Vertical structure of atmosphere in pre-monsoon season over Bangalore

Abstract

This paper discusses the variation of dry bulb and dew point temperature (T and Td) on the days with and without thunderstorm (TSD and NTSD) over Bangalore during pre-monsoon season. The thermodynamic parameters like convective available potential energy (CAPE), convective inhibition energy (CIN), precipitable water content (PWC) and dynamical parameter vertical wind shear difference (VWS) are studied. The mean profiles of T, Td are generated using March–May upper air data of 1730 hrs IST from 2000–2007 for Bangalore. These are also generated on the TSD and NTSD respectively. It is found that the difference between mean profile of T for TSD/NTSD and seasonal mean is negative/positive till 200 hPa. On the other hand, the difference of the seasonal mean of Td and that of Td on the TSD/NTSD is found to be positive/negative till 300 hPa. These results are found to be significant at 99% confidence. It is found that T is less than the mean at surface till 600 hPa on TSD, whereas it is 0.5◦C above average on the NTSD respectively. The difference between the Td on the TSD and mean Td is of the order of 3–5◦C till 300 hPa. On the NTSD, this difference ranges between −1 and −2◦C in the entire troposphere. The mean values of CAPE, CIN, PWC and VWS for Bangalore in pre-monsoon season are found to be 1324, 49.3 J/kg, 30 mm and −0.0007 s−1, respectively. These parameters were used as predictors for forecasting a thunderstorm. The critical success index and Heidke skill score were used for evaluating the forecast skill of the above parameters for 2 years from 2008 to 2009. CAPE and PWC are able to distinguish a TSD from that of a NTSD with 99% confidence. It is found that these scores are 0.44 and 0.35 for CAPE and 0.49 and 0.53 for precipitable water content.