Monsoon Break Period Research Articles

Understanding Recycled Precipitation at Different Spatio‐Temporal Scales Over India: An Eulerian Water Tagging Approach

AbstractLand evapotranspiration has a notable impact on the continental precipitation of India during monsoons. Given the heterogeneous land use and climate, India hosts many transition zones, which are hotspots for land‐atmosphere interactions. This work aims to study the soil moisture‐precipitation feedbacks over India by quantifying recycled precipitation using the Eulerian water vapor‐tagging technique incorporated in the Weather Research and Forecasting model. Eulerian vapor tagging techniques provide the most accurate information on recycled precipitation. We tracked the vapor originating at two spatial scales—entire India and six homogeneous monsoon regions. The latter analysis is conducted to assess inter‐regional vapor exchanges. In this process, we determined the seasonal, monthly, and diurnal variations of recycling ratio and assessed its behavior during active and monsoon break periods. We considered three contrasting monsoon seasons (surplus, normal, and deficit rainfall years) for analysis. Results show a high recycling ratio in Central India, Gangetic plain, and Northeast India. Recycling ratio increases with monsoon progression and peaks in September (∼0.30). The Himalaya and transient low‐pressure systems originating from the Bay of Bengal are vital for precipitation recycling in India. The contrasting monsoons exhibit changes in recycling ratio only at monthly scale. Regional analysis indicates West Central India is the most significant vapor contributor, while Northeast India is the highest recipient of recycled precipitation. High recycling (∼20%) shifts from central India during the active period to the foothills of Himalaya during the break period. Diurnal scale analysis reveals high afternoon recycling due to enhanced convective activity.

2016 Monsoon Convection and Its Place in the Large‐Scale Circulation Using Doppler Radars

AbstractConvective 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 from observations will help inform model development. In this study, 139 days of India Meteorological Department Doppler weather radar data is analyzed for seven sites across India during the 2016 monsoon season. Convective cell‐top heights (CTHs) are objectively identified through the season, and compared with near‐surface (at 2 km height) reflectivity. These variables are analyzed over three time scales of variability during the monsoon: monsoon progression on a month‐by‐month basis, active‐break periods and the diurnal cycle. We find a modal maximum in CTH around 6–8 km for all sites. Cell‐averaged 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. For south‐eastern India, the phase of the diurnal cycle depends on whether the surface is land or ocean, with the frequency of oceanic cells typically exhibiting an earlier morning peak compared to land, consistent with the diurnal cycle of precipitation. Our findings confirm that Indian monsoon convective regimes are partly regulated by the large‐scale synoptic environment within which they are embedded. This demonstrates the excellent potential for weather radars to improve understanding of convection in tropical regions.

Segregation of Forecast Errors in the Planetary Boundary Layer Parameterization Over the State of Odisha and Neighboring Regions in India During Summer Monsoon Season

Planetary boundary layer parametrization (PBL) is a key factor influencing weather forecast skills. This article provides a quantitative illustration of the segregation of forecast errors and their growth arising from different PBLs of the Weather Research and Forecasting (WRF) model. The systematic (root mean square) components of forecast errors arising from four different PBLs of the WRF over the state of Odisha (India) and its surrounding regions are elucidated. Error characterizations are carried out for the forecast lead time up to 96 h (day-4) for two contrasting monsoon seasons, i.e., 2013 (normal) and 2014 (deficit). A total of 1112 simulations are carried out for each initial condition, i.e., May 15 to September 29 for both monsoon seasons using four PBL schemes, i.e., Yonsei University (YSU), Mellor–Yamada–Nakanishi-Niino (MYNN), Asymmetric Convective Model version 2 (ACM2), and medium-range forecast (MRF). The overall results suggest the errors in thermodynamical variables (i.e., temperature and relative humidity) are large compared to the dynamical variable (i.e., wind). For the normal monsoon year (i.e., 2013), the MRF and MYNN exhibit the lowest root mean square error (RMSE) of temperature compared to ACM2 and YSU, whereas MRF (MYNN) shows the lowest (highest) error growth at 24–48 h (72–96 h). The deficit year (2014) has a higher temperature error compared to that of the normal monsoon year (2013), which might be due to frequent monsoon break periods. The spatial distribution of wind exhibits the lowest systematic error for MRF with lead time up to 48 h. The subsequent decrease (increase) of convergence (divergence) of error flux over northern Odisha and increase (decrease) of the same over southern Odisha suggests that the error propagation occurs from north to south. In general, both the convergence and divergence of error energy are found to be weak in MRF and MYNN, attributable to the lower error growth rate and hence the smaller systematic errors compared to ACM2 and YSU for both these monsoon seasons. It is also found that the systematic component of the linear (nonlinear) error growth rate is contributed by the physics (dynamics) components of the model. These findings will provide guidance for the model community and operational agencies to make an optimal choice of PBL parameterizations, particularly for the monsoon forecast.

