Cloud Condensation Nuclei over the Bay of Bengal during the Indian Summer Monsoon

The first measurements of cloud condensation nuclei （CCN） at five supersaturations were carried out onboard the research vessel ＂Sagar Kanya＂ （cruise SK-296） from the south to the head-bay of the Bay of Bengal as part of the Continental Tropical Convergence Zone （CTCZ） Project during the Indian summer monsoon of 2012. In this paper, we assess the diurnal variation in CCN distributions at supersaturations from 0.2% to 1% （in steps of 0.2%） and the power-law fit at supersaturation of 1%. The diurnal pattern shows peaks in CCN concentration （NccN） at supersaturations from 0.2% to 1% between 0600 and 0700 LST （local standard time, UTC＋0530）, with relatively low concentrations between 1200 and 1400 LST, followed by a peak at around 1800 LST. The power-law fit for the CCN distribution at different supersaturation levels relates the empirical exponent （k） of supersaturation （%） and the Nccy at a supersaturation of 1%. The Nccy at a supersaturation of 0.4% is observed to vary from 702 cm-3 to 1289 cm-3, with a mean of 961 ＋ 161 cm-3 （95% confidence interval）, representing the CCN activity of marine air masses. Whereas, the mean Nccy of 1628 ＋ 193 cm-3 at a supersaturation of 1% is higher than anticipated for the marine background. When the number of CCN spectra is 1293, the value of k is 0.57 ± 0.03 （99% confidence interval） and its probability distribution shows cumulative counts significant at k ≈ 0.55 ± 0.25. The results are found to be better at representing the features of the marine environment （103 cm-3 and k ≈ 0.5） and useful for validating CCN closure studies for Indian sea regions.

Seasonal Variation of Urban Heat Island and Its Impact on Air-Quality Using SAFAR Observations at Delhi, India

This paper discussed the urban heat island (UHI) intensity and local air quality by using observational data of project of the System of Air Quality Forecasting and Research (SAFAR) over Delhi during the month of May and December 2013. It is found that UHI magnitudes ~2.2°C and ~1.5°C are formed at the evening traffic hours during May and December respectively. Also, intensity of UHI °C over daytime is referred as Urban Cool Island (UCI) during May and December. The diurnal PM2.5 concentration shows a bimodal pattern with peaks at morning and evening traffic hours during May and December. The planetary boundary layer height (PBLH) values show higher in magnitude during the daytime and lower in magnitude during the night-time. Whereas, the Ground Heat Flux values are lower during the daytime and higher during the night-time. The wind speed shows lower values during the UHI and higher magnitudes during the UCI formation hours. Concentration of PM2.5 and wind speed shows a strong negative correlation during May (r = -0.56, p = 0.002) and December (r = -0.57, p = 0.001) at C V Raman (CVR) site, however, high values in the concentration of PM2.5 during the low wind speed favour the condition for the formation of UCI. The regression analysis indicated that PM2.5 plays a significant role in the daytime cooling and nighttime warming over the urban areas during the low wind speed condition.

Association of Rainfall and Stability Index with Lightning Parameter over the Indo-Gangetic Plains

The Lightning Imaging Sensor (LIS) based satellite lightning grid data for 10 year period (1998-2007) were used to study the association of rainfall and stability index with lightning parameter over Indo-Gangetic plain (IGP) region. The spatial variation of flash rate density (FRD) is found to be (40 fl·km-2·yr-1) higher over northern region of IGP as compare to that of eastern IGP region. The annual variation of FRD exhibits bimodal distributions, while the precipitation rate shows unimodal distributions. The results show that the FRD peaked 2 months (pre-monsoon) in advance to the monsoon months where rainfall peak occurred due to environmental lapse rates more than 7.0°C/km during pre-monsoon which is evident from the temperature profile for correlation coefficient between temperature (700 mb) and FRD with coefficient of 0.70, p ≤ 0.0001 during pre-monsoon. The annual variation of lifted index show negative value over March to September due to intense insolation, convective available potential energy (CAPE) and also availability of moisture. The convective cloud transform into thundercloud with the development of mixed-phase (cloud water + ice) which subsequently produce the lightning. During monsoon, seasonal thermal heating diminishes and even on revival after break monsoon period, K-index is found to be less as the orography does not allow the highly moist air of low temperature to reach to large height above freezing level. They can be referred as maritime clouds of intermediate height with moderate updraft and hence minimum lightning activity during the monsoon season. Lifted index are proved to be indicators of thunderstorm conditions. This is because that rising air parcel is much warmer than its surroundings and can accelerate rapidly and create severe thunderstorms.