C. G. Deshpande

Challenge to the assumption of the unitary diurnal variation of the atmospheric electric field based on observations in the Indian Ocean, Bay of Bengal, and Arabian Sea

Measurements of the atmospheric electric field and conductivity made aboard a research ship, ORV Sagarkanya on her four cruises in the Bay of Bengal and the equatorial Indian Ocean, are reported. In contrast to the generally observed unitary diurnal variation, the 40-day average field curve shows a maximum around 1000 UT and a minimum at 0000 UT with a small secondary peak at 1900 UT. Diversity of the nature of field curves obtained on our different cruises demonstrates the variability of the curve when averaged over shorter periods. Magnitudes of the measured electric field and conductivity and the calculated values of air-Earth current are comparatively much smaller than that reported earlier over oceans. When considered with reference to the classical theory of the global electric circuit, our observations indicate a tendency to accentuate the effect of storm activity over Asia-Australia and Africa-Europe and to attenuate the effects of far-distant storms over the Americas.

Aerosol size distributions in the north and south Indian ocean during the northeast monsoon season

Abstract Measurements of aerosol size distributions (3- to 1000-nm diameter) were made over the Indian Ocean (15°N, 75°E to 70°S, 11°E) during the onward (December 12, 1996 to January 6, 1997) and return (March 9 to April 5, 1997) cruises of the XVI Indian Scientific Expedition to the Antarctic. Observations show that during the January to April period, the environment over the Indian Ocean undergoes a transition from a relatively clean to a fairly polluted one. Our observations of the large concentrations and the North-to-South positive gradient of aerosol concentration over the northern Indian Ocean strongly support the transport of aerosols and trace gases with the seasonal northeasterly winds from the Indian subcontinent to the Indian Ocean. The results also indicate the production of new particles over the oceanic region by the gas-to-particle conversion processes. During this season, because of the persistent northeasterly winds and the shift of the Inter-Tropical Convergence Zone (ITCZ) to the Southern Hemisphere in this region, the air pollutants over the northern Indian Ocean are carried into the Southern Hemisphere with the cross-equatorial flow and reach up to the southern limit of the ITCZ. Some pockets of very high aerosol concentrations have been observed in and around the ITCZ. The nucleation mode particles are observed in great abundance up to 30°S. From the Indian coast to 5°N, aerosol particles are observed to have bimodal size distributions with a maximum in accumulation mode at 133-nm diameter and a minimum for nucleation/Aitken mode particles of 422 nm in the 5°N-Equator belt and are generally trimodal in the Southern Hemisphere. Transport of the nucleation mode particles from the free troposphere to the marine boundary layer (MBL) associated with the large-scale subsidence and the subsequent North-to-South transport of these and continental aerosols with the seasonal northeasterly surface winds over the Indian Ocean is proposed to explain the high concentration of the nucleation mode particles observed up to the ITCZ. Size distributions change to Junges power-law type in regions of high aerosol concentration inside the ITCZ. Comparatively small concentrations with almost uniform size distributions that are typical of the pristine air in the Southern Hemisphere are observed from 42°S to 56°S. Some high peaks in aerosol concentration that can be associated with the low-pressure systems that encircle Antarctica are observed in the 60–70°S latitudinal belt. Further, our observations strongly demonstrate the effect of wind direction on the land-to-ocean transport of the atmospheric aerosols.

Observations of some atmospheric electrical parameters in the surface layer

Measurements of atmospheric potential gradient, conductivity of both polarities and space charge are reported for Pune (18°32′N, 73°51′E, 559 m above msl). Observations show negative excursions of potential gradient at night, frequent observations of excess values of one polarity of conductivity and the presence of very high values of space charge of either polarity at night. Results are discussed in terms of a lower thin layer of positive space charge due to the electrode effect and an upper layer of negative space charge due to radioactive elements and their emanations close to ground and mutual mixing of these two layers under different meteorological conditions.

Physical properties of aerosols at Maitri, Antarctica

Measurements of the submicron aerosol size distribution made at the Indian Antarctic station, Maitri (70‡45′S, 11‡44′E) from January 10th to February 24th, 1997, are reported. Total aerosol concentrations normally range from 800 to 1200 particles cm−3 which are typical values for the coastal stations at Antarctica in summer. Aerosol size distributions are generally trimodal and open-ended with a peak between 75 and 133 nm and two minima at 42 and 420 nm. Size distributions remain almost similar for several hours or even days in absence of any meteorological disturbance. Total aerosol concentration increases by approximately an order of magnitude whenever a low pressure system passes over the station. Based on the evolution of aerosol size-distributions during such aerosol enhancement periods, three types of cases have been identified. The nucleation mode in all three cases has been suggested to result from the photochemical conversion of the DMS emissions transported either by the marine air or by the air from the ice-melt regions around Maitri. Subsidence of midtropospheric air during the weakening of radiative inversion is suggested as a possible source of the nucleation mode particles in the third case. Growth of the nucleation mode particles by condensation, coagulation and/or by cloud processes has been suggested to be responsible for other modes in size distributions.