A climatological analysis of the monsoon break following the summer monsoon onset over Luzon Island, Philippines

AbstractThis study investigates the climatology of the monsoon break following the onset of the summer rainy season over Luzon Island (120–122.5°E, 13–22°N) in the Philippines from 1979–2017. The first post‐onset monsoon break is remarkable in stations located over the north and central Luzon Island and occurs climatologically in early June. Composite analysis of the large‐scale circulation features during the monsoon break period shows that this break is associated with the westward extension of the western North Pacific Subtropical High (WNPSH), which weakened the monsoon southwesterlies and induced enhanced low‐level divergence over Luzon Island. The westward extension of the WNPSH may be facilitated by the phase change of the boreal summer intraseasonal oscillation (BSISO). About 59% (23/39) of the monsoon break cases occurred when suppressed convection, associated with the dry phases of the BSISO, is apparent over the western North Pacific. This suppressed convection favours the westward expansion of the WNPSH.With the occurrence of the monsoon break in early summer, the seasonal march of the early summer monsoon over the Philippines can be divided into three phases: (1) the monsoon onset phase, which occurs between mid to late May under the influence of the westerly/southwesterly low‐level winds, (2) the monsoon break phase, when rainfall decreases over Luzon Island in early June, and (3) the monsoon revival phase, when rainfall increases again due to the intrusion of monsoon southwesterlies over the Philippines. This study highlights the complex features of the summer monsoon onset and the impact of the WNPSH on the local climate of the Philippines in early summer.

Locally forced convection in subkilometre‐scale simulations with the Unified Model and WRF

AbstractThis study evaluates the performance and benefits of kilometre‐ and subkilometre‐scale convection‐permitting simulations over tropical Australia. Focusing on an extended monsoon break period, we can directly compare Unified Model (UM) and Weather Research and Forecasting model (WRF) simulations to C‐band polarimetric radar observations and soundings. We show that the two models have different behaviour, and both are different to observations. Whereas WRF produces daily squall lines whether or not they occurred in observations, the UM primarily generates small but intense storms. The UM and WRF produce qualitatively different surface density currents at different times in the diurnal cycle. Once the density currents are present, the models also show different behaviour in relation to convective initiation. While higher resolution helps in the distribution of total precipitation over the domain, most characteristics do not change with higher resolutions, and model differences are always larger than resolution differences. While Convective Available Potential Energy/Convective Inhibition (CAPE/CIN) does not seem to be important to explain model differences, our findings point to the evolution of density currents in the boundary layer as the most important source of model errors and differences.

