Climatology of local severe convective storms in Assam, India

摘要

In Assam, local severe convective storms (LSCS) are recognized as exceptionally powerful and destructive meteorological events resulting in both death and loss of property, as well as livelihood. A valuable aid to assessing and managing LSCS lies in a reliable database of historical severe storms. In this paper, we investigate the temporal and spatial characteristics of LSCS in the state of Assam lying in the North Eastern Province of India and provide a climatology of LSCS for the state, with respect to: distribution, storm types, frequency, seasonality, and time of occurrence. This was accomplished by developing a LSCS database through a systematic scanning of newspapers and other available sources. This historical database on LSC S dating from 1962 to 2016 was used to identify the areas where the threat and risks from these storms is maximum. Our findings show that LSCS occur throughout the state of Assam, but there are unique geographical areas where the propensity for the occurrence severe to intense local convective storm is much higher. From the monthly distribution of LSCS events, as have been found by previous researchers, the most active month is April. The monthly distributions of hail follow similar pattern as the total number of LSCS events, however, seasonality of lightning differ from other LSCS categories. The probability of LSCS is not uniform throughout the day and tend to reach their most vigorous development during the latter part of the evening and night hours. The observed seasonal pattern of LSCS day's distribution corresponds with the time of the year when convective heating of the lower atmosphere is at its highest in the region, thereby increasing instability of the atmosphere. In addition, climatological mean wind at upper level (200 hPa) show influence of subtropical westerlies and at 925 hPa indicate possible low-level moisture transport for the peak LSCS months. Therefore, high storm frequencies are to be anticipated. Although November, December and January show the lowest frequency during the entire year, it is still surprisingly high considering that convectional heating and instability of the atmosphere has decreased substantially by then. The results from this study could be applied to produce a hazard map of LSCS for the state of Assam. Such a hazard map will benefit numerous stakeholders, in particular, to direct disaster management authority in terms of interventions for LSCS risk in the state of Assam.