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The Indian mesosphere–stratosphere–troposphere (MST) radar observations during the passage of 37 convective systems are utilized to investigate the characteristics of vertical air velocity w in different convection categories (shallow, deep, and decaying) and also the differences in draft core statistics from the wet to dry spell. The radar and optical rain gauge measurements show pronounced differences in core statistics (in terms of their vertical structure, draft strength, size, number, and the elevation angle) and surface rainfall characteristics from the dry to wet spell. The shallow convective cores are preponderant in the dry spell. Composite w profiles, retrieved from all deep cases and also from individual convection cases, depict an upper-tropospheric peak in the wet spell and a bimodal distribution (peaks at 5 and 11–13 km) in the dry spell, illustrating that they are characteristic features of wet and dry spells. The average vertical extents of the cores are nearly equal (about 8 km) in both spells of the monsoon; however, the core-base (and top) altitudes are different. In both wet and dry spells, the composite w profile for all cores show similar vertical variation to that of for updraft cores, while the composite w for downdraft cores do not show much variation with altitude, indicating that the updraft cores dictate the vertical structure of composite w. The core size varies considerably (a factor of 2) with altitude in both spells of the monsoon. Although nearly equal in the lower troposphere in both phases of the monsoon, the core size is larger by 1–2 km in the dry spell in the middle and upper troposphere. Consistent with the longer lifetime (bigger core size) of cores in the dry spell, the cores are more inclined (with a mean elevation angle of 30°) in the dry spell. The surface rainfall distribution is wider and has large number of intense rainfall rates in the wet spell. The mean rainfall rate for the wet spell is also larger by a factor of 2, consistent with earlier studies. [ABSTRACT FROM AUTHOR]

Summer precipitation over the Western Ghats and its adjacent Arabian Sea is an important component of the Indian monsoon. To advance understanding of the physical processes controlling this regional precipitation, a series of high-resolution convection-permitting simulations were conducted using the Weather Research and Forecasting (WRF) Model. Convection simulated in the WRF Model agrees with TRMM and MODIS satellite estimates. Sensitivity simulations are conducted, by altering topography, latent heating, and sea surface temperature (SST), to quantify the effects of different physical forcing factors. It is helpful to put India's west coast rainfall systems into three categories with different causes and characteristics. 1) Offshore rainfall is controlled by incoming convective available potential energy (CAPE), the entrainment of midtropospheric dry layer in the monsoon westerlies, and the latent heat flux and SST of the Arabian Sea. It is not triggered by the Western Ghats. When offshore convection is present, it reduces both CAPE and the downwind coastal rainfall. Strong (weak) offshore rainfall is associated with high (low) SSTs in the Arabian Sea, suggested by both observations and sensitivity simulations. 2) Coastal convective rainfall is forced by the coastline roughness, diurnal heating, and the Western Ghats topography. This localized convective rainfall ends abruptly beyond the Western Ghats, producing a rain shadow to the east of the mountains. This deep convection with mixed phase microphysics is the biggest overall rain producer. 3) Orographic stratiform warm rain and drizzle dominate the local precipitation on the crest of the Western Ghats. [ABSTRACT FROM AUTHOR]

Conditions producing disastrous flooding in Uttarakhand, India, in June 2013 differed from conditions that produced other notorious floods in the Himalayan region in recent years. During the week preceding the Uttarakhand flood, deep convection moistened the mountainsides, making them vulnerable to flooding. However, the precipitation producing the flood was not associated with a deep convective event. Rather, an eastward-propagating upper-level trough in the westerlies extended abnormally far southward, with the jet reaching the Himalayas. The south end of the trough merged with a monsoon low moving westward across India. The merged system produced persistent moist low-level flow oriented normal to the Himalayas that advected large amounts of water vapor into the Uttarakhand region. The flow was moist neutral when it passed over the Himalayan barrier, and orographic lifting produced heavy continuous rain over the region for 2-3 days. The precipitation was largely stratiform in nature although embedded convection of moderate depth occurred along the foothills, where some mild instability was being released. The Uttarakhand flood had characteristics in common with major 2013 floods in the Rocky Mountains in Colorado and Alberta, Canada. [ABSTRACT FROM AUTHOR]

