AS1.7
Orals: Wed, 26 Apr | Room M1
Within the atmospheric modelling community, a large focus in recent years has been on the concept of Convective Self-Aggregation (CSA): In an environment of radiative convective equilibrium, with homogeneous initial conditions and a constant-temperature tropical sea surface, convection can spontaneously aggregate into domain-wide patterns of persistent dry areas and constrained rainy areas over a temporal timescale of weeks to months. CSA, albeit still a modeling paradigm, could reveal the mechanisms behind some of the convective organization observed in the tropics.
This process of forming domain-wide structure can be accelerated to the order of days by imposing oscillating surface temperatures with a large enough amplitude [1]. The ‘diurnally aggregated’ cloud field is similar to CSA as it also constrains the surface rain field to certain parts of the domain. Further, pattern formation was found to initiate first as persistent dry patches in the uppermost layers of the simulated atmosphere. The dry patches subsequently penetrate through to the subcloud layer [2].
In this work we investigate how diurnal surface temperature amplitudes, typical of tropical land, affect the formation of persistent dry patches and the spatio-temporal extent of the emergent mesoscale convective systems. We run a set of cloud resolving simulations initialized with typical profiles of temperature and humidity. We impose a large-amplitude diurnally oscillating surface temperature, which we then set to constant at different times, to see the effect on the diurnally aggregated cloud field. We present the results of this study, which show a strong dependence on the degree of aggregation over ‘land’, in determining the aggregation over ‘sea’, and a form of hysteresis arises.
1. Haerter, Jan O., Bettina Meyer, and Silas Boye Nissen. ‘Diurnal Self-Aggregation’. Npj Climate and Atmospheric Science 3, no. 1 (30 July 2020): 1–11. https://doi.org/10.1038/s41612-020-00132-z.
2. Jensen, Gorm G., Romain Fiévet, and Jan O. Haerter. ‘The Diurnal Path to Persistent Convective Self-Aggregation’. Journal of Advances in Modeling Earth Systems 14, no. 5 (2022): e2021MS002923. https://doi.org/10.1029/2021MS002923.
How to cite: Kruse, I. L. and Haerter, J. O.: Investigating Convective Self-Aggregation in the Transition from Land to Sea, EGU General Assembly 2023, Vienna, Austria, 24–28 Apr 2023, EGU23-6933, https://doi.org/10.5194/egusphere-egu23-6933, 2023.
Squall lines are the consequence of the interaction of low-level shear with cold pools associated with convective downdrafts. Beyond a critical shear amplitude, squall lines tend to orient themselves at an angle with respect to the low-level shear. While the mechanisms behind squall line orientation seem to be increasingly well understood, uncertainties remain on the implications of this orientation. Roca & Fiolleau 2020 show that long lived mesoscale convective systems, including squall lines, are disproportionately involved in rainfall extremes in the tropics. One may then question whether the orientation of squall lines has an impact on rainfall extremes, and if so, why.
Using a cloud-resolving model, we perform idealized simulations of tropical squall lines by imposing a vertical wind shear in radiative-convective equilibrium. Our results show that precipitation extremes in squall lines are 40% more intense in the critical case and remain 30% superior in the supercritical regime. With a theoretical scaling of precipitation extremes (Muller & Takayabu 2019), we show that the condensation rates control the amplification of precipitation extremes in tropical squall lines, mainly due to its dynamic component. The critical case is not only optimal for squall line orientation, but also for the cloud base velocity intensity of new convective cells.
How to cite: Abramian, S., Muller, C., and Risi, C.: Extreme Precipitation in Tropical Squall Lines , EGU General Assembly 2023, Vienna, Austria, 24–28 Apr 2023, EGU23-15870, https://doi.org/10.5194/egusphere-egu23-15870, 2023.
Radiative-convective equilibrium is the simplest possible way to phrase many questions about a deep-convecting atmosphere and is accessible by a wide range of model types. The radiative-convective equilibrium project (RCEMIP) provides a common configuration, but reveals a large spread in the simulated climate across models, including profiles of temperature and relative humidity. Here we use simple models and theory to understand the intermodel spread in CAPE, relative humidity, and their responses to warming.
Across the RCEMIP ensemble, temperature profiles are systematically cooler than a moist adiabat, consistent with theory that they are set by dilute ascent. As horizontal grid spacing is reduced in models with explicit convection from 1 km to 200 m, CAPE and relative humidity increase. Across all models, CAPE increases with warming at a rate (14-19%/K) greater than that expected from the Clausius-Clapeyron relation. We find that there is higher CAPE (greater instability) in models that are on average moister in the mid-troposphere, which is consistent with the simple plume model of Romps (2016) in which both instability and relative humidity depend on entrainment and precipitation efficiency. The sign of the relationship suggests that differences in entrainment drive the intermodel spread. This relationship is true across both models with explicit and parameterized convection.
To more explicitly evaluate the drivers of the intermodel spread, we use the Romps (2016) model to diagnose theory-implied values of entrainment and precipitation efficiency given the simulated values of CAPE and relative humidity. We then decompose the the variability across models in CAPE and relative humidity (and their responses to warming) into contributions from entrainment, precipitation efficiency, and the depth of the convecting layer. Targeted microphysics parameter perturbation experiments with an individual cloud-resolving model in which precipitation efficiency is varied and explicitly diagnosed provide proof of concept for this decomposition technique.
How to cite: Wing, A. and Singh, M.: Control of Tropical Stability and Relative Humidity in Radiative-Convective Equilibrium Simulations, EGU General Assembly 2023, Vienna, Austria, 24–28 Apr 2023, EGU23-3532, https://doi.org/10.5194/egusphere-egu23-3532, 2023.
