water holding capacity of the atmosphere. However, for an assessment of future hurricane risk in the Caribbean it also important to better understand whether and how the overall frequency of tropical cyclones might change.

Projecting future tropical cyclone activity remains challenging because of the weak representation of tropical cyclones in most global circulation models. Here we want to overcome this shortcoming by estimating hurricane risk indirectly based on favorable climatic conditions in the region. These large-scale predictor fields are easier to model and therefore allow for an improved assessment of future hurricane risk.

We define hurricane risk as the accumulated energy that TCs produce in proximity of Caribbean islands. Using novel statistical methods, we identify regions of sea surface temperature, wind speeds and sea level pressure with predictive power for the next weeks hurricane activity. Based on these predictors we construct a classification model to estimate the probability of having TCs in proximity of islands and their strength for the following week. The expected value of seasonal hurricane risk based on these weekly probabilities shows high skill in reproducing the observational record.

Applying the same hurricane risk model to climate projections from global circulation models allows us to estimate future hurricane risk without relying on the ability of climate models to adequately represent tropical.

How to cite: Pfleiderer, P. and Schleußner, C.-F.: Assessing future hurricane risk in the Caribbean based on large-scale predictor fields, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-7306, https://doi.org/10.5194/egusphere-egu2020-7306, 2020

D3019 |
EGU2020-9581<span style="font-size: .8em!important; font-weight: bold; vertical-align: super; color: green!important;"></span>
Liguang Wu

Extreme updrafts (stronger than 10 m s-1) have been observed in the tropical cyclone core region, which have profound implications to tropical cyclone intensification and structure change. Since extreme updrafts in the tropical cyclone are difficult to observe, their features and the associated mechanisms for formation and influences on tropical cyclones remain poorly understood. This study presents an analysis of extreme updrafts in a strong tropical cyclone that was simulated with the large-eddy simulation technique and the finest grid spacing of 37 meters. The simulated tropical cyclone experiences the vertical wind shear of about 5 m s-1 in a typical large-scale evironment in the western North Pacific. The simulated extreme updrafts in the inner core region exhibit the high frequency at the altitudes of ~ 750 m, 6.5 km and 13 km. The extreme updrafts in the inflow and outflow layers are closely associated with the Richardson Number of less than 0.25, indicating their relationship with severe turbulence caused by strong vertical wind shears. The extreme updrafts in the middle layer are associated with the strong convective activity. The details of the structures of the extreme updrafts are discussed.

How to cite: Wu, L.: Large-Eddy Simulation of Extreme Updrafts in the Tropical Cyclone Inner Core, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-9581, https://doi.org/10.5194/egusphere-egu2020-9581, 2020

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EGU2020-13044<span style="font-size: .8em!important; font-weight: bold; vertical-align: super; color: green!important;"></span>
Xiaolei Zou and Xiaoxu Tian

A recently further refined hurricane warm core retrieval algorithm is applied to the NOAA-20 and S-NPP Advance Microwave Temperature Sounder (ATMS), the Advanced Microwave Sounding Unit-A (AMSU-A) and the Fengyun-3D (FY-3D) microwave temperature sounding instrument (MWTS) brightness temperature observations within and around Hurricanes and incorporated into A four-dimensional variational (4D-Var) vortex initialization (VI) system is developed for a nonhydrostatic axisymmetric numerical model with convection accounted for (the RE87 model). It is shown that the temporal evolution of the ATMS and AMSU-A derived maximum warm core temperature anomalies follow more closely with that of the minimum mean sea level pressure and slightly less closely with the maximum sustained wind, and the radii of the ATMS derived warm cores at 4 and 6 K compared favorably with the 34 kt and 50 kt wind radii during the entire life span of Hurricane Irma in 2017. The vertical extend of the warm core toward the lower levels increases with increasing intensity when Irma experiences a strong intensification due to an enhanced latent heat release associated with diabatic processes. The multi-polar-orbiting operational meteorological satellites can well capture the TC inner cores’ diurnal cycle with a maximum around midnight. A model fit to satellite microwave retrievals of tropical cyclone (TC) warm-core temperatures from the above mentioned three polar-orbiting satellites and and total precipitable water (TPW) Global Change Observation Mission  – Water Satellite 1 produced a significantly improved intensity forecast of Hurricane Florence (2018) and Typhoon Mangkhut (2018), with more realistic vertical structures of all model state variables (e.g., temperature, water vapor mixing ratio, liquid water content mixing ratio, tangential and radial wind components, and vertical velocity) are obtained when compared with a parallel run initialized simply by the European Centre for Medium-Range Weather Forecasts ERA5 reanalysis.