Climatological characteristics of precipitation during the active and break spells of the monsoon are studied using 15 years of TRMM measurements. The spatial variation of rain fraction suggests that most of the seasonal rainfall occurs in spells of active monsoon over India, except for the zones along the east coast. The broader reflectivity distribution at higher altitudes and larger average storm height during active spells indicate the high prevalence of deep systems during this spell. The spatial distribution of the occurrence and fraction of different types of rain exhibits large variability from land to ocean and between the spells. The higher occurrence and fraction of stratiform rain during the active spell, particularly over the core monsoon zone, is due to the prevalence of organized mesoscale systems with large stratiform portions. The break spells are characterized by higher occurrence of shallow rain and larger fraction of convective rain. While an evening peak is observed over land during the break spell, the phase of the diurnal cycle exhibits large spatial variability during the active spell. The rainfall peaks from late night to midnight in southeastern India and in the morning near the foothills of the Himalayas during the active spell. The diurnal and semidiurnal components together explain more than 90% of total variance over many of the zones during both spells. The observed differences in precipitation between the spells are discussed in light of the differences in synoptic- and mesoscale mechanisms responsible for the production of precipitation. [ABSTRACT FROM AUTHOR]

Indian summer monsoon rainfall was examined in two different greenhouse gas emission scenarios: the Special Report on Emissions Scenarios (SRES; B1) and a similar greenhouse gas scenario, the new representative concentration pathways (RCPs; RCP4.5). The rainfall change in the climate model projections through remotely induced changes in precipitation processes and through changes in precipitation efficiency processes was discussed. To that end, two model setups were applied: 1) the regional climate model (RCM) Consortium for Small-Scale Modelling in Climate Mode (COSMO-CLM), nested in the global climate model (GCM) ECHAM5/Max Planck Institute ocean model (ECHAM5/MPIOM), applying the greenhouse gas scenario B1; and 2) the RCM nested in a newer version of the GCM, ECHAM6/MPIOM, incorporating the RCP4.5 scenario. Both GCM simulations showed a slight increase in precipitation over central India toward the end of the twenty-first century. This slight increase was the result of two largely compensating changes: increase of remotely induced precipitation and decrease of precipitation efficiency. The RCM with the scenario RCP4.5 followed this trend, but with smaller changes. However, the RCM with B1 showed a decreasing trend in precipitation because of a slightly larger absolute change of the reduced precipitation efficiency compared to the change caused by the remote processes. Changes of these processes in the scenario simulations were larger than the natural variability, as simulated in an unperturbed preindustrial greenhouse gas control (CTL) climate simulation. Results indicated that the projection of the Indian summer monsoon rainfall is still a key challenge for both the GCM and the RCM. [ABSTRACT FROM AUTHOR]

The upgraded version 7 (V7) of the Tropical Rainfall Measuring Mission (TRMM) Multisatellite Precipitation Analysis (TMPA) products is available to the user community. In this paper, two successive versions of the TMPA-3B42 research monitoring product, version 6 (V6) and V7, at the daily scale are evaluated over India during the southwest monsoon with gauge-based data for a 13-yr (1998-2010) period. Over typical monsoon rainfall zones, biases are improved by 5%-10% in V7 over the regions of higher rainfall like the west coast, northeastern, and central India. A similar reduced bias is seen in V7 over the rain-shadow region located in southeastern India. In terms of correlation, anomaly correlation, and RMSE, a marginal improvement is seen in V7. Additionally, in all-India summer monsoon rainfall amounts, mean, interannual values, and standard deviation show an overall improvement in V7. Different skill metrics over typical subregions within India show an improvement of the monsoon rainfall representation in V7. Rainfall frequency in different categories also indicates an overall improvement in V7 across all scales and subregions. Over central India regions associated with the monsoon transients, the sign of the bias has changed toward a positive bias. Even if the bias in the frequency of the occurrence of light rain has improved in V7, the values still show a large difference compared to observations. Though both V6 and V7 are able to represent the anomalous dry/wet regions during contrasting monsoon years, V7 shows some improvement in amplitude of those anomalies over V6. In general, V7 has considerably improved over V6 and will continue to be in demand from various sectors of observed rainfall data users. [ABSTRACT FROM AUTHOR]

Eight years of Tropical Rainfall Measuring Mission (TRMM) Precipitation Radar (PR) data show how convective systems of different types contribute to precipitation of the South Asian monsoon. The main factor determining the amount of precipitation coming from a specific system is its horizontal size. Convective intensity and/or number of embedded convective cells further enhance its precipitation production. The precipitation of the monsoon is concentrated in three mountainous regions: the Himalayas and coastal ranges of western India and Myanmar. Along the western Himalayas, precipitation falls mainly from small, but highly convective systems. Farther east along the foothills, systems are more stratiform. These small and medium systems form during the day, as the monsoon flow is forced upslope. Nighttime cooling leads to downslope flow and triggers medium-sized systems at lower elevations. At the mountainous western coasts of India and Myanmar, small and medium systems are present throughout the day, as an orographic response to the southwesterly flow, with a slight superimposed diurnal cycle. Medium systems are favored over the eastern parts of the Arabian Sea and large systems are favored over the Bay of Bengal when an enhanced midlevel cyclonic circulation occurs over the northern parts of these regions. The systems forming upstream of coastal mountains over the Bay of Bengal are larger than those over the Arabian Sea, probably because of the moister conditions over the bay. The large systems over the bay exhibit a pronounced diurnal cycle, with systems forming near midnight and maximizing in midday. [ABSTRACT FROM AUTHOR]