Convective Precipitation Efficiency Observed in the Tropics

AbstractPrecipitation efficiency refers to the fraction of condensate in the atmosphere that reaches the surface as precipitation. A high‐quality data set of radar‐estimated precipitation rates and convective scale vertical velocity near Darwin, Australia, is used to construct the first estimate of precipitation efficiency at convective scales for a long record of observations in the tropics. It is found that precipitation efficiency increases with precipitation rate and midtropospheric humidity and decreases with increasing convective available potential energy and surface temperature. Precipitation efficiency is largest under moist monsoonal conditions and smallest during monsoon break periods, which are characterized by a drier free troposphere. However, these differences in efficiency do not translate to differences in the instantaneous precipitation rate across the synoptic regimes because of a compensating change in the net condensation rate. This is driven by variations in cloud updraft velocity, which is larger in drier environments than in moist environments.

Scatterometer estimates of the tropical sea‐breeze circulation near Darwin, with comparison to regional models

In tropical coastal environments, simulating the diurnal cycle of wind and precipitation in numerical weather and climate models presents unique challenges due to the interaction of intraseasonal and mesoscale dynamics. This can lead not only to incorrect short‐term weather forecasts but also to unphysical energy and momentum transport by convective processes. In particular, the sea/land‐breeze circulation and its role in initiating convection has been identified as a possible source of errors in the timing and offshore extent of coastal precipitation in the Tropics.In this study, the offshore land/sea breeze around Darwin, Australia, is examined using scatterometer wind observations and two regional atmospheric models. Although the comparison is limited by satellite swath times which cluster around two times of day, useful results are obtained by sub‐sampling the simulated data to match the coverage of the scatterometer data.We find that offshore surface sea‐breeze characteristics (intensity and horizontal spatial extent) from the models and satellite estimates are generally in good agreement, with intensity differences less than 2 m s−1, and offshore extents not varying by more than approximately 150 km. The variation in offshore extent and amplitude of the land/sea‐breeze wind perturbations with monsoon regime is well simulated. Furthermore, despite the simplifying assumptions of linear sea‐breeze theory, the model and scatterometer results are in broad agreement with theoretical values, particularly in the presence of the weak background winds during the monsoon break period.

Observed short‐ and long‐term changes in summer precipitation over South Korea and their links to large‐scale circulation anomalies

ABSTRACTWe investigated short‐ and long‐term changes in total and extreme summer precipitation and the characteristic patterns of atmospheric circulation and sea surface temperature (SST) anomalies related to wet summers over South Korea. Over the last century (1913–2012), the total and extreme summer precipitation showed a significant increase of 2.62 and 4.45% decade−1, respectively. Because extreme summer precipitation increased more rapidly than total summer precipitation, the contribution of extreme precipitation to total precipitation increased significantly by 2.48% decade−1. A two‐peak rainy season was substantially intensified, with a slight shift and a large increase of precipitation during the monsoon break period, indicating a shortening of the monsoon break period. The 20‐year return values of daily maximum precipitation increased at all stations except Mokpo, showing changes in the frequency of extreme events. During the last 40 years (1973–2012), the pattern of change in the precipitation indices was spatially heterogeneous, with a particularly significant increasing trend in extreme summer precipitation over the North‐central portion of western South Korea. When compared with long‐term trends over 1912–2012, recent short‐term trends showed more rapid increases in all precipitation indices, with the exception of the number of consecutive dry days. In addition, the number of consecutive wet days (CWD) increased significantly during the last 40 years, whereas the long‐term trend over the last century for CWD was not significant. These regional trends were confirmed by field significance at the 90% confidence level. From the composite analysis results, the positive anomalous North Pacific subtropical high, intensification of the upper‐level westerly Jet over Korea and its vertical coupling with the anomalous strong low‐level westerlies, and anomalous warm SST values in the western North Pacific region were linked to recent increases in extreme summer precipitation.

