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A defining feature of the Earth’s climate is the annual variation of heavy precipitation and convergent wind circulation in the tropics and subtropics. This dominant mode of hemispherically distributed rainfall is often termed the 'global monsoon', comprising of regional monsoon systems on every continent. Monsoon regions are defined using annual precipitation differences and average seasonality rather than by the dynamical similarities of rainfall dynamics; they thus fail to (i) consider global patterns of extreme rainfall events (EREs), and (ii) take into account spatio-temporal similarities in timing and intensity of monsoonal circulation.

In this work, we investigate the dynamics of the Global Monsoon using the framework of complex networks derived from extreme rainfall events. In particular, we use time-delayed event synchronization applied to the GPCP rainfall dataset to first extract a network of global ERE teleconnections. We then identify regions with similar ERE patterns by applying on the global ERE network a Bayesian hierarchical clustering approach based on the stochastic block model.

Our work presents evidence to place different monsoon regions in a global context and therefore to describe them as a unified system with common underlying dynamics: Besides known teleconnections, our method captures various differently resolved representations of the global weather system. These range from a description containing two clusters separated by the hemispheric equator to a precise representation of distinguishable but connected monsoon regions. We argue that the global monsoon can be regarded as a hierarchical complex system into which regional monsoons are embedded in intermediate levels of the clustering hierarchy.

How to cite: Strnad, F. and Goswami, B.: Investigating the Global Monsoon by networks of extreme rainfall events, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-3245, https://doi.org/10.5194/egusphere-egu21-3245, 2021.

The South Asian monsoon system impacts the livelihoods of over a billion people. While the overall monsoon rainfall is believed to have decreased during the 20^th century, there is a good agreement that the extreme precipitation events have been rising in some parts of India. As an important part of the Indian population is dependent on rainfed agriculture, such a rise in extremes, along with resulting flood events, can be all the more problematic. Although studies tend to link this rise in extreme events with anthropogenic forcing, some uncertainties remain on the exact causes. In order to examine the correlation between anthropogenic forcings and the different trends in extreme events, we have analyzed the high-resolution daily rainfall data in the past century delivered by the Indian Meteorological Department alongside several other economic and ecological estimates. The results from this analysis will be presented in detail.

How to cite: Falga, R. and Wang, C.: Extreme precipitation events during the South Asian summer monsoon season in the past century, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-6418, https://doi.org/10.5194/egusphere-egu21-6418, 2021.

Rain-gauge datasets indicate strong increases in both annual mean and extreme precipitation over large parts of the Mainland Southeast Asia (MSEA) including Vietnam and the northwestern part of the peninsula over the last 40 years. Increasing precipitation is associated with increased monsoon intensity in southeast Asia and a northward shift of the monsoon activity centre towards MSEA. Warming-driven evaporation increases over the three main oceanic moisture sources - the Arabian Sea, the Bay of Bengal, and the South China Sea- may partially explain increasing precipitation in large parts of MSEA. Changes in the patterns of the two main modes of natural variability in the tropical Indian Ocean – the Indian Ocean Basin Mode (IOBM) and the Indian Ocean Dipole (IOD) – contribute to surface warming in these oceanic moisture source regions supplying precipitation to MSEA. Climate model projections show robust wide-spread trends in wet season precipitation with increasing frequency and intensity of extreme precipitation events throughout MSEA over the 21^st century. Similar to observations, the projected precipitation trends are associated with strong warming-driven increases in evaporation in all major oceanic moisture sources supplying precipitation to MSEA.

How to cite: Skliris, N., Marsh, R., Haigh, I., Wood, M., Hirschi, J., Darby, S., Quynh, N. P., and Hung, N. N.: Trends in mean and extreme rainfall over Mainland Southeast Asia associated with warming-driven trends in evaporation , EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-12258, https://doi.org/10.5194/egusphere-egu21-12258, 2021.