How to cite: Zou, X. and Tian, X.: Satellite Microwave TC Warm-core Retrieval for a 4D-Var Vortex Initialization Using a Nonhydrostatic Axisymmetric Model with Convection Accounted for, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-13044, https://doi.org/10.5194/egusphere-egu2020-13044, 2020

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EGU2020-15555<span style="font-size: .8em!important; font-weight: bold; vertical-align: super; color: green!important;"><span title="Early career scientist: an ECS is an undergraduate or postgraduate (Masters/PhD) student or a scientist who has received their highest degree (BSc, MSc, or PhD) within the past seven years. Provided parental leave fell into that period, up to one year of parental leave time may be added per child, where appropriate.">ECS</span></span>
Weicheng Ni, Ad Stoffelen, and Kaijun Ren

Among all kinds of natural disasters, hurricanes are regarded as one of the most destructive, which can cause tremendous losses to the global economic system and ecosystem every year. However, until now, there remain many issues unknown about hurricane dynamics, while hurricanes undergo amplification, shearing, eyewall replacements, diurnal influences, and so forth. Precise morphology parameters, extracted from high-resolution spaceborne Synthetic Aperture Radar (SAR) image, can play an essential role in further exploring and monitoring the hurricane dynamics. Moreover, these morphology parameters may help to build guidelines for the wind calibration of the more plentiful, but lower resolution scatterometer wind field data in hurricane events in order to better link scatterometer wind fields to hurricane categories. In this paper, we have developed a new method for extracting the hurricane eyes from C-band SAR data by constructing Gray Level-Gradient Co-occurrence Matrix (GLGCM) for each image. The hurricane eyewall (HE) area is determined with a 2-dimensional vector, which is automatically generated by maximizing the conditional entropy of HE area in GLGCM. Subsequently, we select the HE pixels based on minimizing the variance of normalized radar cross-section (NRCS) values of the pixel set chosen. The texture information of HE can be adequately preserved in this process. The experimental results prove the effectiveness of our method. Notably, the HE extracted with this automatic method is still in line with the visually observed eyewall even when the hurricane is weak or the eyewall is unclosed. Compared with the morphological analysis and wavelet analysis methods proposed in other papers, the approach developed here is able to accomplish in a simpler way with equally satisfying results. In conclusion, this study can provide a new choice for hurricane eye morphology extraction.

How to cite: Ni, W., Stoffelen, A., and Ren, K.: Hurricane Eye Morphology, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-15555, https://doi.org/10.5194/egusphere-egu2020-15555, 2020

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EGU2020-18605<span style="font-size: .8em!important; font-weight: bold; vertical-align: super; color: green!important;"><span title="Early career scientist: an ECS is an undergraduate or postgraduate (Masters/PhD) student or a scientist who has received their highest degree (BSc, MSc, or PhD) within the past seven years. Provided parental leave fell into that period, up to one year of parental leave time may be added per child, where appropriate.">ECS</span></span>
Da Zhang and Jiahua Zhang

Rapid intensification process (RI) still challenges the tropical cyclone (TC) intensity prediction. The convective structure evolution during RI process were explored by using the Himawari-8 satellite data in the western North Pacific. 39 TCs underwent at least one RI process during 2015-2017 and the RI-onset, RI-continue, and RI-end episodes were identified in each one of the RI events based on JTWC best track data.

The differences between the infrared channel and water vapor channel brightness temperature (IRWV) were calculated and the negative pixel values of IRWV were considered as deep convection areas. The radial and azimuthal profiles and the morphological features were extracted from 3-hour interval images and several key patterns and the rules considering the location, shapes, and magnitude of the IRWV were identified through the whole RI process. The composite analysis shows that each TC appears negative IRWV during the RI process, however, not all TCs demonstrate significant changes either in areas nor patterns, which indicate that the deep convection may not be a necessary condition for RI occurrence. Compared with the Non-RI cases, the development and maintenance of a good spin structure of the negative IRWV were considered as a crucial condition for the TC intensification. The RI-onset periods were mostly connected with the sudden change of IRWV and the inward movement to the inner-core area. The pinhole eye features were normally a sign of continue RI, while the appearance of big eye features, indicates the ending of RI process. It was suggested that the IRWV feature combined with the TC structure feature can be utilized to skillfully predict the episodes of RI. More RI events are expected to involved in the current study and a CART4.5 decision tree algorithm with the aforementioned rules was also under explored.

How to cite: Zhang, D. and Zhang, J.: Topical Cyclone Convection Structure Evolution during Rapid Intensification using Himawari-8 Satellite, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-18605, https://doi.org/10.5194/egusphere-egu2020-18605, 2020

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EGU2020-20341<span style="font-size: .8em!important; font-weight: bold; vertical-align: super; color: green!important;"></span>
Mohan Smith and Ralf Toumi

Remote (r ≤ 1800km) outgoing longwave radiation (OLR) fields are investigated in observations, the ECMWF ensemble forecast and reanalysis data. A large scale dipole pattern of low and high fluxes are found in both the observations and model. Low OLR regions are positioned within the cyclone circulation and high OLR regions are found 500-1500km to the north west of the TC. The position of the high OLR region rotates anticlockwise about the TC center as the TC motion vector rotates clockwise from westward to eastward. There is a strong association between the low level wind divergence fields and the high OLR remote region. We propose this remote high OLR region is of interest regarding TC track forecasts. Sub-ensembles selected upon the location of the remote high OLR region improved track forecasts improved by 15\% at 6hrs lead time. This technique out performs those sub-ensembles selected by the inner 750km TC OLR signal, however the best skill improvement in the study selects sub-ensembles by a 3600x3600km TC centered OLR field.

How to cite: Smith, M. and Toumi, R.: Outgoing Long Wave Radiation Dipole of Tropical Cyclones, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-20341, https://doi.org/10.5194/egusphere-egu2020-20341, 2020