The interactions of ice particles with radiative fluxes in tropical high clouds substantially alter the heating structure within the atmosphere, also known as cloud radiative heating (CRH). CRH influences the upper-tropospheric temperature structure and thus modulates the strength and position of tropical and extratropical circulations. Moreover, it influences the life cycle of tropical high clouds through longwave destabilization of the cloud layer and lifting of clouds by absorption of both shortwave and longwave radiation by ice crystals. A possible change of CRH, for example, due to global warming, can substantially alter the tropical climate.Despite a large body of work that has explored interactions between clouds and radiation, responses of CRH to global warming remain largely unknown. We therefore use idealized SAM cloud-resolving model simulations, the RCEMIP multimodel dataset, and a 15-year-long satellite-derived CRH dataset to explore changes in CRH under different sea surface temperatures.
To a first approximation, the upper tropospheric CRH shifts nearly isothermally to a higher altitude level following a surface warming. In addition, upper-tropospheric CRH in 27 of the 32 analyzed models increase by 0.5 to 10%/K, with a mean value of about 3%/K. Interestingly, the CRH increases despite decreases in upper tropospheric ice water content and cloud fraction. The increase in CRH can be to a large extent explained by an increase in atmospheric transmissivity due to a 2-3 km vertical shift of high clouds, in an environment with decreased air density. Similarly, all models simulate an increase in the upper tropospheric clear-sky radiative cooling in warmer conditions.
Additionally, the CRH response to surface warming can be largely predicted by assuming a nearly isothermal vertical shift of upper tropospheric CRH profiles (as per the fixed anvil temperature hypothesis) following a warmer moist adiabat and by considering the increase in CRH magnitude due to changes in atmospheric density. Therefore, if we know the CRH of a reference climate state, we can, to a good approximation, estimate its response to surface warming.
The modeled CRH vertical shift and increase are confirmed by a 15-year-long satellite-derived tropical CRH dataset. The years with the highest SSTs lead to the most positive CRH that is shifted to higher levels, similarly to what is simulated by RCEMIP models.
How to cite: Gasparini, B., Voigt, A., Mandorli, G., and Stubenrauch, C.: SST-driven changes in cloud radiative heating in RCEMIP models and observations , EGU General Assembly 2023, Vienna, Austria, 24–28 Apr 2023, EGU23-2556, https://doi.org/10.5194/egusphere-egu23-2556, 2023.
Change of the intertropical convergence zone (ITCZ) with global warming has important consequences for the regulation of the tropical climate and for future precipitation projections. Most of the volume of the ITCZ is filled with ice associated with convective anvils, which opens the perspective of using the response of ice clouds to study changes of the tropical convergence zones with global warming. Past studies have shown a decrease of tropical high-cloud fraction with surface warming, whereby the response of anvil clouds is used to interpret the response of ice clouds as a whole. However, tropical clouds organize over a very wide range of scales, meaning that the response of ice clouds is more complex. In particular, precipitating deep convection may represent a small volume of total cloudiness, but it concentrates most of the ascending motion in the tropics and is therefore of crucial importance for the dynamics. Here we show how the high resolution in next generation convection-resolving climate models and in observations can be leveraged to directly measure the response of precipitating deep convection with surface warming in the ice signal. For this purpose, we use the first year-long simulations of global warming ever performed with a Global Storm Resolving Model (GSRM) at 3 km resolution. These simulations use the eXperimental System for High-resolution prediction on Earth-to-Local Domains (X-SHiELD), developed at the Geophysical Fluid Dynamics Laboratory (GFDL). By tracking the response of tropical clouds to surface warming from the response of ice water path (IWP), the vertical integral of ice mixing ratio, we show that the response of precipitating deep convection can be identified at high resolution and that this response, marked by an increase in frequency of very deep convective cores and a decrease in frequency of more moderate convection, is robust in model and active sensor observations. We discuss this result and show how it promotes a simple view of the changes of tropical convergence zones in ice-based coordinates.
How to cite: Bolot, M., Harris, L., Cheng, K.-Y., Blossey, P., Bretherton, C., Clark, S., Kaltenbaugh, A., Merlis, T., Zhou, L., and Fueglistaler, S.: New GSRM global warming simulations and active sensors reveal robust changes of tropical convergence zones in cloud ice space, EGU General Assembly 2023, Vienna, Austria, 24–28 Apr 2023, EGU23-4150, https://doi.org/10.5194/egusphere-egu23-4150, 2023.
The convective mass flux within tropical convection influences the large-scale circulation, drives cloud radiative forcing, has integral links to the production of fresh water, and impacts extreme weather. CMF forms the focus of the recently selected Investigation of Convective Updrafts (INCUS) mission to be launched in 2026. This NASA mission is comprised of 3 spacecraft, all of which will carry a Ka-band cloud radar. One spacecraft will also carry a passive microwave radiometer. The 3 smallsats are to be separated by time intervals of 30, 90 and 120 seconds, thus allowing for the rapid and systematic sampling of the same storm with all three spacecraft. These time intervals (delta-ts) also facilitate the investigation of the magnitude and evolution of CMF, which will be examined as a function of storm type, storm lifecycle and environmental properties. INCUS will therefore provide the first global systematic investigation into CMF and its evolution within deep tropical convection.
A wide range of research tasks have been conducted in preparation for the INCUS mission and the development of the INCUS algorithms including: (1) running and analyzing extensive suites of large-domain, high-resolution model simulations; (2) examining ground-based Doppler radar observations obtained using adaptive scanning techniques during several recent field campaigns; and (3) evaluating anvil characteristics using passive microwave radiometer and geoIR data. This talk will focus on three specific highlights arising from these modeling and observational analyses. First, we will examine the temporal scales of updraft variability. Second, we will analyze the relationship between ice water path cores and convective updrafts. Finally, we will demonstrate proof of the INCUS delta-t concept linking changes in reflectivity to CMF through the use of ground-based radar analyses.
How to cite: van den Heever, S., Haddad, Z., Dolan, B., Freeman, S., Grant, L., Kollias, P., Leung, G., Luo, J., Marinescu, P., Posselt, D., Rasmussen, K., Sai, P., Schulte, R., Stephens, G., Storer, R., and Takahashi, H.: Tropical Convection through the Lens of the INCUS Mission, EGU General Assembly 2023, Vienna, Austria, 24–28 Apr 2023, EGU23-11285, https://doi.org/10.5194/egusphere-egu23-11285, 2023.