Madden Julian Oscillations in Total Column Ozone, Air Temperature and Surface Pressure Measured over Cochin during Summer Monsoon 2015

The intra-seasonal variability plays a major role in the inter-annual variability of weather parameters such as rainfall, temperature and pressure which lead to extreme weather events in certain years. The active (more rainy days) and break (less rainy days) periods of Indian summer monsoon heavily depend on the intra-seasonal variability of weather parameters such as wind, pressure and temperature oscillations during the monsoon season. In the present analysis daily total column ozone, surface temperature and surface pressure measured over Cochin using Microtop II Ozonometer (sun Photometer) were used to study the Intra-Seasonal Variations (ISV) of the above parameters during the monsoon season, 2015. The dominant and significant intra-seasonal oscillations (ISOs) were identified using an advanced statistical method called the Discrete Mayer’s Wavelet (DMW) analysis. Two major ISOs such as Madden Julian Oscillations (MJO, 30 - 60 days) and quasi-bi weekly (12 - 16 days) oscillations were found in TCO, surface temperature and pressure. In TCO an additional mode of ISO with quasi tri-weekly periodicity was also found (16 - 22 day). It is observed that MJO mode is the dominant among all other modes and its positive and negative phases correlate with positive and negative anomalies of the above parameters. The ISO mode in the surface pressure shows an out of phase relation with the Indian summer monsoon rainfall which indicates the active and break periods of Indian summer monsoon. The contribution of MJO mode is dominant in the tropical atmosphere, which modulates the intra-seasonal variability. It is found that for the year 2015 total column ozone, surface pressure and surface temperature show an annual range of 30 DU, 4 hPa and 1°C, respectively.

Gravity waves generated by convection during TWP‐ICE: I. Inertia‐gravity waves

AbstractGravity waves are analyzed in radiosonde soundings taken during the Tropical Warm Pool‐International Cloud Experiment (TWP‐ICE) campaign. The properties of the inertia‐gravity waves are analyzed in Part I, whereas Part II focuses on high‐frequency gravity waves. Two groups of inertia‐gravity waves are detected: group L (Long vertical wavelength) in the middle stratosphere during the suppressed monsoon period and group S (Short vertical wavelength) in the lower stratosphere during the monsoon break period. Waves belonging to group L propagate to the southeast with a mean intrinsic period of 35 h and have vertical and horizontal wavelengths of about 5–6 km and 3000–6000 km, respectively. Ray‐tracing calculations indicate that these waves originate from a deep convective region near Indonesia. Waves belonging to group S propagate to the south‐southeast with an intrinsic period, vertical wavelength, and horizontal wavelength of about 45 h, 2 km, and 2000–4000 km, respectively. These waves appear to originate from convection in the vicinity of New Guinea.

Atmospheric moisture distribution and transport over the Tibetan Plateau and the impacts of the South Asian summer monsoon

In this study, by using the ECMWF ERA-Interim reanalysis data from 1979 to 2010, the spatial distribution and transport of total atmospheric moisture over the Tibetan Plateau (TP) are analyzed, together with the associated impacts of the South Asian summer monsoon (SASM). Acting as a moisture sink in summer, the TP has a net moisture flux of 2.59×107 kg s−1 during 1979–2010, with moisture supplies mainly from the southern boundary along the latitude belts over the Bay of Bengal and the Arabian Sea. The total atmospheric moisture over the TP exhibits significant differences in both spatial distribution and transport between the monsoon active and break periods and between strong and weak monsoon years. Large positive (negative) moisture anomalies occur over the southwest edge of the TP and the Arabian Sea, mainly due to transport of easterly (westerly) anomalies during the monsoon active (break) period. For the whole TP region, the total moisture supply is more strengthened than the climatological mean during the monsoon active period, which is mainly contributed by the transport of moisture from the south edge of the TP. During the monsoon break period, however, the total moisture supply to the TP is slightly weakened. In addition, the TP moisture sink is also strengthened (weakened) in the strong (weak) monsoon years, mainly attributed by the moisture transport in the west-east directions. Our results suggest that the SASM has exerted great impacts on the total atmospheric moisture and its transport over the TP through adjusting the moisture spatial distribution.

