A Severe Hailstorm at Pokhara: CAPE Stability Index Calculations

Deepak Aryal^1, , Yolanda N. Rosoff¹ and Lochan Prasad Devkota¹

¹Central Department of Hydrology and Meteorology, Tribhuvan University, Kathmandu, Nepal

Cite this paper:
Deepak Aryal, Yolanda N. Rosoff and Lochan Prasad Devkota. A Severe Hailstorm at Pokhara: CAPE Stability Index Calculations. Journal of Geosciences and Geomatics. 2015; 3(5):142-153. doi: 10.12691/jgg-3-5-5

Abstract

A severe hailstorm, with hail stones estimated at 1 kg, at the city of Pokhara, 800m asl in the Nepal Himalayas, on May 18, 2005 is investigated in this paper. Upper air data obtained from Gorakhpur combined with Department of Hydrology and Meteorology (DHM) surface data at Pokhara provided the basis for stability index calculations, such as convective available potential energy (CAPE). Strong updrafts in which large hail is formed are directly related to CAPE. This paper attempts to calculate CAPE values in complex terrain, where data for low tropospheric levels is not available. Temperatures and dewpoint temperatures above the surface were estimated from the DHM observations of cumulus cloud base heights at Pokhara. This proved to be a valid technique for providing essential weather data at 850hPa. The Pokhara area is a known location of moisture convergence. This, plus exceptionally cold air aloft from the northwest on May 17 and 18, resulted in extreme instability, with CAPE values reaching and possibly exceeding 6800J/Kg.

Keywords:
CAPE lifting index hailstorm radiosonde geopotential height

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