The semiarid region of the Sahel was marked during the 20^th Century by significant modulations of its rainfall regime. Part of these modulations has been associated with the internal variability of the climate system, mediated by changes in oceanic sea surface temperature (SST). We show here that the external forcings, and in particular anthropogenic aerosols, might have played a role more important than previously thought in setting these variations. The study is based on the recent simulations performed for CMIP6 with the IPSL-CM6A-LR coupled model. As in most coupled models, the maximum precipitation is limited to the southern Sahel during boreal summer in the IPSL-CM6A-LR model. A novel definition of the Sahel precipitation region is proposed in order to take this bias into account. Our results show that external forcings induce decadal modulations of Sahel precipitation that correlate significantly at 0.6 with the observed precipitations and that the anthropogenic aerosols explain more than 70% of these modulations. These results confirm recent results of CMIP6 highlighting an important role of aerosol forcing for the decadal climate in and around the North Atlantic ocean.

How to cite: Ndiaye, C. D., Mignot, J., and Mohino, E.: Forced modulations of Sahel rainfall at decadal timescale over the 20th Century., EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-4105, https://doi.org/10.5194/egusphere-egu21-4105, 2021.

The intraseasonal characteristics of rainfall have important implications for agriculture in the Sahel. For example, the development and yield of millet, sorghum and maize depend not only on the rainfall seasonal total amounts, but also on the onset of the rainy season and the seasonal distribution of rainy days as well as the occurrence of dry spells. However, the decadal variability of intraseasonal rainfall characteristics in the Sahel and in particular in Senegal has received little attention in the literature so far. In this study, we analyze the decadal modulations of the intraseasonal characteristics of the monsoon season in Senegal over the period 1918-2000. From daily rainfall data measured at different stations in Senegal, we have defined indices characterizing, among others, the number of rainy days, the average intensity of rainy days, the starting day and ending day of the rainy season. The spatial patterns of the mean indices generally show a north/south gradient and their temporal modulations show a clear decadal signal. Application of EOF (Empirical Orthogonal Function) analysis provides a main mode of variability showing same-signed loads throughout the territory. The associated PCs show strong decadal variability for most indices with a strong link to the Atlantic Multidecadal Variability. The exception are the indices related to the duration of the monsoon season, which show a weaker decadal variability with a clear trend.

How to cite: Badji, A., Diakhaté, M., Gaye, A. T., Mignot, J., and Mohino, E.: Decadal Variability of Rainfall in Senegal : beyond total seasonal amounts, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-12507, https://doi.org/10.5194/egusphere-egu21-12507, 2021.

Investigating the synchrony and interdependency of heavy rainfall occurrences is crucial to understand the underlying physical mechanisms and reduce physical and economic damages by improved forecasting strategies. In this context, studies utilizing functional network representations have recently contributed to significant advances in the understanding and prediction of extreme weather events.

To thoroughly expand on previous works employing the latter framework to the East Asian Summer Monsoon (EASM) system, we focus here on changes in the spatial organization of synchronous heavy precipitation events across the monsoon season (April to August) by studying the temporal evolution of corresponding network characteristics in terms of a sliding window approach. Specifically, we utilize functional climate networks together with event coincidence analysis for identifying and characterizing synchronous activity from daily rainfall estimates with a spatial resolution of 0.25° between 1998 and 2018. Our results demonstrate that the formation of the Baiu front as a main feature of the EASM is reflected by a double-band structure of synchronous heavy rainfall with two centers north and south of the front. Although the two separated bands are strongly related to either low- or high-level winds which are commonly assumed to be independent, we provide evidence that it is rather their mutual interconnectivity that changes during the different phases of the EASM season in a characteristic way.

Our findings shed some new light on the interplay between tropical and extratropical factors controlling the EASM intraseasonal evolution, which could potentially help improving future forecasts of the Baiu onset in different regions of East Asia.

Further details: F. Wolf, U. Ozturk, K. Cheung, R.V. Donner: Spatiotemporal patterns of synchronous heavy rainfall events in East Asia during the Baiu season. Earth System Dynamics (in review). Discussion Paper: Earth System Dynamics Discussions, (2020)

How to cite: Wolf, F., Ozturk, U., Cheung, K., and Donner, R. V.: Spatiotemporal patterns of synchronous heavy rainfall events in East Asia during the Baiu season, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-3062, https://doi.org/10.5194/egusphere-egu21-3062, 2021.