The vertical structure of large-scale tropical circulations is determined by their complex coupling with both deep and shallow convection. This is reflected in our theoretical frameworks for understanding the tropical precipitation distribution, which consist of “deep” theories that focus on overturning circulations that extend throughout the depth of the troposphere and “shallow” theories that focus on low-level convergence driven by boundary-layer pressure gradients. While both types of theories suggest links between low-level thermodynamic fields and the precipitation distribution, shallow theories highlight the importance of the distribution of surface temperature, while deep theories additionally highlight the importance of the low-level humidity.
Here we use idealised cloud-permitting simulations to elucidate the physical factors that control the vertical structure of tropical circulations. We first demonstrate how the influence of convective entrainment on the lapse rate can act to change the vertical structure of deep tropical circulations, with implications for the behaviour of precipitation in the current and future climate. We further investigate the interaction between deep and shallow tropical circulations by simulating an idealised overturning circulation over varying surface conditions. By independently varying the sea-surface temperature and moisture availability, the low-level temperature and moisture distributions are manipulated such that the predictions of “deep" and “shallow" theories of the circulation may be distinguished. The results provide insight into the relative roles of oceanic SST gradients and land-ocean contrasts in determining the climatological precipitation distribution in the tropics.
How to cite: Singh, M.: Shallow and Deep Circulations in the Tropical Atmosphere, EGU General Assembly 2023, Vienna, Austria, 24–28 Apr 2023, EGU23-3078, https://doi.org/10.5194/egusphere-egu23-3078, 2023.
We investigate microphysical uncertainties in hailstorms using statistical emulation in a single model framework with the objective to disentangle the relative contributions from aerosols, microphysical parameters and environmental conditions to the uncertainty in cloud-, precipitation- and hail-related parameters.
Our selected case study is the Andreas hailstorm on 28 July 2013 in the Neckar Valley and over the Swabian Jura in Southwest Germany. We perform model simulations on cloud-resolving scale with the numerical weather prediction model ICON coupled with the aerosol module ART (ICON-ART). We use a two-moment cloud microphysics scheme with a representation of ice nucleation by dust aerosols.
We generated a perturbed parameter ensemble (PPE) to sample uncertainties in cloud-, precipitation- and hail related parameters. Six parameters from the categories aerosols, microphysics and environmental conditions were jointly perturbed, namely the cloud condensation nuclei (CCN) and ice nuclei (IN) concentrations, the riming efficiency of graupel and hail, the convective available potential energy (CAPE) and vertical wind shear. The defined parameter ranges are based on forecast analysis and literature. We used the maximin Latin hypercube algorithm to distribute the parameters well-spaced in the six-dimensional parameter uncertainty space. For these six parameters, an ensemble of 90 members was generated and in addition a smaller independent ensemble of 45 members serves for validation.
We used the Gaussian process emulation and developed emulators for hail- and precipitation related output variables. To quantify contributions to the uncertainty in the output variables from the perturbed parameters individually as well as interactions between them, a variance-based sensitivity analysis was performed. We will present first results, which reveal the importance of the CCN concentration for controlling the number concentration of hail particles as well as the CCN concentration and environmental conditions for controlling the amount of hail and precipitation in the model. The geographical distribution of hail and precipitation shows a large variety among the ensemble members, with storm tracks shifted further to the north or south compared to the reference simulation. The path of the storm track is thereby mainly controlled by CAPE and the vertical wind shear, however, aerosol parameters seem to be important for the development of multiple storm tracks.
How to cite: Frey, L., Hoose, C., Kunz, M., Miltenberger, A., and Kuntze, P.: Using statistical emulation to quantify microphysical uncertainties for the Andreas hailstorm in 2013, EGU General Assembly 2023, Vienna, Austria, 24–28 Apr 2023, EGU23-14317, https://doi.org/10.5194/egusphere-egu23-14317, 2023.
This study focuses on the deep-inflow mixing features of the orographically locked diurnal convection, involving interactions between local circulation and the thermodynamic environment of the convection. Under the weak synoptic weather regime, orographically locked diurnal convection is a typical summertime phenomenon in Taiwan, a tropical island in the Asian monsoon region. Numerical simulations are carried out using the vector vorticity equation model with high-resolution Taiwan topography (TaiwanVVM), which can appropriately simulate the characteristics of diurnal convection and the evolution of boundary layer and local circulation. The semi-realistic approach, simplified by observed soundings as the uniform initial condition over the entire domain, emphasizes the decisive environmental factors that modulate the development of convection, representing the variability of the background environment by the ensembles. The analyses by the deep-inflow mixing framework, including the locally-derived convective structures and the upstream moist static energy (MSE) transport, improve the understanding of the interactive physical processes in the boundary layer development and local circulation evolution of orographically locked diurnal convection over complex topography. The convective structures of the deep-inflow mixing, increasing vertical velocity and convective mass flux with height through a deep lower-tropospheric inflow layer, are found in strong convective updraft columns within heavily-precipitating systems over precipitation hotspots. While the topography constrains the location of the convection, enhanced convective development is associated with higher upstream MSE transport through this deep-inflow layer via local circulation, augmenting the rain rate by 35% in precipitation hotspots. The results highlight the importance of non-local dynamical entrainment of the deep-inflow, transporting MSE via local circulation to supply the growth of orographically locked diurnal convection. Thus, the deep-inflow mixing framework can serve as the theoretical basis for describing the orographic locking feature of diurnal convection over complex topography. Guided by the simulations, the Storm Tracker mini-radiosondes are released upstream of the precipitation hotspot, targeting observations within the most common deep-inflow path. Initial field measurements support the presence of high MSE transport within the deep-inflow layer when organized convection occurs at the precipitation hotspot.
