Are precipitation concentration and intensity changing in Bangladesh overtimes? Analysis of the possible causes of changes in precipitation systems.

A time series of premonsoon season (March–May) rainfall (PSR) resulting from premonsoon thunderstorms has been prepared for the period 1901–1992 over Gangetic West Bengal (GWB) and its neighbourhood. Though premonsoon rainfall is sporadic, unlike monsoonal rainfall, statistical tests confirm the series to be normal. Spatial distribution pattern of premonsoon season rainfall shows a northwest–southeast gradient, indicating the importance of close proximity of the sea, the Bay of Bengal, besides the land surface characteristics over the southeastern part of the study area. This is used to study premonsoon rainfall variability over the area through empirical orthogonal function analysis. The first eigenvector explains the interannual variability, while the second is linked with local circulation.



Statistical analyses have been made to study the temporal characteristics of the series. Results reveal neither long-term trends nor periodicities. However, short-term fluctuations are present. A negative tendency from 1915 onwards until the early 1970s may be a result of gradual deforestation. Moreover, a further rise in the 1970s and later, which is significant at the 5% level, is possibly related with land use changes, as well as local circulation. Copyright © 2000 Royal Meteorological Society

Premonsoon season rainfall variability over Gangetic West Bengal and its neighbourhood, India

Variation of atmospheric thermodynamical structure parameters between days of thunderstorm occurrence and non-occurrence is presented based on data sets obtained during Severe Thunderstorm-Observations and Regional Modeling (STORM) experiments conducted over Kharagpur (22.3°N, 87.2°E) in pre-monsoon season of 2009 and 2010. Potential instability (stable to neutral) is noticed in the lower layers and enhanced (suppressed) convection in the middle troposphere during thunderstorm (non-thunderstorm) days. Low-level jets are observed during all days of the experimental period but with higher intensity on thunderstorm days. Convective available potential energy (CAPE) builds up until thunderstorm occurrence and becomes dissipated soon after, whereas convective inhibition (CIN) is greatly decreased prior to the event on thunderstorm days. In contrast, higher CAPE and CIN are noticed on non-thunderstorm days. Analysis of thermodynamic indices showed that indices including moisture [humidity index (HI) and dew point temperature at 850 hPa (DPT850)] are useful in differentiating thunderstorm from non-thunderstorm days. The present study reveals that significant moisture availability in the lower troposphere in the presence of convective instability conditions results in thunderstorm occurrence at Kharagpur.

Thermodynamical Structure of Atmosphere during Pre-monsoon Thunderstorm Season over Kharagpur as Revealed by STORM Data

Using NASA’s TRMM Lightning Imaging Sensor (LIS) data from 1998 to 2014, this paper presents a 17-year lightning climatology of Bangladesh, at 0.5° × 0.5° spatial resolution. Diurnal, seasonal, monthly and annual variations in the occurrence of lightning flashes were explored. The diurnal regime of lightning is dominated by afternoon/evening events. Overall, peak lightning activity occurs in the early morning (0200 LST) and evening (1900 LST). The distribution of lightning flash counts by season over Bangladesh landmass is as follows: pre-monsoon (69.2%), monsoon (24.1%), post-monsoon (4.6%) and winter (2.1%). Flash rate density (FRD) hotspots were primarily located in the north and north-eastern parts of Bangladesh, with a maximum of 72 fl km−2 year−1. Spatially, the distribution of FRD increases from the Bay of Bengal in the south to relatively higher elevations (of the Himalayan foothills) in the north. A spatial shift in FRD hotspots occurs with change in season. For example, in monsoon season, hotspots of lightning activity move in a south-westerly direction from their pre-monsoon location (i.e. north-eastern Bangladesh) towards West Bengal in India. South and south-eastern parts of Bangladesh experience high lightning activity during post-monsoon season due to regional orographic lifting and low-pressure systems (i.e. cyclone) in the Bay of Bengal. To the best of our knowledge, this is the first study focused on LIS-based lightning climatology over Bangladesh. This baseline study, therefore, is an essential first step towards effective management of lightning-related hazards in Bangladesh.

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Spatial and temporal analysis of a 17-year lightning climatology over Bangladesh with LIS data

Organizational modes of squall-type Mesoscale Convective Systems during premonsoon season over eastern India

Surfaces fluxes, turbulent kinetic energy and Flux Richardson number are calculated for three typical sea breeze days characterizing three types of sea breeze onset at an inland station Kharagpur (22°21′ N, 87°19′ E), 80 km inland, one of the sites for MONTBLEX (MONsoon Trough Boundary Layer EXperiment), in India. The sea breeze onset is associated with a decrease in momentum and heat fluxes and an increase in moisture flux. The turbulent kinetic energy shows quite a significant value even in the late afternoon. The surface layer becomes nearly stable even before sunset, due to the passage of the sea breeze.

Sea breeze activity at an Inland station Kharagpur (India): a case study

Using data from the MONsoon Trough Boundary Layer EXperiment (MONTBLEX), the thermodynamic structure of the atmospheric boundary layer (ABL) under the influence of a monsoon depression has been studied. When the depression was in the vicinity of the observing station, the soundings showed an increase in potential temperature, the sub-cloud layer was well mixed, the wind speed increased to 35 m/s, and the monsoon boundary layer was convectively more unstable at night than in the daytime. Cloud-top processes, which lead to an apparent breakdown of the boundary layer, seem to explain this.

B. Chakravarty

Papers

Sea breeze activity at an Inland station Kharagpur (India): a case study

Thermodynamic structure of the monsoon boundary layer under the influence of a large-scale depression