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The diurnal cycle of tropical convection over the Indian region has been analysed in this study for the period of March to June from the thunderstorm reports of ground-based observatories throughout India during 2016–2020. The analyses indicate that during this period of the year, when the land progressively heats up diurnally, under conditions of deficient moisture, synoptic systems interact with the semi-permanent features of the atmosphere over the region to define areas of moisture and wind convergence, which in turn determine the frequency and diurnal cycle of thunderstorm activity. While the easterly and westerly waves are the major synoptic scale weather systems that affect the Indian region during March to May, the atmospheric changes are associated with the onset of the monsoon season in June. Amongst the semi-permanent features, the gradual intensification and migration of the shallow heat low to northwest India during May and June reinforce the western disturbances over this region, thereby intensifying the thunderstorm activity during the afternoon to evening hours over the western Himalayas and northwest Indian region. The role of the low-level anticyclones over the Arabian Sea and the Bay of Bengal is seen in the east–west-oriented moisture gradient across the Indian subcontinent which makes the east Indian subcontinent generally more prone to thunderstorm activity during this season. The east–west-oriented discontinuity line across north India is particularly intense during the morning hours along the foothills of the Himalayas. Its location directs moisture from the Bay of Bengal into the Himalayas causing early initiation of thunderstorm activity over the Himalayas. The discontinuity line moves southwards to the north Indian plains later in the day, although the western end becomes less marked. The north–south-oriented discontinuity line across the Indian subcontinent between the two anticyclones intensifies during the afternoon hours due to land heating and combines with the east–west wind discontinuity to become a T-shaped maximum convergence zone for thunderstorm activity during the afternoon hours over the Indian region, which intensifies as the months progress. With the onset of the southwest monsoon over the south peninsula and east Indian regions in June, the change in wind pattern from an easterly to a southwesterly flow regime over the south peninsula is reflected in an abrupt shift of the afternoon maximum of thunderstorm activity over the inland regions of the south peninsula to an early morning maximum over the southwest peninsular coast. Simultaneously, with the weakening of the anticyclone over the north Bay of Bengal and gradual strengthening of the southerly moisture flow into east and northeast India, there was in-phase increase in thunderstorm activity during the afternoon hours over the plains of east-central India, east India, and adjoining southern parts of northeast India during June. The diurnal pattern of tropical convection significantly affects human lives over the Indian subcontinent. [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]

Nowcasting in the India Meteorological Department (IMD) is being provided for T + 0 to T + 2 h, using the Warning Decision Support System (WDSS-II) software. Prior to operational nowcasting over the Indian region, the parameters of the nowcast algorithm tool of the software were optimized, and accuracy was evaluated for various weather systems over Delhi. This optimization is demonstrated in this study with reference to three weather systems over Delhi, with each case representing one of three typical types of cloud systems over the region. These are-(a) convective lines associated with winter and early pre-monsoon weather systems, (b) deep convective cells that form in the pre-monsoon (April-June) and post-monsoon season (October-November) and (c) wide convective echoes that form during the monsoon season. The efficacy of the algorithm was assessed on a frame-by-frame basis as well as holistically for entire convective episodes. The important findings of the frame-by-frame study are (1) the inability of the inbuilt growth-decay algorithm to capture the evolution of storm cells, (2) setting of the threshold of detection of storms and tracking storms and (3) number of scales through which storms should be tracked. The holistic capabilities of the nowcast algorithm were tested for entire convective episodes using Model Evaluation Tools software. The results indicate that the advection algorithm tends to move the convective areas faster than observed at all time scales. Hence the multi-scale segmentation approach (over the two-scale approach) increases the smoothening of the output, at the cost of decreased nowcast skill. The inter-event comparison indicates that the low-intensity convective line zones, which are characteristic of winter and early pre-monsoon weather systems, have the most rapid temporal change in the overall area under convection. This leads to larger area errors during nowcasting of these systems. On the other hand, pre-monsoon systems comprised mostly isolated cells that reach great heights and move very fast, but do not have much horizontal area growth. The error in the nowcasting of these systems is mostly in respect of location error, as well as error in forecast of the intensity of the cells. The overall error in nowcasting is least for the monsoon systems over the Delhi region. [ABSTRACT FROM AUTHOR]

The intra-seasonal variability of the Indian summer monsoon, which manifests in the form of "active" and "break" phases in rainfall, is investigated with respect to the variability of the convective and stratiform precipitating cloud pattern over the region. Long period data from TRMM PR satellite (2A23 and 3B42 datasets) for the monsoon season of 2002 to 2010 over the Indian subcontinent is used for this purpose. The study reveals that the most significant spatial variation in convective and stratiform cloud amount in relation to the active and break phase occurs over the monsoon trough region in central India. The active phase is characterized by positive convective (~5%) and stratiform (~20%) precipitating cloud anomalies over this region. However, the maximum of the former precedes the latter by 1-2 days leading up to the active phase, indicating that the stratiform build up, is due to the gradual organization of the convective cloud systems over the region. The days leading up to the break phase are marked by negative anomalies in the convective and stratiform fractions of cloudiness over this region, which are in phase with each other, unlike the lead-up to the active phase. Analysis of the pattern of atmospheric heat source and sinks over the region from the NCEP-NCAR re-analysis data indicates that the engine for the growth/decay of convection over the monsoon trough region lies primarily in the Bay of Bengal and adjacent east India. The active phase is preceded by a heating pattern that promotes large scale, organized convective cloud growth over the Bay of Bengal preceding the actual onset, while the heating pattern leading up to the break phase promotes the formation of isolated convective clouds and decay of cloud organization over the monsoon trough region. [ABSTRACT FROM AUTHOR]