How to cite: Chang, Y.-H., Chen, W.-T., Wu, C.-M., Kuo, Y.-H., and Neelin, J. D.: Identifying the Deep-inflow Mixing Features in Orographically Locked Diurnal Convection, EGU General Assembly 2023, Vienna, Austria, 24–28 Apr 2023, EGU23-4079, https://doi.org/10.5194/egusphere-egu23-4079, 2023.
The initiation of deep convection in a diurnal cycle is still one of the most important uncertainties in a climate numerical model. This is partially due to our poor understanding of the physical mechanisms leading to the transition from shallow to deep convection. In this work, we discuss the role of shallow cumulus clouds in the initiation of deep convection. By using a simple entraining plume model, we show that the interaction between an active and a passive shallow cumulus helps the former to reach higher altitudes and, in the right conditions, may initiate deep convection. It is also shown that the organization of passive and active clouds due to the formation of cold pools may act as a positive feedback. Furthermore, based on the proposed mechanism, a stochastic triggering function is derived, which can be implemented in climate models. As an important feature, the stochastic function is scale-aware, which makes it suitable for simulations at the gray-zone.
How to cite: Vraciu, C.-V.: The role of passive shallow cumuli in the transition from shallow to deep convection, EGU General Assembly 2023, Vienna, Austria, 24–28 Apr 2023, EGU23-3561, https://doi.org/10.5194/egusphere-egu23-3561, 2023.
Posters on site: Wed, 26 Apr, 16:15–18:00 | Hall X5
Idealized high-resolution models show spontaneous aggregation of tropical convection on the beta-mesoscale driven by radiative feedbacks, and the resulting drying implies a potentially important impact on climate sensitivity missing in classic convective parameterization schemes. Here, we combine multiple state-of-the-art observations and reanalysis of the tropical atmosphere and ocean in a 1000 x 700 km region in the tropical Western Pacific warm pool region, along with numerical models and machine learning techniques to demonstrate that in boreal summer, while radiative and surface fluxes act to cluster convection, the convection remains in a random configuration as evidenced by very limited spatial variability in total column humidity. Instead, in the winter/spring period, when the warm pool is displaced southwards, the region lies on the warm pool boundary with stronger north-south surface temperature gradients. Convection usually remains strongly organized in these periods but is interspersed with occasional random episodes. This entails a sudden flipping into the random state associated with the southerly flow anomalies that advect convection and humidity over the cooler sea surface temperature (SST) regions. Observations and models suggest that this advection of humidity is the principal driver of organization and disorganization of convection and that diabatic feedbacks instead always act to try and cluster convection. Results also indicate that when convection is organized, the atmosphere is significantly drier than when convection is random and that the Longwave (LW) clear-sky top of atmosphere flux is significantly larger in the organized state, principally due to the moisture differences between both configurations. The LW all-sky flux difference between both states is less significant compared to the LW clear-sky because it is largely driven by the cloud cover, which, although smaller for the organized state, does not differ significantly. These differences between organized and random convective states, and the role of the diabatic processes in providing forcing for aggregation, mostly reproduce the findings of idealized models. However, this study indicates that in the real tropical atmosphere diabatic forcing is inadequate to lead to aggregation on its own over homogeneous SSTs, and instead, spatial SST gradients and large-scale dynamics are key to driving aggregation and determining its breakup over the warm pool region.
How to cite: Casallas, A., Tompkins, A., and De Vera, M.: No evidence of spatial feedbacks causing convective clustering in the Tropical Western Pacific, EGU General Assembly 2023, Vienna, Austria, 24–28 Apr 2023, EGU23-12581, https://doi.org/10.5194/egusphere-egu23-12581, 2023.
The shallow cumulus clouds are ubiquitous in the atmosphere, populating a large part of the subtropical oceans. They may play a strong climate feedback due to their cooling effect on the Earth atmosphere. As a result, a large number of studies investigated the organization of cumulus clouds and their interaction with the climate. However, the organization of passive shallow clouds and their impact on the atmospheric convection and climate change received very limited attention. In this work, we perform a series of large eddy simulations in order to investigate how the organization and the total cloud cover depends on the relative humidity of the environment. We show that although the active cumulus clouds only show a weak correlation with the relative humidity, the passive clouds are very sensitive to it. We show thus that the cloud cover of the shallow cumuli is very sensitive to the relative humidity which could be very important in the context of the climate change. Furthermore, we formulate a conceptual picture to explain the organization of passive shallow cumulus clouds.
How to cite: Marin, A. and Vraciu, C.-V.: On the organization of passive shallow cumulus clouds, EGU General Assembly 2023, Vienna, Austria, 24–28 Apr 2023, EGU23-5898, https://doi.org/10.5194/egusphere-egu23-5898, 2023.
Convectively induced turbulence (CIT) is a serious aviation hazard and it is challenging to forecast the CIT in the region near convection. Previous studies used reginal model with high resolution or global model with low resolution and selected empirical indices to diagnose the turbulence. In this study, we used The Model for Prediction Across Scales (MPAS) to simulate some cases of CIT reported near Hong Kong. MPAS allows us to use convection-permitting resolution in the interested area while including the global-large scale circulation with coarser resolutions in other regions. The eddy dissipation rate (EDR) is computed to diagnose the potential occurrence of CIT. We compared three methods for calculating EDR from the resolved flow in the MPAS, the first one based on second order structure function, the second one based on Scale-Similarity in Large Eddy Simulation (LES) and the third is Near Cloud Turbulence (NCT) diagnostics by using Convective Gravity Wave Drag. Comparing with the NOAA Graphical Turbulence Guidance (GTG) product and flight data suggests that computing EDR with Scale-Similarity is more effective and accurate than second order structure function and NCT diagnostics. Resolution is also an important factor in forecast, we tested the method in mesh with different resolutions but similar distributions, the results from low resolution simulations can generate a useful turbulence pattern forecast, but the intensity is weak, highlighting the value of high resolution simulations that can resolve convection. We evaluated the sensitivity to several model physics and numeric options in simulations. Those variations can change the EDR prediction by influencing the intensity and the life cycle of the convection. No particular scheme produces systematically more intense turbulence than others, suggesting varying model physics captures some stochasticity of convection. Compared with flight records of EDR along the flight routes, MPAS could produce in three out of five cases showing maximum EDR is close to the observed intensity of turbulence (EDR>0.4). However, in the other two cases, the results are not satisfactory mainly because of significant location biases of the predicted convection. We also add initial condition perturbation-based large ensemble in one case and find it possible to improve the prediction of the failed cases by influencing the position of the convection. Further work should be conducted to prioritize the ensemble members since only a few members can capture the turbulence and doing the average will erase them easily.