IMPROVE is motivated by the effects of orography on Indian precipitation as part of the diurnal cycle of convection, contributing to water supply, as well as its role in extreme events.  IMPROVE considers two focal regions.  The Western Ghats, which intercept the monsoon flow across the Arabian Sea, receive some of the most frequent and heaviest rainfall during summer as well as being subject to extremes such as the 2018 Kerala floods.  Meanwhile, the Himalayas play a vital role in separating dry midlatitude flows from tropical airmasses and are subject to extremes during the summer monsoon, as well as in winter due to the passage of western disturbances.  This presentation summarizes the key results of IMPROVE.  Firstly, we examine the impact of orography on the observed convective diurnal cycle and assess its simulation in models at a range of resolutions including convection-permitting scales.  MetUM and WRF model experiments are used to identify key mechanisms and test their capability at simulating scale interactions between forcing at the large scale from the BSISO and newly identified regimes of on- and offshore convection near the Western Ghats.  An additional aspect to this work is the construction of a two-layer analytical model to test the behaviour of sheared flow perpendicular to a ridge analogous to the Western Ghats.  Secondly, the role of orography in extreme events is considered.  For the Western Ghats, this focuses on the interaction between monsoon low-pressure systems and the southwesterly flow in enhancing local rainfall.  For the Himalayas, we focus on characterising interactions between tropical lows and western disturbances in enhancing the orographic precipitation.  The work in IMPROVE works towards a deeper understanding of orographic rainfall and its extremes over India and uncovering why such mechanisms may be poorly represented in models.

How to cite: Turner, A., Fletcher, J., Hunt, K., Phadtare, J., Griffiths, S., Ross, A., Schiemann, R., and Stein, T.: Indian Monsoon Precipitation over Orography: Verification and Enhancement of understanding – Outcomes of the IMPROVE project, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-15748, https://doi.org/10.5194/egusphere-egu21-15748, 2021.

Accurate predictions of heavy precipitation in India are vital for impact-orientated forecasting, and an essential requirement for mitigating the impact of damaging flood events. Operational forecasts from non-convection-permitting models can have large biases in the intensities and spatial structure of heavy precipitation, and while convection-permitting models can reduce biases, their operational use over large areas is not yet feasible. Statistical postprocessing can reduce these biases for relatively little computational cost, but few studies have focused on postprocessing monsoonal rainfall and the associated severe flooding events. As part of the Weather and Climate Science for Service Partnership India (WCSSP India), the HEavy Precipitation Forecast Postprocessing over India (HEPPI) project assesses the value of multiple postprocessing methods in this context. 

Here, we present an evaluation of two postprocessing approaches to determine their suitability for heavy rainfall in India: Univariate Quantile Mapping (UQM) and Ensemble Model Output Statistics (EMOS). For each method, we apply the statistical postprocessing to daily precipitation in the NCMWF 12km forecast for the 2018 and 2019 monsoon seasons individually at each grid cell within the forecast. UQM leads by construction to rainfall distributions close to the observed ones, while EMOS optimises the spread of the postprocessed ensemble without guaranteeing realistic rainfall distributions. The choice of method is therefore to some degree dependent on end user requirements.

We use three rainfall observation data sets and different parametric distributions for UQM to determine the best setup. Mixed distributions, where gamma distributions are fitted separately to the bottom 90% and the top 10% of rainfall events are found to be the best choice because they are a better fit for the high rainfall values.

In several case studies, an overestimation of west coast rainfall in the forecasts is corrected by UQM. Although errors linked to forecasting rainfall in the wrong location or where no rainfall has been observed at all cannot be corrected by local statistical postprocessing, the overall forecast performance is improved by the UQM approach adopted here.

