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ABSTRACT Tropical Rainfall Measuring Mission Precipitation Radar data ( 2A23 V7), for a period of 9 years (2002-2010), is used to analyse the evolution of the monthly cloud climatology over the Indian subcontinent (lat. 6.5°-38.5°N, long. 66.5°-100.5°E). The analysis of the cloud data suggests that over most regions and seasons, the stratiform cloud fraction is associated with outflows from and remnants of convective cells. However, the stratiform cloud cover over the North Indian subcontinent during winter season is not accompanied by any convective cloud maximum zone, indicating minimal role of ground processes in the growth of cloud cover over the region during this season. Mesoscale factors and land surface processes play a major role in the development of cloud maxima during March to May over the Indian subcontinent, over the eastern Himalayas and adjoining plains, as well as southwest peninsular India. As the summer monsoon season approaches, the spatial spread and growth of both cloud fractions gradually increase (stratiform fraction of cloud cover increases rapidly, while the convective fraction rises gradually) over east India, Bay of Bengal and over south Arabian Sea, indicating increased organization of the cloud systems with longer lasting stratiform outflows with the advancement of the season. The largest cloud maxima during the monsoon season are observed at the windward side of hills along the west coast of peninsular India and the west coast of Myanmar. The respective values of convective and stratiform cloud fractions indicate that the cloud cover over Myanmar is associated with more organized cloud systems with larger stratiform outflows, as compared with clouds along the west coast of peninsular India. Similarly, during October to December, the increasing stratiform cloud fractions over southeast peninsular India indicate increasing organization of clouds, compared with values during the monsoon season over this region. [ABSTRACT FROM AUTHOR]

Widespread very heavy to extremely heavy rainfall occurred over Uttarakhand and its neighbouring states during 16-18 June 2013, which caused flash floods, landslides, large-scale loss of lives and damage to property. The present study analyses the dynamical and thermodynamical features associated with this torrential rainfall episode. The analysis suggests that due to strong interaction between an oncoming midtropospheric trough in the westerlies and the strong lower-tropospheric southeasterly monsoon wind flow in association with a monsoon low-pressure system over the North Indian region, a lower tropospheric wind convergence zone developed over Uttarakhand and its neighbouring regions. A strong Bay of Bengal current of air with wind speed of 40 kts in the northern periphery of the monsoon low, pumped a lot of moisture into the region. Supported by strong orographic effect due to high terrain and strong moisture feeding from both the Arabian Sea and the Bay of Bengal, a large-scale quasistationary regenerative mesoscale convective system (MCS) developed over the zone of convergence. The episode was comparable to the 2010 Leh flash flood episode, in that the synoptic conditions were similar in both cases. However, while the Leh episode resulted from repeated surges of westward travelling MCS across the Tibetan Plateau, the present case was due to quasistationary regenerative MCS over the region. [ABSTRACT FROM AUTHOR]

In the present study, the diurnal variations in the time of initiation of rainfall, during two contrasting monsoon seasons of 2008 (below normal) and 2009 (normal) over the Indian subcontinent and surrounding oceanic areas has been analyzed. Harmonic analysis was used to detect the spatial variation of the diurnal cycle of the time of initiation of rainfall, as obtained at half-hourly intervals from the Kalpana 1 satellite. In general, the diurnal cycle in the time of initiation is strongest in regions where convective clouds are predominant, while it is weaker in regions where the clouds are predominantly stratiform with long-lived medium to high cloud cover. In the interior of the subcontinent, the time of maximum mainly occurred in the afternoon to evening hours, with a distinct southeast to northwest gradation. Substantial spatial variations were detected in the diurnal patterns between a normal and below normal monsoon years. Spatially, rainfall is initiated later in 2009 compared to 2008 over most of the interior of the Indian subcontinent. The most distinct difference was observed over the core monsoon region in central India, where the diurnal patterns were stronger in 2009 compared to 2008. On the other hand, over the oceans surrounding the Indian subcontinent, the initiation times are generally earlier in 2009. [ABSTRACT FROM AUTHOR]

In general, the overall differences in activity and timing of convection are a result of the influence of large-scale regional and synoptic flow patterns on the local mesoscale environment. The linkage between the space-time variability of observed clouds and rainfall, with large-scale circulation patterns and mesoscale variables over north India during the pre-monsoon season (March to May) is the focus of this paper. We use harmonic analysis to identify the first hour of rainfall for 42 stations spread over the north Indian region during the pre-monsoon summer season (March to May), from 1980 to 2000. The variability is observed to be systematic, with large regions having similar timing for occurrence of rainfall. The stations located in the foothills of the Himalayas have a late night to early morning maximum of first hour rainfall. In the northwestern plains, the first hour of rainfall mostly starts in the early afternoon to evening hours. Further eastward, the rainfall occurs in the late evening hours. Overall, there is a gradient in the occurrence of first rainfall events from late afternoon hours in the southern sections of the north Indian region to nocturnal maxima in the higher altitude regions. Five of these stations, located in different regions of homogenous timing of rainfall occurrence, were selected to analyze in detail the variable trigger for convection. Our results indicate that convective episodes occur mostly in association with the passage of westerly troughs over this region. These upper atmosphere troughs enable moisture to flow from the surrounding oceanic regions to the dry inland regions and also provide some dynamic support to the episodes of convection. However, the actual occurrence of convection is triggered by local factors, giving rise to the mesoscale structure of the weather systems during this season. Specifically, over the plains of northwest India, convection is triggered in a moistened environment by diurnal solar heating. The late night to early morning convection over the foothills is triggered by the orography, when the moistened airflow is normally incident on the mountain slopes. Further eastward, the primary trigger for localized moist convection is downdrafts from south-eastward propagating convective systems that originate at a north-south dry line over north India. These systems propagate with a speed of about 15 m s. The above results are supported by geostationary satellite brightness temperature data for March to May 2008. [ABSTRACT FROM AUTHOR]