Short time variations in atmospheric electrical parameters

Measurements of atmospheric space charge at 0.1, 1 and 2m above the ground surface, the potential gradient at the ground and the positive and negative conductivity at 12cm above the ground have been made at a frequency of 1 Hz at different times of the day. Short time variations in each of these parameters are studied. Night-time observations of space charge at the three levels show features typical of space charge movement in a stable stratified atmosphere, while daytime observations are typical of space charge movement in a well mixed turbulent atmosphere. Short time variations of a parameter generally seem to follow the magnitude of the parameter. The short time variations of the potential gradient and conductivity are in accordance with Ohms law.

Atmospheric electricity measurements at Pune during the solar eclipse of 18 March 1988

Measurements of atmospheric electric potential gradient, conductivity of both polarities and space charge at the ground surface at Pune during the partial solar eclipse of 18 March 1988 have been made. In spite of no appreciable change in atmospheric temperature at the ground surface, all the atmospheric electric parameters showed remarkable changes during the period of eclipse. Results do not support any vertical transport of charge, either by conduction or by convection, near to the ground surface.

Extension of atmospheric aerosols over ocean around peninsular India in the southwest monsoon season

Measurements of atmospheric electric conductivity aboard ORV Sagarkanya were conducted during her cruise around the Indian peninsula during the southwest monsoon season when the persistent southwesterly surface winds exist over the region. Observations show that extension of aerosol pollution over sea may be highly dependent on the wind direction with respect to the coastline. The conductivity values near the western coast are more than twice the values at roughly the same distance away from the eastern coast in the Bay of Bengal. The increase in conductivity with distance observed along all the three legs of the cruise in the Bay of Bengal, as the ship moves away from the coast, exhibits the importance of the age of air mass in determining the values of conductivity over sea.

Atmospheric electric conductivity measurements over the Indian Ocean during the Indian Antarctic Expedition in 1996–1997

The atmospheric electric conductivity measurements made over the Indian Ocean with a Gerdiens apparatus mounted aboard MV Polarbird during the XVI Indian Scientific Expedition in 1996–1997 are reported. Simultaneous three-hourly measurements of aerosol concentrations of 13–1000 nm size and some meteorological parameters are also reported. Latitudinal variation of conductivity along the cruise route shows a minimum at ∼28°S. Further, the variations in conductivity in the 10°N–20°S and 60°–70°S latitudinal belts show opposite trends on the outward and return cruises, which fall near to the onset and withdrawal phases of the northeast monsoon season, respectively. The results are explained on the basis of the well-known northward shift of the subsidence leg of the southern Hadley cell and of the position of the Intertropical Convergence Zone during the months of March–April in this region and the observations of the cyclonic systems near the continent of Antarctica during the period of outward cruise.

Measurements of atmospheric electric field and conductivity in the locality of a gas well flame

Measurements of the atmospheric electric field and conductivity of both polarities are made at the Earths surface 1.5 kms from a very large flame produced due to a gas well blowout. The results show a single periodic diurnal variation of electric field with a maximum in the afternoons and very low values of electric field during nights. The observation is explained with the help of Vonnegut et al.s [1995] results that a flame burning under the influence of the fair weather electric field releases negative charge into the atmosphere. Possible implication of the results in explaining the development of inverted dipole in forest fire clouds is discussed.

Breakup Modes of the Drops Suspended in a Vertical Wind Tunnel in Presence of the Horizontal Electric Field

The influence of strong horizontal electric field (EH) on different stages of deformation and eventual breakup of the large water drops of 6.6, 7.0 and 7.25 mm diameter has been observed in a vertical wind tunnel using high-speed photography. Dumbbell, filament and bag modes of drop breakup are observed when EH = 0. However, drops elongate in horizontal direction, mostly develop sharp curvature at their ends, eject a fine jet spray of tiny droplets and ultimately break up into several droplets in EH = 500 kVm-1. Extreme elongation upto 29 mm is observed for a 7.0 mm diameter drop. Results show that the breakup time, i.e. the time from the drops extreme prolate shape to its breakup in its final oscillation, ranges from 13 - 41 ms when EH = 0 and 57 – 105 ms when EH = 500 kVm-1. So, although the lifetime of the drop since its suspension to breakup is reduced, its elongation and breakup time increase in EH. It suggests that the effect of EH in final oscillation before breakup overcomes the effect of hydrodynamic and aerodynamic forces in elongating the drop. Also, no breakup of bag type is observed in EH = 500 kVm-1. Moreover, the fragments formed after the drop breakup and tiny droplets ejected by their fragments, carry electrical charges of polarity determined by the induced charge on the parent drop in EH. The significance of the results is discussed in modifying the drop growth and the radar echo– precipitation relationships in thunderclouds.