How to cite: Chen, H., Shi, X., Leung, C. Y., Cheung, P., and Chan, S.: Using MPAS model to forecast the Convectively Induced Turbulence, EGU General Assembly 2023, Vienna, Austria, 24–28 Apr 2023, EGU23-1439, https://doi.org/10.5194/egusphere-egu23-1439, 2023.
The difficulty to accurately represent atmospheric convection in numerical weather forecasts contributes to persistent biases in weather and climate simulations – particularly tropical precipitation. Convection-permitting global forecasts are an improvement on global models with parametrized convection schemes, however it is not yet clear whether they improve forecast skill to match or improve upon the current approach of nesting a convection-permitting high-resolution regional model inside a global model with parameterized convection. This is far less computationally expensive than running a global convection-permitting model.
To test this, the Met Office is coordinating a UK K-scale project nesting high resolution (2.2 km) limited area models (LAMs) within global models that have between 5 and 10 km grid resolution. We compare these nested regional models with two different global simulations, run with parameterised and explicit convection science configurations. The 2.2 km resolution LAMs encompass a variety of domains focussing on both tropical land and ocean regions.
Our current work seeks to investigate if and where we see differences in model evolution between the high-resolution nested LAM approach and the explicit convection global driving model. We focus on an active MJO event in January 2018 where enhanced convection propagated across the Indian Ocean and impacted the Maritime continent. For high-impact events such as this, do we see a marked change in the model forecast when explicitly simulating convection globally rather than in a regional limited area model (as currently used in operational forecasts)? Further, are differences between the global convection permitting and LAM forecasts more pronounced over ocean-dominated regions where the amplitude of the diurnal cycle of convection is smaller?
This talk will summarise our findings in the context of the wider K-scale project, evaluating how our recent work contributes to the development of more accurate weather forecasts.
How to cite: Macholl, J., Jones, R., and Lewis, H.: Globally modelling explicit convection: how does it compare with the nested limited area model approach at high horizontal resolutions?, EGU General Assembly 2023, Vienna, Austria, 24–28 Apr 2023, EGU23-7082, https://doi.org/10.5194/egusphere-egu23-7082, 2023.
The cold pools, created below cumulonimbus from the evaporation of precipitation, generate the strong winds responsible for the large dust storms called “haboobs” which appear in the Sahel in summer. Most global climate models do not take into account these types of dust emissions due to lack of parameterization of cold pools and associated gusts (Marsham et al. 2011 ; Pantillon et al. 2015).
The introduction of a parameterization of cold pools in the LMDZ climate model has improved the representation of convection, and in particular of the diurnal cycle of continental precipitation in the tropics (Rio et al. 2009). The aim of this work is to develop a parameterization of gusts related to cold pools in order to take into account ‘‘hoobobs’’ in LMDZ model. To do this, we use Large Eddy Simulations (LES) performed on an oceanic domain and in Radiative-Convective Equilibrium (RCE) mode. We use a LES of an oceanic RCE case, easier to analyze because the temperatures are uniform on the surface and therefore the cold pools easier to detect. Before developing a gust parameterization, we evaluate the cold pools parameterization in LMDZ on this RCE case, which has never been done so far. If the comparison confirms the relevance of the scheme and its qualitative match to the LES behavior, is also led to substantial improvements and adjustments to this scheme. Next, we analyze the wind distributions in the LES in order to construct a parametrization based on a probability distibution function of the subgrid scale distribution of the wind which will allow us to take into account the effect of gusts on dust storms. The parametrization relates the moments of the distribution to large-scale wind speed, the spreading speed of the cold pools and the surface fraction covered by the latter. In the following, we will test the parametrization on the LMDZ model by focusing on dust storms in the Sahel during the rainy season.
How to cite: Thiam, M. L., Hourdin, F., Grandpeix, J.-Y., Rio, C., and Gaye, A. T.: Parametrization of dust storms in the Sahel by cold pools, EGU General Assembly 2023, Vienna, Austria, 24–28 Apr 2023, EGU23-7224, https://doi.org/10.5194/egusphere-egu23-7224, 2023.
The intrinsic failure of eddy-diffusion parameterizations in representing upward transport of heat in the convective boundary layer, recognized since the 70s, has lead to various propositions of parameterizations like counter-gradient terms and third order closures to account for the asymmetry of the vertical transport. An approach that is now well recognized consists in combining a mass flux parameterization of the organized structures of the convective boundary layer with a local TKE closure for small scale turbulence. The idea traces back to a proposition by Chatfield and Brost (1987) and is since often referred to as the Eddy Diffusion Mass Flux (EDMF) approach. The “thermal plume model” developed for LMDZ was the first EDMF scheme published and tested in a climate model (Hourdin et al., 2002). It was first introduced in the LMDZ5B atmospheric component of the IPSL model for CMIP5. However, this first version suffered from youth problems. It is only for CMIP6A, about 20 years after the development of the parameterization, that a first satisfactory version of the model was delivered. Through years, and more often with this last version, the key role of the representation of shallow convection on many component of the system has been realized: 1) the ventilation of air by the subsiding air around thermal plumes dries the surface, reinforcing the near surface evaporation. Representing this convection correctly both over trade winds and subsiding regions in the tropics, together with the associated cumulus and stratocumulus clouds, is one of the key for the reduction of the East Tropical Ocean warm bias; 2) the preconditioning of the deep convection by a phase of shallow convection is a key for a correct representation of the phasing of the diurnal cycle of convective rainfall over continents; 3) the strong diurnal cycle of the convective boundary layer in desert areas is essential to well represent the maximum of near surface wind in the morning, responsible for a maximum of dust emission, when the momentum of the nocturnal low level jet is brought suddenly back toward the surface when reached by the developing dry convection. 4) the thermal plume model being active about on half of the globe all the time, it controls the transport of all trace elements, with some non linear effects when the emissions themselves show a diurnal cycle. In this presentation, we review these lessons learned with LMDZ, identify the issues which should require further developments, and expose how new machine assisted techniques allow to reconcile improvement of parameterizations at process scale and climate model improvement.