The Indian mesosphere–stratosphere–troposphere (MST) radar measurements during the passage of 60 convective systems are used to study the vertical air velocity (w) characteristics of tropical convection. The upand downdraft cores and various stages/types of convection (shallow, deep, and decaying) are discerned from radar time–intensity maps of w. The characteristics of cores (speed, size, orientation, vertical extent, gravity wave activity, etc.) at different stages of convection are discussed with the help of three case studies. The cores stratified based on the type of convection are mostly erect in nature in all types of convective systems, except for deep updraft cores. A considerable percentage (35%) of deep updraft cores show inclined structure with elevation angles as low as 0°–20°. The variation of the horizontal wind field with height and the internal dynamics of mesoscale convective systems (MCSs) are thought to be responsible for this geometry. Further, the vertical extent of draft cores is limited in all types of convection, except for deep updraft cores. About 77% of deep updraft cores have a vertical extent greater than 10 km and ∼23% of these cores reach an altitude of 16 km. The size (overpass time) of the core shows an increasing trend with altitude up to 10–12 km and then decreases. Among different types of convection, the size of core is larger for deep updraft cores and smaller for shallow updraft cores. The variation of w distribution with height is different for different convection categories. The mode (and also the mean) of the distribution shows low-level descent (below 3 km) and mid–high-level ascent in shallow and deep convection categories, while nearly uniform distribution is seen in decaying convection. Strong updrafts are seen in deep convective systems in the upper troposphere (of the order of 15–20m s-1), followed by shallow and decaying systems, while downdrafts are generally weaker in all types of convection. The variability (within the cores and also with altitude) and the number of data points are larger in updraft cores than in downdraft cores corresponding to shallow and deep convection. Contrasting the composite w profile at Gadanki with those obtained elsewhere revealed interesting features: the absence of subsidence at higher levels, the presence of low-level subsidence, a single ascent peak in the middle troposphere, etc. Further, the magnitude of composite w derived from wind profiler measurements is larger than that obtained with other techniques. [ABSTRACT FROM AUTHOR]

Raindrop size distribution (DSD) parameters are retrieved from dual-frequency (UHF and VHF) wind profiler measurements made at Gadanki, India, in a summer monsoon season. The convoluted UHF spectra are first corrected for vertical air motion and spectral broadening (using VHF measurements) and later are used for deriving DSD parameters. Two distinctly different case studies, a mesoscale convective system and a pure stratiform precipitation system, have been considered for a detailed study. DSD parameters obtained in these case studies reveal systematic variations of DSD from case to case and also from one rain regime to another within the same precipitating system. A statistical study has been carried out using the profiler data collected during the passage of 16 rain events. The retrieved DSD profiles are divided into separate rain regimes (stratiform and convection), based on reflectivity, to examine salient microphysical characteristics and the vertical variability of DSD in different precipitation regimes. The distribution of DSD parameters is, in general, wider in the convective rain regime than in the stratiform regime, particularly below 2.4 km. The vertical variation of the gamma parameter distribution in the stratiform rain regime is minimal, indicating that the microphysical processes (growth and decay), which alter the rain DSD, may be in equilibrium. On the other hand, the distribution in the convective rain regime appears to be more complex, with the mean profile of the shape parameter varying significantly with height. The observed vertical variability of the gamma parameters and the median volume diameter in the convective rain regime is attributed to two major microphysical processes: evaporation and breakup. The role of other processes, like drop sorting and collision–coalescence, in altering the DSD parameters is also discussed. The present statistics, representing continental monsoon rainfall, are compared with the existing statistics at Darwin, Australia, and the results are discussed in light of DSD differences in oceanic and continental monsoon precipitation. [ABSTRACT FROM AUTHOR]