Lightning distribution with respect to the monsoon trough position during the Indian summer monsoon season

The location and shift in the position of the maximum lightning activity with respect to the position the of monsoon trough has been investigated in the region of 20°N–32°N and 75°E–95°E during the Indian summer monsoon seasons of 2002 and 2003 from the lightning imaging sensor (LIS) data from the Tropical Rainfall Measuring Mission (TRMM) satellite. The polynomial curve fitting the maximum lightning activity followed the position of the monsoon trough in both the normal monsoon and drought years. Observations of the maximal lightning activity in the TRMM‐LIS data collected during a period of 13 years, from 1998 to 2010, confirm these findings in the monsoon season. When the seasonal position of the monsoon trough moved from the plains to the submontane region of the Himalayas during a monsoon break period, the polynomial fitting of the maximum lightning activity also moved from the central Indian plains to that region. An empirical orthogonal function (EOF) analysis is applied to two different data sets of precipitation and observed high lightning flash rates over the monsoon trough region for the period of 1995–2005. The first and second components, EOF1 and EOF2, of precipitation values are out of phase with those of the flash rates. Our results imply that deep electrified convective systems during monsoon periods are responsible for the lightning activity in the monsoon trough region and that the lightning activity in this region can be used as a proxy for the location and shift of the seasonal monsoon trough.

CloudSat–CALIPSO characterizations of cloud during the active and the break periods of Indian summer monsoon

Here we report cloud macrophysical (cloud top height (CTH), cloud base height (CBH), cloud geometrical depth), microphysical (liquid and ice water contents, water paths, effective radii and number concentration) and radiative (heating rate) properties over the North Central India (18–28°N, 65–88°E) region (core monsoon zone) during the active and the break periods (2006–2010) of the Indian summer monsoon (ISM). Synergetic data from the CloudSat radar and the CALIPSO lidar are used. Analysis shows that the CTH and CBH have bimodal distributions during both the phases of monsoon. We have classified clouds into four type's viz., high-, mid-, low- and vertically extended deep clouds. The low-level clouds and optically thick cirrus are more dominant of the total observations and they occur more frequently during the active period compared to that of the break period. In contrast, the high-level clouds and optically thin cirrus are more frequent during the break phase of monsoon. The integrated depolarization ratio of high-level cloud exhibits bimodal distribution. It is observed that there is a significant variation in macrophysical, microphysical, optical and radiative properties of all the four types of clouds during the active and the break periods. As little observational evidence exists in the vertical structure of clouds during the active and the break periods of the ISM, the current results would be useful in understanding the characteristics of monsoon clouds, which have implications in the Earth's radiation budget and global climate models.

Comparison of Rainfall Characteristics and Convective Properties of Monsoon Precipitation Systems over South China and the Yangtze and Huai River Basin

AbstractRainfall characteristics and convective properties of monsoon precipitation systems over South China (SC) and the Yangtze and Huai River basin (YHRB) are investigated using multiple satellite products, surface rainfall observations, NCEP reanalysis, and weather maps. Comparisons between SC and YHRB are made for their monsoon active periods and their subseasonal variations from the premonsoon to monsoon and further to postmonsoon periods. The principal findings are as follows. (i) During the monsoon active period, region-averaged rain accumulation is greater in SC due to more frequent occurrence of precipitation systems; however, heavy rainfall contribution is greater in YHRB. These differences are related to more intense convective motion over the YHRB in association with the flatter land and more concurrent presence and stronger intensity of the low-level vortices and surface fronts. (ii) Largely in agreement with the subseasonal variations of the atmospheric thermodynamic conditions, convective intensity is enhanced progressively from the premonsoon to the monsoon and further to the postmonsoon period in both regions, as suggested by most convection proxies, except for lightning flash rate, which decreases substantially over SC but increases slightly over the YHRB from the premonsoon to the monsoon period. (iii) Compared to the monsoon active period, precipitation storms in both regions during the postmonsoon and monsoon break periods are more controlled by local instability due to solar heating but less controlled by larger-scale weather systems. This scale difference in the driving mechanisms leads to the smaller horizontal extent of the precipitation systems during the postmonsoon and monsoon break periods and also to the more pronounced afternoon peaks in precipitation system occurrence in the postmonsoon period.

