D. M. Lal

Effect of relative humidity and sea level pressure on electrical conductivity of air over Indian Ocean

[1]xa0The 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 × 10−14 Sm−1 in the region of high aerosol concentration and it is 0.783 × 10−14 Sm−1 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.

Rain‐Induced Soil NOx Emission From India During the Onset of the Summer Monsoon: A Satellite Perspective

[1]xa0Rain-induced soil NOx emission in the rural regions in India has been investigated using satellite observations of daily tropospheric NO2 columns from the Ozone Monitoring Instrument (OMI). We selected three rural regions with low population density where industrial and transportation activities are notably less in comparison to other regions of India. We show that OMI-derived tropospheric NO2 columns capture the NOx pulses from soil during the onset of the rainy season (June) over the regions considered in our analysis. Strong enhancements in tropospheric NO2 columns after the active spell of precipitation and subsequent decrease during the break spell have been observed. We argue that this enhancement is due to rain-induced soil NOx pulsing after the onset of summer monsoon rainfall over the dry soil in the study regions. Relating OMI tropospheric NO2 columns to surface NOx emission, soil emission accounts for an average emission flux of ∼23–28 ng N m−1 s−1 during the pulsing event. Lightning is unlikely to account for the enhanced OMI tropospheric NO2 columns over the study regions.

Influence of aerosol on clouds over the Indo-Gangetic Plain, India

Using Total Ozone Mapping Spectrometer Aerosol Index (AI) and NCEP/NCAR reanalysis clouds data for the period 1979–1992, the influence of aerosol on the clouds (low and high cloud cover) over the Indo-Gangetic Plain (IGP) in India has been brought out for the first time in the present study. AI shows increasing tendency over the IGP suggesting that aerosol loading over this region increased significantly during the study period. In our analysis, High Cloud Cover (HCC) shows increasing trend and Low Cloud Cover (LCC) shows decreasing trend over the IGP during the same period. During pre-monsoon season when aerosol loading is more, HCC increases in positive correlation with AI. On the other hand, LCC show decreasing trend and is anti-correlated with AI. During summer monsoon, aerosol shows increasing trend but their effect on HCC and LCC is not seen to be significant. Similarly, the role of humidity on aerosol induced changes in HCC and LCC over the IGP region was also analyzed. In the low to moderate humid areas of IGP region (western and middle IGP), increasing AI leads to increase in HCC and decrease in LCC. On the other hand, in high humid areas (eastern IGP), increase in AI does not show any significant effect on HCC, but LCC shows positive trend. Therefore, we strongly argue that increasing aerosol loading enhances Cloud Condensation Nuclei over the region which in turn, alters the microphysical properties of clouds by reducing the size of cloud droplets, and atmospheric humidity controls the aerosol effect on clouds. During the recent period (2005–2010), similar features have also been observed over the IGP region.

Effects of environmental conditions on inducing charge structures of thunderstorms over Eastern India

It is well known that environmental conditions like convective instability, aerosol loading, and availability of moisture content affect the polarity of charge structures of thunderstorms. The electrical characteristics of thunderstorms observed during the pre-monsoon season of year 2009, over Eastern India were studied to identify the effects of different environmental conditions on charge structures of thunderstorms occurring over this region. Electric field and Maxwell current data suggest that at least one of these thunderstorms had an inverted charge structure. Doppler RADAR, radiosonde, and Moderate Resolution Imaging Spectroradiometer (MODIS) Aerosol Optical Depth (AOD) data have been used to compare the microphysical and dynamical characteristics of these thunderstorms. The thermo dynamical structure observed by radiosonde during the day on which an inverted polarity thunderstorm was observed showed very high CAPE in the mixed-phase region compared to other thunderstorm days. Furthermore, the AOD peaked 1xa0day before this thunderstorm. The back trajectories of winds also suggest that the aerosols might have been transported from a desert region on that day. It has been proposed that the large ice nuclei concentration can produce dominant positive charge in the lower portion of the mixed-phase region by maintaining ice saturation.

Radiative Impact of Fireworks at a Tropical Indian Location: A Case Study

During Diwali festival, extensive burning of crackers and fireworks is made. Weeklong intensive observational campaign for aerosol study was carried out at a representative urban location in the eastern Indo-Gangetic Plain (IGP), Varanasi (25.3°N, 83.0°E), from October 29 to November 04, 2005 (Diwali on November 01, 2005), to investigate behavioral change of aerosol properties and radiative forcing between firework affected and nonaffected periods. Results show a substantial increase (~27%) in aerosol optical depth, aerosol absorption coefficients, and aerosol scattering coefficients during affected period as compared to non-affected periods. Magnitudes of radiative forcing at top of atmosphere during affected and non-affected periods are found to be