As in UQM, we use multiple observational datasets to determine the best EMOS setup. We select the gamma distribution, due to its suitability for both low and heavy rainfall events. Unlike in UQM, mixed distributions are unnecessary as the distribution is fitted across ensemble members at each timestep. EMOS and UQM are verified against observations and compared to each other using a variety of metrics including case studies, the Receiver Operating Characteristic and the Continuous Rank Probability Score.

How to cite: Angus, M., Widmann, M., Orr, A., and Leckebusch, G.: A comparison of two postprocessing approaches as part of the HEavy Precipitation forecast Postprocessing over India (HEPPI) project.  , EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-12403, https://doi.org/10.5194/egusphere-egu21-12403, 2021.

The climate of southern arid central Asia (SACA, 35.25°-45°N，46.25°-80°E) is controlled by the subtropical high and it exhibits typical Mediterranean characteristics. Given the lack of systematic research on the physical mechanism of precipitation variations in the major precipitation seasons of SACA, we analyzed the physical mechanism of winter precipitation variations during 1979-2017. The results suggest that two water vapor pathways influence the winter precipitation in SACA and that they are closely related to a low-latitude high-pressure anomaly and the mid-high latitude North Atlantic Oscillation (NAO). Specifically, at low latitudes, the northern Indian Ocean heated by El Nino  causes the anomalous intensification of the subtropical high throughout the low latitude region, especially over the Indian subcontinent, resulting in increased water vapor transport from the northern Indian Ocean to SACA. At middle and high latitudes, the negative NAO phase leads to the southward movement of the water vapor pathway, which causes it to pass over a greater number of upwind water bodies, resulting in the transport of more westerly-associated water vapor to SACA. Further analysis showed that there is a northwest-southeast teleconnection wave train, from the North Atlantic to Central Asia and to the Indian subcontinent, which allows wave fluxes originating in the North Atlantic and the northern Indian Ocean to propagate from high and low latitudes, respectively, to the study area. At the same time, high-latitude cold air advection, brought by the low-pressure system in Central Asia, converges with the flow of warm water vapor from the low-latitude northern Indian Ocean, generating an ascending motion and reducing atmospheric static stability, which results in increased precipitation in SACA. Therefore, the key to determining the origin of precipitation variations in SACA is understanding the interaction of large-scale circulation systems at low and mid-high latitudes. In the future, with continued global warming, strong El Nino events will occur more frequently, and the subtropical high is like to intensify and move northward. As a result, there will be a tendency for the NAO and the Arctic Oscillation (AO) to remain in a negative phase; consequently the Arctic vortex in the eastern hemisphere is likely to provide favorable water vapor and dynamic conditions promoting increased winter precipitation in SACA. Overall, our findings are valuable for understanding the regional response of precipitation in arid areas against the background of ongoing global warming.

How to cite: Xie, T., Huang, W., Zheng, F., Chen, J., and Chen, F.: Physical mechanism of winter precipitation variations in southern arid Central Asia, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-1899, https://doi.org/10.5194/egusphere-egu21-1899, 2021.

An intermediate complexity moist General Circulation Model is used to investigate the forcing of the Asian monsoon and the associated upper level anticyclone by land-sea contrast, net horizontal heat transport by the ocean, and topography. The monsoonal pattern is not simply the linear additive sum of the response to each forcing; only when all three forcings are included simultaneously does the monsoonal circulation extend westward to India. This nonadditivity impacts the location of the upper level anticyclone, which is shifted eastward and weaker if the forcings are imposed individually. Sahelian precipitation, and also austral summer precipitation over Australia, southern Africa, and South America, are likewise stronger if all forcings are imposed simultaneously. The source of the nonlinearity can be diagnosed using gross moist stability, but cannot be accounted for using the land-sea breeze paradigm. This non-additivity implies that the question of which forcing is most important is ill-posed.

How to cite: Garfinkel, C., White, I., Adam, O., Gerber, E., and Jucker, M.: Nonlinear Interaction between the Drivers of the Monsoon and Summertime Stationary Waves, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-336, https://doi.org/10.5194/egusphere-egu21-336, 2020.