A climatological perspective of deep convection penetrating the TTL during the Indian summer monsoon from the AVHRR and MODIS instruments

Abstract. The impact of very deep convection on the water budget and thermal structure of the tropical tropopause layer is still not well quantified, not least because of limitations imposed by the available observation techniques. Here, we present detailed analysis of the climatology of the cloud top brightness temperatures as indicators of deep convection during the Indian summer monsoon, and the variations therein due to active and break periods. We make use of the recently newly processed data from the Advanced Very High Resolution Radiometer (AVHRR) at a nominal spatial resolution of 4 km. Using temperature thresholds from the Atmospheric Infrared Sounder (AIRS), the AVHRR brightness temperatures are converted to climatological mean (2003–2008) maps of cloud amounts at 200, 150 and 100 hPa. Further, we relate the brightness temperatures to the level of zero radiative heating, which may allow a coarse identification of convective detrainment that will subsequently ascend into the stratosphere. The AVHRR data for the period 1982–2006 are used to document the differences in deep convection between active and break conditions of the monsoon. The analysis of AVHRR data is complemented with cloud top pressure and optical depth statistics (for the period 2003–2008) from the Moderate Resolution Imaging Spectroradiometer (MODIS) onboard Aqua satellite. Generally, the two sensors provide a very similar description of deep convective clouds. Our analysis shows that most of the deep convection occurs over the Bay of Bengal and central northeast India. Very deep convection over the Tibetan plateau is comparatively weak, and may play only a secondary role in troposphere-to-stratosphere transport. The deep convection over the Indian monsoon region is most frequent in July/August, but the very highest convection (coldest tops, penetrating well into the TTL) occurs in May/June. Large variability in convection reaching the TTL is due to monsoon break/active periods. During the monsoon break period, deep convection reaching the TTL is almost entirely absent in the western part of the study area (i.e. 60 E–75 E), while the distribution over the Bay of Bengal and the Tibetan Plateau is less affected. Although the active conditions occur less frequently than the break conditions, they may have a larger bearing on the composition of the TTL within the monsoonal anticyclone, and tracer transport into the stratosphere because of deep convection occurring over anthropogenically more polluted regions.

The Role of Entrainment in the Diurnal Cycle of Continental Convection

Abstract In continental convective environments, general circulation models typically produce a diurnal cycle of rainfall that peaks close to the noon maximum of insolation, hours earlier than the observed peak. One possible reason is insufficient sensitivity of their cumulus parameterizations to the state of the environment due to weak entrainment. The Weather Research and Forecasting (WRF) model, run at cloud-resolving (600 and 125 m) resolution, is used to study the diurnal transition from shallow to deep convection during the monsoon break period of the Tropical Warm Pool–International Cloud Experiment. The WRF model develops a transition from shallow to deep convection in isolated events by 1430–1500 local time. The inferred entrainment rate weakens with increasing time of day as convection deepens. Several current cumulus parameterizations are tested for their ability to reproduce the WRF behavior. The Gregory parameterization, in which entrainment rate varies directly with parcel buoyancy and inversely as the square of the updraft speed, is the best predictor of the inferred WRF entrainment profiles. The Gregory scheme depends on a free parameter that represents the fraction of buoyant turbulent kinetic energy generation on the cloud scale that is consumed by the turbulent entrainment process at smaller scales. A single vertical profile of this free parameter, increasing with height above the boundary layer but constant with varying convection depth, produces entrainment rate profiles consistent with those inferred from the WRF over the buoyant depth of the convection. Parameterizations in which entrainment varies inversely with altitude or updraft speed or increases with decreasing tropospheric relative humidity do not perform as well. Entrainment rate at cloud base decreases as convection depth increases; this behavior appears to be related to an increase in vertical velocity at downdraft cold pool edges.