Most attempts at predicting south Asian monsoon variability have concentrated on seasonally averaged rainfall over the Indian subcontinent some months in advance using regional and remote boundary effects as predictors. Overall, about 30% of the variance of mean seasonal monsoon rainfall can be explained, but the statistics appear to be nonstationary and correlations vary strongly on interdecadal time scales. Model intercomparisons show that climate models have difficulty in simulating even gross-scale features of the monsoon such as mean summer rainfall, and there is little demonstrated skill when the models are used in predictive mode. Even if the statistics were stable and model predictions were skillful it is argued that the information is not readily downscalable because the mean rainfall does not define the timing or number of intraseasonal variations or even the spatial distributions of the seasonal mean rainfall. Based on these concerns, it is argued that skillful and timely forecasts of intraseasonal variability possess a greater potential utility for agriculture and water resource management and should be the highest priority for prediction within the monsoon regions. A physically based empirical Bayesian prediction scheme is developed for forecasting regional intraseasonal variability of the monsoon. Ten predictors are chosen that depict the morphology of the monsoon intraseasonal mode. The scheme employs a wavelet-banding technique and linear regression to forecast 5-day average rainfall variability over regions of south Asia 15–30 days (i.e., six 5-day lags) in the future. Hindcasts conducted for the central Indian region for the period 1992–2002 show considerable skill out to 30 days in both the timing and amplitude of the intraseasonal oscillations. Skill, albeit reduced, is also found in smaller regions such as the Indian states of Rajasthan and Orissa. The use of wavelet analysis to sort time series and isolate each band from the noise generated in other bands, together with the careful choice of predictors, are the defining elements of the scheme. Anomaly correlations of rainfall in the 28–80-day band in central India are 0.88, 0.76, 0.73, 0.66, and 0.58 for 10, 15, 20, 25, and 30 days, respectively. Similar skill is found for forecasting the discharge of the Ganges and Brahmaputra into Bangladesh. The potential utility of these forecasts for applications in agriculture and water resource management is discussed together with the possible use of the empirical scheme as a diagnostic tool and as a guide for the development of a new type of numerical model. [ABSTRACT FROM AUTHOR]

ABSTRACT The ensemble of seasonal (120 days) simulations of the Northern Hemispheric summer for the reanalysis period of the European Centre for Medium-Range Weather Forecasts has been examined to assess the extent to which the characteristic features of the Indian summer monsoon can be reproduced in these simulations. The present simulations could reproduce a better distribution of the seasonal average precipitation over India in comparison with the earlier Atmospheric Model Intercomparison Project simulations. The interannual variation in the seasonal total of the spatially averaged precipitation over India has predictability. The 10-day-average precipitation values did not show any impact of the El Nino or La Nina events or any periodicity in the amount of precipitation. The intraseasonal variability did not produce any distinct pattern for 10-day-average rainfall during the excess or deficient years. The simulated patterns over India correspond to the weak phase of summer monsoon with excess precipitation over the northeastern part of the country and adjoining China. The cloud cover is less over the central parts of India and near-ground maximum temperature is higher. A simulated motion field reproduces the typical features of the Indian summer monsoon including the low-frequency seasonal migration of the Tibetan anticyclone at 200 hPa. [ABSTRACT FROM AUTHOR]

The monsoon depressions that form over India during the summer are analyzed using simulations from the Laboratoire de Météorologie Dynamique general circulation model. This type of synoptic system often occurs with a frequency of one to two per month and can produce a strong Indian rainfall. Two kinds of analyses are conducted in this study. The first one is a subjective analysis based on the evolution of the precipitation rate and the pattern of the sea level pressure. The second one is an objective analysis performed using the TRACK program, which identifies and tracks the minima in the sea level pressure anomaly field and computes the statistics for the distribution of systems. The analysis of a 9-yr control run, which simulates strong precipitation rates over the foothills of the Himalayas and over southern India but weak rates over central India, shows that the number of disturbances is too low and that they almost never occur during August, when break conditions prevail. The generated disturbances more often move north, toward the foothills of the Himalayas. Another analysis is performed to study the effect of the Tibetan Plateau elevation on these disturbances with a 9-yr run carried out with a Tibetan Plateau at 50% of its current height. It is shown that this later integration simulates more frequent monsoon disturbances, which move rather northwestward, in agreement with the current observations. The comparison between the two runs shows that the June–July–August rainfall difference is in large part due to changes in the occurrence of the monsoon disturbances. [ABSTRACT FROM AUTHOR]

The Indian summer monsoon rainfall is the net result of an ensemble of synoptic disturbances, many of which are extremely intense. Sporadic systems often bring extreme amounts of rain over only a few days, which can have sizable impacts on the estimated seasonal mean rainfall. The statistics of these outlier events are presented both for observed and model-simulated daily rainfall for the summers of 1986 to 1989. The extreme events cause the wet-day probability distribution of daily rainfall to be far from Gaussian, especially along the coastal regions of eastern and northwestern India. The gamma and Weibull distributions provide good fits to the wet-day rainfall distribution, whereas the lognormal distribution is too skewed. The impact of extreme events on estimates of space and time averages can be reduced by nonlinearly transforming the daily rainfall amounts. The square root transformation is shown to improve the predictability of ensemble forecasts of the mean Indian rainfall for June 1986-89. [ABSTRACT FROM AUTHOR]