Chatfield, R. B., & Brost, R. A. (1987). A two-stream model of the vertical transport of trace species in the convective boundary layer. Journal of Geophysical Research, 92, 13,263–13,276
Hourdin, F., Couvreux, F., & Menut, L. (2002). Parameterisation of the dry convective boundary layer based on a mass flux representation of thermals. Journal of the Atmospheric Sciences, 59, 1105–1123
How to cite: Hourdin, F. and Rio, C.: On the importance of dry and cloudy boundary layer convection and of its parameterization in climate models, EGU General Assembly 2023, Vienna, Austria, 24–28 Apr 2023, EGU23-9438, https://doi.org/10.5194/egusphere-egu23-9438, 2023.
Understanding how convective storms respond to changes in their environment on a local scale is critical to begin to elucidate how Earth’s changing climate will affect storms globally. There is now a vast amount of storm-scale observational data, including from geostationary and low-earth orbiting satellites and ground-based observing systems. However, employing these datasets to build comprehensive databases of convective storms and the local environments that form them requires new analysis methodologies. Here, in preparation for the NASA INCUS satellite mission, we have used the tobac tracking package to identify, track and analyze storms and their environments with these big datasets. Using tobac to track storms with geostationary satellite and ground-based radar data, we have built a comprehensive, months-long database of convective storms over their entire lifetime. For each individual convective storm, the database contains their formation environments (including convective available potential energy, wind shear, etc.), evolution over time, and, where applicable, additional data, such as those from low earth orbiting satellites. In this presentation, we will employ this vast database of clouds and storms to quantify the relationship, on a storm scale, between thermodynamic and dynamic environments and storm properties, including lifetime, growth rate, and ice and liquid water paths.
How to cite: Freeman, S., Schulte, R., Leung, G., and van den Heever, S.: Tracking Convective Storms and their Environments with the tobac Tracking Package, EGU General Assembly 2023, Vienna, Austria, 24–28 Apr 2023, EGU23-9855, https://doi.org/10.5194/egusphere-egu23-9855, 2023.
Tropical deep connective clouds (DCCs) have large top of atmosphere (ToA) cloud radiative effects (CREs) in both the shortwave (SW) and longwave (LW), which both have average magnitudes of greater than 100 Wm-2. Due to the opposite sign of the two components, the overall ToA CRE is generally assumed to average to approximately 0 Wm-2. Although there are a number of mechanisms that contribute to this balance, the fact that the daytime only SW CRE balances with the LW CRE indicates that the diurnal lifecycle of DCCs is a key component of this balance. Understanding how the diurnal cycle of DCCs influences their CRE is vital for understanding how any changes in their diurnal cycle of these clouds may influence the climate.
A year-long dataset of retrieved cloud properties and derived broadband radiative fluxes has been produced by the ESA Cloud CCI project using temporally highly resolved satellite observations. Using a novel method, we are able to detect and track both isolated DCCs and large, mesoscale convective systems (MCSs) over their entire lifecycle. We explicitly retrieve the cloud properties and CREs of DCCs over Africa, and how these properties change over the lifecycle of approximately 100,000 observed clouds. We find that the mean anvil SW CRE greatly varies depending on the initiation time of day and the lifetime of the DCC, whereas the LW CRE is consistent throughout the diurnal cycle and varies primarily with cloud top temperature.
As a result of our study we can confirm that the mean observed ToA CRE of all DCCs (integrated over area and lifetime) is indeed approximately 0 Wm-2, but very few DCCs individually have mean CREs near this value. Instead, we find that DCCs occurring during the daytime have a large cooling effect, and those at nighttime have a warming effect, resulting in a bimodal distribution. While MCSs make the largest contribution to the overall effect due to their large areas and lifetimes, because they tend to exist during both nighttime and daytime the overall magnitude of their ToA CREs tend to be smaller than those of isolated DCCs. As a result, factors which influence the diurnal cycle of deep convection – such as changes in CAPE generation or convective inhibition – may have a more important influence on the properties of isolated DCCs rather than larger MCSs.
How to cite: Jones, W., Stengel, M., and Stier, P.: The Diurnal Cycle of the Cloud Radiative Effect of Deep Convective Clouds over Africa from a Lagrangian Perspective, EGU General Assembly 2023, Vienna, Austria, 24–28 Apr 2023, EGU23-13744, https://doi.org/10.5194/egusphere-egu23-13744, 2023.
The spatial and temporal variability of air temperature represents the imprint of various meteorological processes, ranging from microscale turbulence to synoptic-scale weather systems. Convective cold pools, formed by evaporatively cooled downdrafts of precipitating clouds, are known to be an important source of mesoscale variability over mid-latitude land. Cold pools both directly perturb the near-surface temperature field and influence variability by controlling larger-scale convective organization. However, their impact on the sub-mesoscale (100 m to 10 km) temperature variability is unclear due to insufficient observational data. Consequently, the validation of sub-mesoscale variability in numerical weather prediction (NWP) and Large-Eddy Simulation (LES) models is also impeded.
In this study, we apply the variogram framework to determine sub-mesoscale temperature variability in observations as well as in idealized and realistic simulations of cold pool events. The basis of the analyses are actual and virtual observations of a dense network of 99 surface measurement stations as part of the Field Experiment on Submesoscale Spatio-Temporal Variability in Lindenberg (FESSTVaL) conducted in eastern Germany during summer 2021. The observed variogram averaged over the lifetime of a cold pool shows enhanced temperature variance at scales between about 1 km and 15 km compared to well-mixed boundary layer conditions, although the magnitude of the perturbation strongly varies for single time steps. Except for the intensification phase, the cold pool generally reduces the temperature variability at sub-km scales compared to pre-cold pool conditions. This suggests smoothing of sub-km temperature gradients by enhanced mixing near the surface as well as damped turbulent surface fluxes.