Relationship between Lightning Activity over Peninsular India and Sea Surface Temperature

AbstractThis paper presents a study of spatiotemporal variation of lightning activity over Peninsular India (8°–22°N, 72°–88°E) by using monthly satellite-based lightning flash grid (1° × 1°) data for a period of 10 yr (1998–2007). The data are examined in terms of spatial, annual, and seasonal distribution of the lightning activity. It is found that lightning activity is higher over south Peninsular India and eastern India. On a seasonal time scale, the lightning activity shows two maxima—first in the month of May and then in the month of September. The lightning activity in the monsoon period is noticed to be considerable because of the occurrence of the low-level jet and increase in the monsoon break period. During the postmonsoon, the activity is mainly due to the presence of the convective nature of the disturbed weather during the northeast monsoon season over most parts of the east coast of south Peninsular India. The relationship between lightning activity over Peninsular India and sea surface temperature in the bordering seas (Arabian Sea and Bay of Bengal) is also examined. The results disclose a significant link between them.

WRF and GISS SCM simulations of convective updraft properties during TWP‐ICE

The Weather Research and Forecasting (WRF) model, running at cloud‐resolving model resolution (1.3 km and 0.6 km), is used to simulate cumulus updraft speeds associated with three distinct convective regimes sampled during the intensive observing period of the Tropical Warm Pool–International Cloud Experiment (TWP‐ICE) near Darwin, Australia. The WRF model produces strong updrafts during a monsoon break period and weaker updrafts during an active monsoon period, consistent with observational proxies of convective strength. It also captures the observed feature of midlevel convection during a suppressed monsoon period. The ability of the WRF model to differentiate the updraft speeds among three subperiods is robust to changes in its microphysics and turbulence schemes, resolution, and forcing procedure. For comparison to the parameterized diagnostic updraft speeds in the Goddard Institute for Space Studies Single Column Model (GISS SCM), we define an equivalent mean updraft speed for deep convection in the WRF simulation as the ratio of the domain average upward flux of hydrometeors to the domain average hydrometeor water content. Parameterized convective updraft speeds diagnosed from the thermodynamic structure in the SCM can reproduce the WRF difference between the active and break period updraft strength and the shallower suppressed monsoon convection, but only if a free parameter that regulates entrainment strength is allowed to vary. SCM updraft speeds are consistently too strong in the upper troposphere compared with the WRF. Hydrometeor profiles in both the WRF and the SCM are sensitive to assumptions about the ice phase microphysics.

Aerosol and thermodynamic effects on tropical cloud systems during TWPICE and ACTIVE

Abstract. Regularly occurring storms over the Tiwi Islands, north of Darwin, Australia are used as a laboratory for investigating the relative importance of thermodynamic parameters, shear and aerosols on the amount and intensity of convection over the islands during the pre-monsoon and monsoon break periods of the 2005–2006 summer wet season. Storm systems on individual days are characterised by simple metrics derived from polarimetric radar data. The analysis shows clear dependencies on thermodynamic and shear parameters. The shear dependence was unexpected, as high shear implied less activity, but this is likely an island effect. There are some indications of a dependence of storm intensity on aerosol, but mid-level moisture differences may also play a role.

Suppression of south Asian summer monsoon precipitation in the 21st century

We used a high‐resolution nested climate modeling system to investigate the response of South Asian summer monsoon dynamics to anthropogenic increases in greenhouse gas concentrations. The simulated dynamical features of the summer monsoon compared well with reanalysis data and observations. Further, we found that enhanced greenhouse forcing resulted in overall suppression of summer precipitation, a delay in monsoon onset, and an increase in the occurrence of monsoon break periods. Weakening of the large‐scale monsoon flow and suppression of the dominant intraseasonal oscillatory modes were instrumental in the overall weakening of the South Asian summer monsoon. Such changes in monsoon dynamics could have substantial impacts by decreasing summer precipitation in key areas of South Asia.