The objectives of the Winter Fog Experiment (WIFEX) over the Indo-Gangetic Plains of India are to develop better now-casting and forecasting of winter fog on various time-and spatial scales. Maximum fog occurrence over northwest India is about 48 days (visibility <1000 m) per year, and it occurs mostly during the December-February time-period. The physical and chemical characteristics of fog, meteorological factors responsible for its genesis, sustenance, intensity and dissipation are poorly understood. Improved understanding on the above aspects is required to develop reliable forecasting models and observational techniques for accurate prediction of the fog events. Extensive sets of comprehensive groundbased instrumentation were deployed at the Indira Gandhi International Airport, New Delhi. Major in situ sensors were deployed to measure surface micrometeorological conditions, radiation balance, turbulence, thermodynamical structure of the surface layer, fog droplet and aerosol microphysics, aerosol optical properties, and aerosol and fog water chemistry to describe the complete environmental conditions under which fog develops. In addition, Weather Forecasting Model coupled with chemistry is planned for fog prediction at a spatial resolution of 2 km. The present study provides an introductory overview of the winter fog field campaign with its unique instrumentation.

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The nature of large scale stratospheric circulation is studied using the cosmic ray produced isotopes Par, Be", Sa6 and Nas2 as tracers. Supplementary information obtained from observations of the distribution of the bomb-produced Na** and radongenic Pbp10 is taken into account. The activities of these tracer elements have been measured in the stratospheric air, up to altitudes of 20 km, during 1960-64. Data are fairly extensive for studying the characteristics of the mean circulation in the stratosphere as well as seasonal changes in patterns of mixing/transport of air in certain regions of the stratosphere.The interpretation of the data on cosmic ray tracers is based on a comparison of their observed activities with the expected production rates due to cosmic rays. For this purpose, the work of La1 & Peters is extended to evaluate the variations in the relative production rates of the isotopes Par, Be', S" and Nag* in the atmosphere. These have to be taken into account when isotope data are compared for different altitudes and latitudes in the stratosphere where relative isotope production rates are different because of the markedly different prevailing energy spectrum of nucleons.The analysis allows us to distinguish three zones in the lower stratosphere (below 20 km), well separated from the tropopause, having distinct circulation patterns. These regions are separately well mixed either vertically or horizontally; the mean time of residence of aerosols in these regions differs appreciably too. The most stable region in the Stratosphere is found to be 18-20 km region at 0-30" latitude, where apparent residence times are of the order of twenty months. Polar regions are observed to exhibit an enhanced vertical mixing during November-February. Combining these results with the observations of dispersion of bomb-produced Naxz, which appeared in significant amounts from early 1962 onwards all over the stratosphere, we deduce that in the polar regions, vertical mixing occurs rapidly during November-February so that any activity injected in this region a t 20 km or so mixes downwards a t the rate of about 1.6 km month-'. It is concluded that the observed spring peaks in the troposphere are merely the consequence of this phenomena which is triggered in upper levels (above 20 km) of the stratosphere during October-November.The observations of concentrations of PbP1O in the stratosphere are discussed. The analysis reveals that an appreciable gravitational settling of Pb'l0 seems to have occurred, a t least during the period over which data were collected, from the stratospheric air between 56'-75' latitude. These observations imply that the residence time of air in this region of the atmosphere is appreciably higher than that deduced from tracers which attach themselves to aerosols. Lastly, the Pbglo data indicate that appreciable amounts of tropospheric radon presumably enter the equatorial stratosphere; this conclusion rests on the Observation that Pbz10 concentrations are higher in this region compared to that in the ...

[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.

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