This paper reviews recent progress made by Chinese scientists on the pathways of influence of the Northern Hemisphere mid–high latitudes on East Asian climate within the framework of a “coupled oceanic–atmospheric (land–atmospheric or seaice–atmospheric) bridge” and “chain coupled bridge”. Four major categories of pathways are concentrated upon, as follows: Pathway A—from North Atlantic to East Asia; Pathway B—from the North Pacific to East Asia; Pathway C—from the Arctic to East Asia; and Pathway D—the synergistic effects of the mid–high latitudes and tropics. In addition, definitions of the terms “combined effect”, “synergistic effect” and “antagonistic effect” of two or more factors of influence or processes and their criteria are introduced, so as to objectively investigate those effects in future research.

How to cite: Li, J., Zheng, F., Sun, C., Feng, J., and Wang, J.: Pathways of Influence of the Northern Hemisphere Mid–high Latitudes on East Asian Climate: A Review, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-4225, https://doi.org/10.5194/egusphere-egu21-4225, 2021.

China receives most of its rainfall during the East Asian summer monsoon (EASM). The EASM is a complex, multi-phase and multi-scale phenomenon, influenced by both tropical and mid-latitude dynamics and by the presence of major orography, such as the Tibetan Plateau. The EASM front, displaying a steep gradient in equivalent potential temperature, neatly separates tropical and extratropical air masses as the monsoon marches northwards, particularly in the Mei Yu stage. Many questions are still open on the dynamics of EASM evolution. Recent work on the Indian monsoon has indicated a new approach, focusing on the interaction between competing air masses that shapes monsoon progression. Drawing from that approach, we apply Eulerian and Lagrangian methods to the ERA5 reanalysis dataset to provide a comprehensive study of the seasonal evolution of the EASM and of its front. 

A new frontal detection algorithm is used to perform a front-centred analysis of EASM evolution, allowing to clearly identify and depict the four main stages of evolution of the EASM, in agreement with recent studies. The dynamics of interaction between monsoon and mid-latitude air masses at the EASM front are then investigated, highlighting the key tropical and extratropical processes, at both upper and lower levels. The sub-tropical westerly jet (STWJ) over east Asia has a primary role in controlling the strength and the poleward progression of the EASM front, in particular during Mei Yu. This upper-level mid-latitude forcing acts in conjunction with the low-level moist-air advection from the tropics, modulated by the seasonal cycle of the South Asian monsoon and by the location of the Western North Pacific subtropical high. The Mei Yu stage is distinguished by an especially clear interaction between tropical and extratropical air masses that converge at the EASM front, with the importance of remote moisture sources for the advection of moist tropical air also highlighted. Composites of the years with highest and lowest latitude of the EASM front at Mei Yu are also assessed, outlining the processes behind the interannual variability of the poleward progression of the EASM front. Their analysis reveals the influence of the STWJ on the strength of the mid-latitude flow impacting on the northern side of the EASM front. In turns, this affects the extent of the warm moist advection on the southern side and the distribution and intensity of resultant rainfall over China.

Thus, using a mix of diagnostics tools and methods of analysis, in this study we identify the key airmasses, and related processes, that characterise seasonal EASM progression and variability. Clarifying their roles and joint influences in the evolution of this complex, multi-scale and multi-stage phenomenon we also highlight the dynamics of the tropical-extratropical interaction that occurs at the front, particularly during its Mei Yu northward migration.

How to cite: Volonté, A., Schiemann, R., Turner, A., and Vidale, P. L.: The interaction of tropical and extratropical airmass controlling East Asian summer monsoon progression, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-11045, https://doi.org/10.5194/egusphere-egu21-11045, 2021.

How to cite: Seshadri, A. K. and Dixit, V.: Kinetic energy generation in cross-equatorial flow and the Somali Jet, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-12128, https://doi.org/10.5194/egusphere-egu21-12128, 2021.

