The spatial distribution of convective available potential energy (CAPE) and lightning activity in different seasons over the Indian region have been studied to find out the dependence of lightning activity on CAPE. It is observed that the lightning activity over the Indian region is not controlled by CAPE alone during pre-monsoon season. The prevailing meteorological conditions and orography over northern India, central India, northeast Pakistan and Bangladesh provide favourable conditions for formation of thunderstorms, and hence, lightning activity is higher in spite of lower value of CAPE over these regions compared to other parts of Indian region. During the monsoon season, lightning activity and CAPE are found to be better correlated with each other compared to other seasons over central and north India. It has been found that the high mountains of Himalayas generate strong updrafts necessary for the deep convective events by interacting with prevailing winds and the diabatic heating and radiative cooling of mountaintops create conditions favourable for convections. The diurnal variation of lightning activity at stations over the foothills of Himalayas showing a strong peak in lighting activity after midnight supports the idea that radiative cooling at mountaintops can create a moisture convergence at foothills and trigger the convections.

[1] The electrical conductivity measured over the Indian Ocean (15°N, 77°E to 20°S, 58°E) during the Indian Ocean Experiment (INDOEX-1999) from 20 January to 12 March 1999 has been analyzed. The conductivity values over two oceanic regions, one with very low aerosol concentration and another with very high aerosol concentration, are studied in relation with meteorological parameters such as relative humidity and sea level pressure. The average conductivity is as low as 0.295 Â 10 À14 Sm À1 in the region of high aerosol concentration and it is 0.783 Â 10 À14 Sm À1 in the region of very low aerosol concentration. In both the regions, conductivity shows an inverse relation with relative humidity and this effect is more in the presence of high aerosol concentration. The hydrate growth of aerosol particles in high-humidity condition may be responsible for the inverse relation between conductivity and relative humidity. Size distributions of aerosol particles measured in the same cruise during high-humid conditions are also analyzed to show that sizes, rather than numbers, of aerosol particles increase with an increase in humidity. The relationship between conductivity and sea level pressure in these two regions is also studied and it shows good correlation in the region where the background aerosol concentration is low and no correlation in the region where aerosol concentration is high. The inverse relation between sea level pressure and electrical conductivity is attributed to the possible transportation of ultrafine particles from free troposphere, with subsiding motions associated with high pressure. The positive correlation between ultrafine particles and sea level pressure supports this idea.

[1] The evolution of lightning and the shape of recovery curves after multiple-discharge flashes in a thundercloud have been studied from the surface measurements of electric field and Maxwell current near a tropical thundercloud. Observations suggest a tripole structure of the cloud and that its lower positive charge center (LPCC) plays a dominant role in initiating/triggering an intracloud (IC) or cloud-to-ground (CG) lightning discharge. IC discharges in the initial stage of thundercloud are followed by CG discharges from the LPCC and then by two distinct groups of multiple-discharge flashes. Each flash in the first group consists of an IC discharge triggered by a CG discharge and in the second group a CG discharge triggered by an IC discharge. Flashes in each group are bunched together for $15-20 min and occur with almost a regular periodicity of 1-1.5 min. The Maxwell current during every such flash in both groups has a bipolar transient and a positive overshoot that subsequently relaxes back to its predischarge value. The magnitudes of overshoot for the flashes in the first group are found to be much lower than those for the flashes in the second group. From a small portion of the recovery curves of such multiple-discharge flashes, one can conclude that the rate of charge buildup in the main negative charge center is higher than that in the LPCC.

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