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Diurnal variation of convective storms (CSs) during monsoon season and associated physical mechanisms are significantly important for accurate forecast of short-time and extreme precipitation. The diurnal cycle of CSs is investigated using ground-based X-band radar, Tropical Rainfall Measuring Mission Precipitation Radar, and reanalysis data during the summer monsoon (June–September of 2014) over complex mountain terrain of Western Ghats, India. Diurnally, CSs show a bimodal distribution in the coastal areas, but this bimodality became weak along the upslope regions and on the mountain top. The first occurrence mode of CSs is in the afternoon–evening hours, while the second peak is in the early-morning hours. The diurnal cycle’s intensity varies with location, such that it reaches maximum in the afternoon–evening hours and early morning on the mountain top and coastal areas, respectively. Two possible mechanisms are proposed for the observed diurnal variation in CSs (a) the radiative cooling effect and (b) the surface wind convergence induced by the interaction between land-sea breeze, local topography and large-scale monsoon winds. It is also observed that the CSs developed on the mountain top during afternoon–evening hours are deeper than those along the coast. The higher moisture in the lower- and mid-troposphere, higher instability and strong upward motion facilitate deeper CSs during afternoon–evening hours.

The Indo-Gangetic plains (IGP), which hosts 1/7th of the world population, has undergone significant anomalous changes in hydrological cycle in recent decades. In present study, the role of aerosols in the precipitation changes over IGP region is investigated using Coupled Model Inter-comparison Project-5 (CMIP5) experiments with adequate representation of aerosols in state-of-the art climate models. The climatological sea surface temperature experiments are used to explore the relative impact of the aerosols. The diagnostic analysis on representation of aerosols and precipitation over Indian region was investigated in CMIP5 models. After the evaluation, multi-model ensemble was used for further analysis. It is revealed from the analysis that aerosol-forcing plays an important role in observed weakening of the monsoon circulation and decreased precipitation over the IGP region. The significant cooling of the continental Indian region (mainly IGP) caused by the aerosols leads to reduction in land sea temperature contrast, which further leads to weakening of monsoon overturning circulation and reduction in precipitation.

Analysis of the microphysical structure of deep convective clouds using in situ measurements during the Cloud Aerosol Interaction and Precipitation Enhancement Experiment (CAIPEEX) over the Indian peninsular region is presented. It is shown that droplet size distributions (DSDs) in highly polluted premonsoon clouds are substantially narrower than DSDs in less polluted monsoon clouds. High values of DSD dispersion (0.3–0.6) and its vertical variation in the transient and monsoon clouds are related largely to the existence of small cloud droplets with diameters less than 10 μm, which were found at nearly all levels. This finding indicates the existence of a continuous generation of the smallest droplets at different heights. In some cases this generation of small droplets leads to the formation of bimodal and even multimodal DSDs. The formation of bimodal DSDs is especially pronounced in monsoon clouds. Observational evidence is presented to suggest that in-cloud nucleation at elevated layers is a fundamental mechanism for producing multimodal drop size distribution in monsoon clouds as well as in most deep convective clouds. These findings indicate that inclusion of continued nucleation away from the cloud base into numerical models should be considered to predict microphysics and precipitation of clouds in monsoons and other cloud-related phenomena.

Assessment of Sea Salt (SS) and Non-Sea Salt (NSS) aerosols in rainwater is important to understand the characterization of marine and continental aerosols and their source pathways. Sea salt quantification based on standard seawater ratios are primarily constrained with high uncertainty with its own limitations. Here, by the novelty of k -mean clustering and Positive Matrix Factorization (PMF) analysis, we segregate the air masses into two distinct clusters (oceanic and continental) during summer monsoon period signifying the complex intermingle of sources that act concomitantly. The rainwater composition during strong south-westerly wind regimes (cluster 2-oceanic) was profoundly linked with high sea salt and dust, whereas north-westerly low wind regimes (cluster 1-continental) showed an increase in SO 4 2− and NO 3 − . However, SO 4 2− abundance over NO 3 − in rain-water depicted its importance as a major acidifying ion at the region. The satellite-based observations indicate the presence of mid-tropospheric dust at the top (3–5 km) and marine sea salt at bottom acts as a “sandwich effect” for maritime clouds that leads to elevated Ca 2+ , Na + , Mg 2+ , and Cl − in rainwater. This characteristic feature is unique as sea spray generation due to high surface winds and dust aloft is only seen during this period. Furthermore, four source factors (secondary inorganic aerosol, mixed dust & sea salt, biomass burning & fertilizer use, and calcium neutralization) derived from PMF analysis showed contribution from local activities as well as long-range transport as dominant sources for the rainwater species.

Tropospheric Biennial Oscillation (TBO) is characterized by a tendency for a relatively stronger monsoon to be followed by a relatively weaker one (positive) or vice-versa (negative). This study examines the distribution of different convective systems occurring during TBO phases over the Indian monsoon region. During negative TBO phase, convection is preferential over the Arabian Sea (AS), whereas during positive TBO phase, it is favoured over the land areas and Bay of Bengal (BoB). The isolated shallow convection (ISC) is dominated over the AS and Indian west coast during negative TBO years. A relatively stable environment (statically) capped with drier mid-troposphere results in abundant ISC over the AS. Broad stratiform rain (BSR) dominates over the central and east coast of India, BoB and Myanmar coast during positive TBO years and wide convective core (WCC) are present along the orographic regions, i.e., Myanmar coast and Western Ghats during negative TBO phase. The anomalous easterlies induced by the upper-ocean temperature gradient interact with the mean monsoon winds during positive TBO to provide pathways for developing BSR echoes. The deep-wide convection (DWC) are higher along the Himalayan foothills during positive TBO years. The moist low-level flow from the AS is trapped by dry mid-level flow from high latitudes, resulting in orographic lifting along the Himalayan foothills and form DWC.