Monsoons were traditionally considered to be land-based systems. Recent definitions of monsoons based on either the seasonal reversal of winds or the local summer precipitation accounting for more than 50% of the annual precipitation suggests that monsoon domains extend over oceanic regions as well. The concept of global monsoon combines all the monsoon domains into a single entity. Modern observations show that the variations in precipitation are nearly coherent across all the individual monsoon domains on decadal timescales. Using a transient simulation of the global climate over the last 22,000 years as well as reanalysis data of the modern climate, we have shown that tropical precipitation has different characteristics over land and ocean grids. This is due to the differences in the energetics of monsoon over land and ocean grids. With a lower thermal heat capacity, the net surface energy flux over land is negligible, whereas it is quite large over the ocean. In fact, the orbital scale variability of net energy flux into the atmosphere over the ocean is controlled by the surface energy flux. Another major difference between land and ocean grids of the global monsoon is in the vertical profile of the vertical pressure velocity. It is bottom-heavy over land and top-heavy over the ocean. This results in smaller vertical transport of moist static energy (which has a minimum in the lower troposphere) over land, and a larger vertical transport over the ocean. These differences between the land and ocean, suggest that the land and ocean grids should not be combined as is traditionally done. Global monsoon-land and global monsoon-ocean should be studied separately.

How to cite: Chakraborty, A., Jalihal, C., and Srinivasan, J.: Different energetics of global monsoon over land and ocean, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-14022, https://doi.org/10.5194/egusphere-egu21-14022, 2021.

Convective cloud development during the Indian monsoon helps moisten the atmospheric environment and drive the monsoon trough northwards each year, bringing a large amount of India’s annual rainfall. Therefore, an increased understanding of how monsoon convection develops in observations will help inform model development. In this study, 139 days of India Meteorological Department Doppler weather radar data is analysed for 7 sites across India during the 2016 monsoon season. Convective cell-top heights (CTH) are objectively identified through the season, and compared with near-surface (at 2 km height) reflectivity. These variables are analysed over three time scales of variability during the monsoon: monsoon progression, active-break periods and the diurnal cycle. We find a modal maximum in CTH around 6–8 km for all sites. Reflectivity increases with CTH, at first sharply, then less sharply above the freezing level. Bhopal and Mumbai exhibit lower CTH for monsoon break periods compared to active periods. A clear diurnal cycle in CTH is seen at all sites except Mumbai. The phase of the diurnal cycle depends on the surface type being land or ocean for south-eastern India, with the frequency of oceanic cells typically exhibiting an early morning peak compared to those over land, consistent with the observed diurnal cycle of precipitation. The cell characteristics discovered are discussed in light of the differences in large-scale synoptic and mesoscale mechanisms responsible for different cell regimes. Our findings confirm that Indian monsoon convective regimes are partly regulated by the large-scale synoptic environment within which they are embedded.

How to cite: Doyle, A., Turner, A., and Stein, T.: 2016 Monsoon Convection and its place in the Large-Scale Circulation using Doppler Radars, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-947, https://doi.org/10.5194/egusphere-egu21-947, 2021.

The East Asian Summer Monsoon (EASM) is an inherently multiscale phenomenon and new generations of global convection-permitting climate models hold great promise in representing such multiscale monsoon interactions.

Motivated by the recent availability of multi-year simulations with the HadGEM3 global climate model at about 10km resolution and different treatments of convection, the COSMIC project has delivered new process-based and decision-relevant metrics of diurnal and intraseasonal variability, and of the seasonal progression of the EASM: The newly developed BASMATI (Basin-Scale Model Assessment ToolkIt) tool is used for the scale-selective evaluation of the diurnal cycle of precipitation over Asian river basins and it is used to show that the phase of the diurnal cycle is much better represented in a convection-permitting setup of the global model, whereas mean precipitation biases in this setup are substantial and point to the need for further tuning of this new model version. Furthermore, a new automated method for identifying the EASM front has been developed and applied to ERA5 reanalysis data in a detailed description of the seasonal progression of the front. Lagrangian trajectory analysis is employed to identify air-mass convergence at the EASM front and highlights the specific conditions of converging warm and moist tropical and cooler subtropical air masses during the Mei Yu season. These results offer a new framework for studying the seasonal EASM progression and its representation in models. Finally, the different metrics developed in COSMIC are used for a statistical and dynamical characterisation of the exceptional precipitation and flooding affecting different parts of Asia, and the Yangtze river basin in particular, in June/July 2020.This poster provides a project overview and complements two separate conference papers discussing COSMIC results in greater detail.