Idealized cold pool simulations at LES grid spacings capture the overall variogram shape and evolution well but show the largest uncertainty for sub-km scales as compared to the observed variograms. The results are sensitive to the sampled lifetime stage of the cold pool, its environmental conditions, and the model representation of dissipation time scales and turbulent surface fluxes. These findings can help to identify the spatial and temporal scales of variability that are relevant to correctly simulate convective processes in the atmosphere and their interaction with the land surface.
How to cite: Kirsch, B., Grant, L. D., Falk, N. M., Neumaier, C. A., Bukowski, J., Ament, F., and van den Heever, S. C.: Sub-mesoscale temperature variability in observed and simulated convective cold pools, EGU General Assembly 2023, Vienna, Austria, 24–28 Apr 2023, EGU23-13748, https://doi.org/10.5194/egusphere-egu23-13748, 2023.
The characteristics of isolated deep convection initiation (DCI) and its relation to topography in the North China area are studies statistically and numerically. The infrared brightness temperature data from satellite Himawari-8 are utilized to identify DCI events in three summers. A total of 2534 DCI events are obtained and their locations show clustering over mountains and hills, suggesting the significance of local topography. Topography is described with elevation and relief amplitude. DCI events and grid boxes are counted. DCI events per grid box increases with elevation and relief amplitude. Among different types of topography, DCI is favored in mountains and hilly areas. Moreover, the morning cloud cover condition also shows notable impact on the relation of DCI and topography. For the regime characterized with less morning clouds (regime one), DCI strongly depends on elevation and relief amplitude, while for the regime with more morning clouds (regime two), topography shows a moderate impact on DCI. The time of DCI events are also recorded, and regime one shows a stronger diurnal variation and a peak occurring 2 hours earlier than that of regime two. The synoptic patterns show the difference of large-scale environment between the two regimes, which can explain their differences in DCI to some extent. To clarify the mechanism of topographic effect in DCI process, quasi-idealized numerical simulation in North China is conducted with WRF. The averaged 6-hourly ERA-Interim reanalysis data, which can maintain the major patterns of large-scale circulations, are inputted into the model as initial and boundary conditions. The elevation and relief amplitude of the study domain is varied in the model. The preliminary result shows that the speed of upscale convection growth changes with elevation and relief amplitude, which indicates that mechanisms involving topography-induced variation of solar heating may exist and need further numerical study. We suggest that special attention should be paid to elevation and relief amplitude (or topography type), as well as morning cloud cover condition when forecasting DCI in the North China area and mountainous areas around the world.
How to cite: Lu, G., Ren, Y., Fu, S., and Xue, H.: The Relationship Between Isolated Deep Convection Initiation and Topography in the North China Area, EGU General Assembly 2023, Vienna, Austria, 24–28 Apr 2023, EGU23-13971, https://doi.org/10.5194/egusphere-egu23-13971, 2023.
Convection over the Maritime Continent has large impacts for local extreme weather, as well as for global weather and climate. However, this convection and its impacts are poorly represented in current weather and climate models. This is largely due to complex multi-scale interactions between convection and the ocean, intricate island coastlines and topography, equatorial waves, and larger-scale dynamics such as the Madden-Julian oscillation (MJO). In order to better understand the modulation of convection by the MJO, and its representation in current models, we developed a new modelling suite that couples the Met Office Unified Model to a thermodynamic mixed-layer ocean model with additional corrections to account for ocean dynamics. This allows two-way interactions between the atmosphere and ocean on convection-relevant timescales without the expense of a full dynamical ocean model. We present results from simulations of 10 DJF seasons over the Maritime Continent at grid spacings of 12km (with a mass flux convection scheme) and 2km (without).
We investigated the modulation of large-scale convective heating and moistening by MJO phase in both models. We show that:
- The 2km suite has more variability by MJO phase, and this variability is more realistic than that in the 12km suite when compared to observational data;
- There is more variability in the type of convection (defined by the shape of heating/moistening profiles) between MJO phases in the 2km suite;
- The dominant variation is between different types of convection in the two suites.
We also present preliminary results on the modulation by the MJO of basic properties of the cloud field like feature size, and feature isotropy.
How to cite: Shipley, D., Howard, E., and Woolnough, S.: Modulation of Maritime Continent convection by the MJO: differences between parametrized and explicit convection, EGU General Assembly 2023, Vienna, Austria, 24–28 Apr 2023, EGU23-14672, https://doi.org/10.5194/egusphere-egu23-14672, 2023.
Two structural assumptions are frequently employed in convective parameterisation: the diagnostic and quasi-equilibrium assumptions. The former assumes an instantaneous relationship between the large-scale environment (“macrostate”) and subgrid-scale convective activity, while the latter postulates that convective processes are almost in equilibrium with slowly evolving large-scale forcing at all times. Both assumptions do not take into account the role of convective memory (“microstate” memory), which is defined as the dependence of convection on its own history. Here, we present the memory behaviour of three convection schemes by comparing their responses in two idealised RCE experiments in single-column models (SCMs) to those of a cloud-resolving model (CRM). Three main findings from these tests will be discussed. First, when the large-scale environment is held constant (“FixMacro”), precipitation remains invariant in time with the Zhang-McFarlane scheme, confirming that the scheme does not parameterise convective memory and is fully diagnostic. The org scheme (Mapes & Neale, 2011) displays similar behaviour to the CRM in that precipitation increases in the first moments after FixMacro starts, with larger entrainment rates associated with slower growth. However, its logarithmic growth shape differs from that of the CRM, which displays exponential growth, and can be explained using the scheme’s governing equations. Second, when the prognostic convective memory variable is set to zero at one time step (essentially wiping out microstate memory), the org scheme displays remarkably similar behaviour to the CRM, with precipitation dropping to zero and then recovering to its RCE value over a recovery time scale tmem. In comparison, precipitation in the LMDZ cold pool scheme (Grandpeix & Lafore, 2010) responds in the opposite direction: it grows and then falls back to its RCE value. Finally, the mean and temporal variance of the org variable were found to correlate strongly with memory strength (tmem), indicating that org has captured important aspects of convective memory. Overall, our results indicate that the org and LMDZ cold pool schemes partially, but do not fully capture CRM memory behaviour and are limited by their structural assumptions. They also demonstrate the usefulness of our simple idealised experiments to probe the memory behaviour of convection schemes.