In situ aircraft measurements of cloud microphysical properties and aerosol during the 1st phase of the Cloud Aerosol Interaction and Precipitation Enhancement EXperiment (CAIPEEX‐I) over the Indian sub‐continent provided initial opportunities to investigate the dispersion effect and its implications for estimating aerosol indirect effects in continental cumuli. In contrast to earlier studies on continental shallow cumuli, it is found that not only the cloud droplet number concentration but also the relative dispersion increases with the aerosol number concentration in continental cumuli. The first aerosol indirect effect estimated from the relative changes in droplet concentration and effective radius with aerosol number concentration are 0.13 and 0.07, respectively. In‐depth analysis reveals that the dispersion effect could offset the cooling by enhanced droplet concentration by 39% in these continental cumuli. Adiabaticity analysis revealed aerosol indirect effect is lesser in subadiabatic clouds possibly due to inhomogeneous mixing processes. This study shows that adequate representation of the dispersion effect would help in accurately estimating the cloud albedo effect for continental cumuli and can reduce uncertainty in aerosol indirect effect estimates. Key Points Aerosols increase the relative dispersion of continental cumuli Droplet dispersion offsets the cooling up to 39% in continental cumuli AIE is lesser in sub‐adiabatic clouds due to inhomogeneous mixing

Relationship between the Southern Annular Mode (SAM) and the India summer monsoon rainfall (ISMR) has been examined based on the data period 1949–2013. While the entire data period indicates a significant increasing trend in SAM, recent decades 1983–2013 indicate no trend. The relationship between the two strengthened considerably since 1983. Results reveal that the February–March SAM is significantly related with the subsequent ISMR. A positive (negative) SAM during February–March is favorable (unfavorable) for the ensuing summer monsoon rainfall over the Indian sub-continent. The delayed response is relayed through the central Pacific Ocean. We propose a hypothesis that states: when a negative (positive) phase of February–March SAM occurs, it gives rise to an anomalous meridional circulation in a longitudinally locked air–sea coupled system over the central Pacific that persists up to the subsequent boreal summer and propagates from the sub-polar latitudes to the equatorial latitudes inducing a warming (cooling) effect over the central equatorial Pacific region. In turn, this effect concomitantly weakens (strengthens) the monsoon rainfall over the Indian sub-continent. Thus, the February–March SAM could possibly serve as a new precursor to foreshadow the subsequent behavior of the Indian summer monsoon.

In the present work, the variability, trends and relationships of aerosol, cloud and rainfall are explored for three regions of the Indian subcontinent: the Indo-Gangetic Plain (IGP), central India (CI) and the North Indian Ocean (NIO). The investigated properties include the monthly averages of satellite retrievals for the aerosol optical depth (AOD), angstrom exponent (AE), aerosol index (AI), cloud effective radius (Re), cloud optical thickness (COT), cloud liquid water path (LWP) and rain rate (RR). Inter-annual and seasonal variability and long-term trends for these properties were analyzed over the regions selected. The analysis showed apparent regional and temporal variations in all seven parameters. Strong increases in aerosol properties over the IGP and CI regions over the last two decades have enabled the study of the response to cloud and rainfall properties. Further, aerosol–cloud–rain rate relationships during the summer monsoon were verified using linear fit analysis (using AOD [and AI], Re, LWP and RR) for the three regions, respectively. A negative relation between AOD and Re was seen over the three regions, whereas AOD and RR showed a negative relation over IGP and NIO. It was found that the AI property, which represents the aerosol number, is inversely correlated with Re but positively correlated with RR during the monsoon season over the IGP and CI. AI and LWP showed a negative relation for CI but a positive relation for IGP and NIO. We believe that this study will help in assessing regional and global climate impacts due to aerosols.

Information on the spatial distribution of rainfall is required for many applications, including water and flood management. Weather radars can provide quantitative rainfall estimation over an area. Taking the example of an X-band radar installed at Mandhardev in the Western Ghats, we show that radar reflectivity data can have significant bias despite following standard calibration procedures. We report a technique to identify bias in the X-band radar reflectivity factor using collocated disdrometer observations and propose a methodology to correct it. Simultaneous data collected on 83 days during June–September of 2018 are used. Our results show that bias in the radar reflectivity factor reduced from –8.3 to –0.8 dB after correction. The estimated 83-day accumulated rainfall using uncorrected radar reflectivity data is 80% less compared to that of the disdrometer and the difference between the two reduces to 2% with correction. Bias in the estimated rainfall reduces from –30 to –0.7 mm day –1 and the RMSE reduces by 42%. There are days where daily rainfall from the two instruments agrees well with each other, while large differences exist on some days. We show that the sampling issue is one of the major sources of error, and its contribution depends on the nature/type of precipitating clouds, whereas uncertainties in the Z–R relationships account for about 15% difference.

This observational study during the 29-year period from 1979 to 2007 evaluates the potential role of Eurasian snow in modulating the North East-Indian Summer Monsoon Rainfall with a lead time of almost 6 months. This link is manifested by the changes in high-latitude atmospheric winter snow variability over Eurasia associated with Arctic Oscillation (AO). Excessive wintertime Eurasian snow leads to an anomalous cooling of the overlying atmosphere and is associated with the negative mode of AO, inducing a meridional wave-train descending over the tropical north Atlantic and is associated with cooling of this region. Once the cold anomalies are established over the tropical Atlantic, it persists up to the following summer leading to an anomalous zonal wave-train further inducing a descending branch over NE-India resulting in weak summer monsoon rainfall.