How to cite: Schiemann, R., Turner, A., Muetzelfeldt, M., Volonté, A., Klingaman, N., and Vidale, P. L.: COSMIC project: COnvective-Scale Modelling In China, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-12314, https://doi.org/10.5194/egusphere-egu21-12314, 2021.

Monsoon low pressure systems (LPS) are synoptic scale tropical disturbances that form in the Indian subcontinent over the quasi-stationary monsoon trough axis during the monsoon period (June to September). In a recent study, we showed that 60-70% of monsoon rainfall and 78% of extreme precipitation events in India are associated with LPS. Global circulation models (GCMs) have been used to understand the behavior of tropical disturbances in the past. It has been found that model resolution plays a key role in simulating the climatology of tropical storms, with finer resolution (of the order of 20-100km) required to better represent the genesis and propagation of these storms. As GCMs can be run at these finer resolutions today, various characteristics of LPS in the Indian subcontinent can be studied. It has been found that most CMIP5 GCMs show a southward latitudinal shift in the monsoon trough location and hence in the LPS tracks and associated characteristics. This shift has been attributed to a weaker simulated meridional tropospheric temperature gradient (MTG) in the models. However, the cause of weaker MTG in models is not known. In this study, we investigate the reason for the weaker MTG and hence the southward latitudinal shift of LPS tracks in the Climate Earth System Model (CESM1.2.2). A present-day control simulation is performed at 0.9°×1.25° horizontal resolution, and output is saved at 6-hourly intervals for LPS track analysis. We find that CESM is capable of simulating the general behavior of monsoon over the Indian subcontinent in terms of seasonality, propagation of monsoon rainfall, and mean monsoon winds. LPS are tracked in the CESM outputs by our recently proposed Automated Tracking Algorithm using Geopotential Criteria (ATAGC). A southward latitudinal shift is observed in the median track of LPS in CESM present-day simulations. The value of MTG is also significantly smaller compared to the observed MTG. The results from investigations on the likely causes for the weaker MTG in CESM will be presented at the meeting.

How to cite: Thomas, T. M., Bala, G., and Srinivas, V. V.: An investigation of the bias in the median track of Monsoon Low Pressure Systems over the Indian subcontinent in CESM1.2.2 simulations, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-3879, https://doi.org/10.5194/egusphere-egu21-3879, 2021.

The onset, persistence and variability of summer monsoon rainfall impacts over a billion people. Advance knowledge is critical for agricultural planning and hazard mitigation, yet forecasting remains a challenge. Sources of error that have been identified in forecast models include the representation of the land surface and subsequent coupling with the boundary layer and convection. This study presents an analysis of land-atmosphere coupling in the operational Indian 4km convective scale regional model configuration of the Unified Model (NCUM-R), used by NCMRWF to provide daily forecasts. An earlier study (Barton et al, QJRMS 2019) analysed the coupling in this model for a single forecast when research aircraft observations were available. It revealed rapidly evolving biases in the monsoon trough linked to errors in the representation of soil moisture. Our current work aims to understand whether this behavior is typical of the monsoon season. This matters because the trough is an important dynamical feature and a key driver of regional rainfall. Here we provide a more comprehensive analysis by assessing the impact of initial soil moisture state on a full season of operational three day forecasts. NCUM-R output is evaluated by comparison to ERA5 reanalysis (atmospheric temperature and pressure) and satellite observations from AMSR2 (land surface temperature) and SMAP (soil moisture).  Correlations between surface and atmospheric variables in the model are computed using linear regression. Our results suggest that systematic biases in the evolution of atmospheric temperature and pressure over three days are indeed linked to errors in the initial soil moisture state. These biases likely impact rainfall predictions derived from the forecasts throughout the monsoon season. This work highlights the importance for realistic soil moisture initialisation in high resolution operational forecasts.