How to cite: Hwong, Y. L., Colin, M., Aglas, P., Muller, C., and Sherwood, S.: Evaluating Memory Properties in Convection Schemes Using Idealised Tests, EGU General Assembly 2023, Vienna, Austria, 24–28 Apr 2023, EGU23-4968, https://doi.org/10.5194/egusphere-egu23-4968, 2023.
The representation of cumulus convection is a known source of uncertainty within current weather and climate models. Where model resolution is too coarse to accurately resolve convection, parameterisations are required to estimate the impact of small-scale convective processes. High resolution large eddy simulations (LES) can be used to diagnose many aspects of convective processes, such as heat and momentum budgets and rates of entrainment. However, LES is computationally expensive, making it impossible to use within operational models. This study aims to bridge the gap between current coarser models and LES by developing a stochastic Lagrangian model to represent an ensemble of air parcels. Vertical velocity, liquid water potential temperature, and total specific humidity are predicted following the ensemble of parcels. The random motions associated with turbulence are represented by a stochastic term within the w-tendency equation. The mean fields which the parcels interact with are defined by an ensemble average of nearby parcels. Several fixers have been developed to ensure that conservation properties are respected. At the current stage of development, the model can represent dry convective boundary layer and shallow convection cases. A theoretical study of the stochastic differential equations is useful to verify the self-consistency of the model and also as a tool for calibrating various parameters within the model. A key question for this project is how well the stochastic parcel model can replicate the statistics of LES results. This will act as a measure of the model’s success, allowing for a deeper understanding on accurately modelling convective processes. Due to the Lagrangian nature of the model, analysis can be conducted upon how the parcels’ characteristics change over time as the parcels experience smaller-scale convective processes such as entrainment. Ultimately, results from this model may yield better understandings of small-scale convective processes. This can create potential for improvements to parameterisations in operational models, reducing model uncertainty generated by convective processes.
How to cite: Hutton, T., Thuburn, J., and Beare, R.: A Lagrangian View to the Evolution of Convective Updrafts, EGU General Assembly 2023, Vienna, Austria, 24–28 Apr 2023, EGU23-9002, https://doi.org/10.5194/egusphere-egu23-9002, 2023.
Posters virtual: Wed, 26 Apr, 16:15–18:00 | vHall AS
While a poleward shift of the jet stream and storm track in response to increased greenhouse gases appears to be robust, the magnitude of this change
is uncertain and differs across models, and the mechanisms for this change are poorly constrained. An intermediate complexity GCM is used to explore
the factors governing the magnitude of the poleward shift and the mechanisms involved. The degree to which parameterized subgrid-scale convection is inhibited has a leading-order effect on the poleward shift, with a simulation with more convection (and less large-scale precipitation) simulating a significantly weaker shift, and eventually no shift at all if convection is strongly preferred over large-scale precipitation. Many of the mechanisms that have been proposed to lead to the poleward shift are present in all simulations (even those with no poleward shift), and hence we can conclude that these mechanisms are not of leading-order significance for the poleward shift in any of the simulations. In contrast, the thermodynamic budget is able to diagnose the reason the jet and storm track shift differs among the simulations, and helps identify midlatitude latent heat release as the crucial differentiator. These results have implications for intermodel spread in the jet, hydrological cycle, and storm track response to increased greenhouse gases in intermodel comparison projects.
How to cite: White, I., Garfinkel, C., Keller, B., Lachmy, O., Gerber, E., and Jucker, M.: Sensitivity of projected storm track and jet latitude changes to the parameterization of convection: implications for mechanisms of the future poleward shift, EGU General Assembly 2023, Vienna, Austria, 24–28 Apr 2023, EGU23-4711, https://doi.org/10.5194/egusphere-egu23-4711, 2023.
Mesoscale Convective Systems (MCSs) represent some of the most intense and destructive thunderstorms in the world. Understanding the physical processes that drive these storms and influence their characteristics is vital for hazard prediction and mitigation.
A significant amount of research in the “natural laboratory” of West Africa has shown that soil moisture heterogeneity on different spatial scales can influence the location of convective initiation (10s of km) and the intensification of remotely triggered storms (100s of km).
Previous studies have demonstrated that the control of soil moisture state on convective initiation identified in West Africa is also important elsewhere in the world whereas very little is known about the influence of surface conditions on travelling storms in other regions.
In the current work we combine satellite observations and reanalysis data to characterise the impact of pre-storm soil moisture conditions on the atmospheric environment and characteristics of mature storms in seven MCS hotspot regions, West Africa, South Africa, South America, Great Plains, India, China and Australia.
We observe a clear latitudinal dependence of the coupling signal with distinct differences between regions where convection is predominately driven by monsoonal or frontal dynamics. However our results suggest that in all regions, large-scale (100s of km) soil moisture gradients are having an impact on convection within mature MCSs through moderation of the climatological temperature gradient in the lower atmosphere, which influences factors that favour convection such as shear and convergence.
How to cite: Barton, E., Klein, C., Taylor, C., Marsham, J., and Parker, D.: Response of Mature Storms to Soil Moisture State in Global Hotspot Regions, EGU General Assembly 2023, Vienna, Austria, 24–28 Apr 2023, EGU23-13504, https://doi.org/10.5194/egusphere-egu23-13504, 2023.