Relationship between aerosol and lightning over Indo-Gangetic Plain (IGP), India

The relationship between aerosol and lightning over the Indo-Gangetic Plain (IGP), India has been evaluated by utilising aerosol optical depth (AOD), cloud droplet effective radius and cloud fraction from Moderate Resolution Imaging Spectroradiometer. Lightning flashes have been observed by the lightning Imaging sensor on the board of Tropical Rainfall and Measuring Mission and humidity from modern-era retrospective-analysis for research and applications for the period of 2001–2012. In this study, the role of aerosol in lightning generation over the north-west sector of IGP has been revealed. It is found that lightning activity increases (decreases) with increasing aerosols during normal (deficient) monsoon rainfall years. However, lightning increases with increasing aerosol during deficient rainfall years when the average value of AOD is less than 0.88. We have found that during deficient rainfall years the moisture content of the atmosphere and cloud fraction is smaller than that during the years with normal or excess monsoon rainfall over the north-west IGP. Over the north-east Bay of Bengal and its adjoining region the variations of moisture and cloud fraction between the deficient and normal rainfall years are minimal. We have found that the occurrence of the lightning over this region is primarily due to its topography and localised circulation. The warm-dry air approaching from north-west converges with moist air emanating from the Bay of Bengal causing instability that creates an environment for deep convective cloud and lightning. The relationship between lightning and aerosol is stronger over the north-west sector of IGP than the north-east, whereas it is moderate over the central IGP. We conclude that aerosol is playing a major role in lightning activity over the north-west sector of IGP, but, local meteorological conditions such as convergences of dry and moist air is the principal cause of lightning over the north-east sector of IGP. In addition, atmospheric humidity also plays an important role in regulating the effect of aerosol on the microphysical properties of clouds over IGP region.

Analysis of sunlight absorption spectra related to atmospheric trace gases in the tropics

ABSTRACT Zenith sky-scattered light intensity spectra of wavelength ranges of 325–500 nm have been recorded with UV-visible spectrometer over tropical station Pune (18° 31′ N, 73° 55′ E). Zenith scattered light spectra in the spectral range of 346–358 nm are analysed to find out differential optical depth (DOD) for the period 15–18 November 2010. In DOD spectra, depths are noticed at relevant wavelength due to the absorption by atmospheric gases such as NO2 (nitrogen dioxide), O3 (ozone), BrO (bromine monoxide), and OClO (chlorine dioxide). These DOD spectra are analysed by a matrix inversion technique to calculate individual DOD spectrum of the gases. The observed and calculated DODs are found to be in a good agreement. The coefficient of determination (R2) between observed and calculated DODs of NO2, O3, BrO, OClO, O4 (oxygen dimer), and Ring effect are observed to be 0.55, 0.77, 0.73, 0.75, 0.82, and 0.91, respectively. Filling-in of solar Fraunhofer lines in the observed zenith scattered sunlight is known as ‘Ring effect’. The slant column densities of the above gases are found to be increased due to increasing absorption path length with solar zenith angles. The vertical column densities (VCDs) of O3 and NO2 derived using ground-based spectrometer are compared with the Ozone Monitoring Instrument (OMI) on board Aura satellite during the period 1 March–31 December 2010. The day-to-day variations are found to be similar; however, the percentage differences in VCDs of O3 between ground-based spectrometer and satellite-based OMI are observed to be varying from 1% to 15%, while for NO2, they vary from 1% to 10%. Also, the seasonal mean values of VCDs of O3 and NO2 are discussed. The O3 mean values in the rainy season are found to be higher than that of in the summer and winter seasons from both ground- and satellite-based measurement. Whereas, the NO2 mean values in the winter season are found to be higher than that of in the summer and rainy seasons from both the measurement techniques. The VCDs of O3 are observed to be lowest in winter season due to the loss of ozone within NO2 and O3 reaction active during the winter season.

Relationship between Size of Cloud Ice and Lightning in the Tropics

The association of lightning flashes with mean cloud ice size over continental and oceanic region in the tropical areas has been analyzed using the observations from various satellite platforms (MODIS, TRMM, and LIS) for the period 2000–2011. We found that frequency of lightning in general is higher over the continental region compared to oceanic region, whereas larger size of cloud ice is observed over the oceanic regions compared to the continental regions. Relationship between lighting and cloud ice size shows similar features over both continental and oceanic regions. For the first time, we show that total lighting increases with increase in the cloud ice size; attends maximum at certain cloud ice size and then decreases with increase in cloud ice size. Maximum lightning occurred for the mean cloud ice size of around 23–25u2009µm over the continental region and mean cloud ice size of around 24–28u2009µm over the oceanic region. Based on our observation we argue that the relation between lightning and mean cloud ice size follow the curve linear pattern, and not linear.