How to cite: Barton, E., Taylor, C., Jayakumar, A., Mitra, A., and Arulalan, T.:  Land-atmosphere coupling in operational NCUM forecasts during the 2020 monsoon season, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-5463, https://doi.org/10.5194/egusphere-egu21-5463, 2021.

This study examined the future changes of Asian monsoon precipitation to global warming on the regional scale, focusing on tropical cyclones along the monsoon trough. This is because the Asian monsoon precipitation is closely associated with tropical disturbances. To reproduce convective precipitation and tropical disturbances, this study used outputs of high-resolution climate simulations. First, two sets of approximately 30-yr simulations under present-day (control) and warmer climate conditions (global warming) were conducted by the 14-km Nonhydrostatic Icosahedral Atmospheric Model (NICAM) with explicitly calculated convection, which were analyzed (Takahashi et al. 2020). Overall, the Asian summer monsoon was well simulated by the model. Precipitation increased as a result of global warming along the monsoon trough, which was zonally elongated across northern India, the Indochina Peninsula, and the western North Pacific Ocean. This increased precipitation was likely due to an increase in precipitable water. The spatial pattern of the increased precipitation was associated with enhanced cyclonic circulations over a large area along the monsoon trough, although it was difficult to determine whether the large-scale monsoon westerly was enhanced. This enhancement can be explained by future changes in tropical disturbance activity, including weak tropical cyclones. In addition to this result, this study will provide the results of future changes in the Asian monsoon precipitation by high-resolution models. 

• Takahashi, H. G., Kamizawa, N., Nasuno, T., Yamada, Y., Kodama and, C., Sugimoto, S., & Satoh, M. (2020). Response of the Asian Summer Monsoon Precipitation to Global Warming in a High-Resolution Global Nonhydrostatic Model, Journal of Climate, 33(18), 8147-8164,

How to cite: Takahashi, H.: Role of tropical cyclones along the monsoon trough in the future changes of the Asian monsoon precipitation by high-resolution models, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-13986, https://doi.org/10.5194/egusphere-egu21-13986, 2021.

The Boreal Summer Intraseasonal Oscillation (BSISO) is a major mode of intraseasonal variability in the Indian summer monsoon. The characteristic pattern includes northward/north-eastward propagating anomalies of convection and circulation over the Indian longitudes, and concurrent eastward propagating anomalies that move through the tropics from the equatorial Indian ocean. In the Indian monsoon region, the BSISO interacts with other processes to affect the rainfall variability on a range of spatial and temporal scales. Convection-permitting simulations are known to improve the representation of some of these smaller-scale processes, but until recently, it has not been feasible to use convection-permitting simulations to model the entire BSISO because of the temporal and spatial scales on which it occurs. Here we assess how well a global multi-year convection-permitting simulation with a coarse grid-spacing of ~10km at the equator models the BSISO. Using Empirical Orthogonal Function (EOF) analysis, we show that overall, the convection-permitting simulation does not give a substantially better representation of the BSISO, when compared with a simulation which parametrises convection. In the observations, the first two EOF eigenvectors and their Principal Component (PC) time series describe the BSISO. The characteristic northwest-to-southeast slope of the observed EOF 1 and 2 patterns is not captured in the parametrised simulation but is better captured in the convection-permitting simulation. However, the convection-permitting simulation does not capture the observed relationship between the PC1 and PC2 time series that describe the strength and phase of the BSISO. The observed pattern is of a fairly constant phase difference between the PC1 and PC2 time series, but in the convection-permitting simulation, there are periods of both negative and positive phase differences. Our results demonstrate that the BSISO is very sensitive to the representation of convection and future higher resolution runs will provide useful routes for understanding scale interactions in the BSISO.

How to cite: Willetts, P., Fletcher, J., and Marsham, J.: The representation of the boreal summer intraseasonal oscillation in a global convection-permitting simulation, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-16255, https://doi.org/10.5194/egusphere-egu21-16255, 2021.