NISCAIR Online Periodicals Repository Collection: IJRSP Vol.41(4) [August 2012]

Forecasting and nowcasting convective weather phenomena over southern peninsular India – Part II: Severe local storms

Sun, 29 Jul 2012 22:58:59 GMT

Title: Forecasting and nowcasting convective weather phenomena over southern peninsular India – Part II: Severe local storms

Authors: Suresh, R

Abstract: A few case studies of severe thunderstorms especially hail storms, over Chennai and its neighbourhood within 250 km radius that occurred during 2002-2007 have been analysed in this paper. Pre-convective thermodynamic parameters have been computed using Chennai, Karaikal, Bangalore, Machilipatnam and Hyderabad RS/RW data; and empirical thresholds that are in vogue have been validated for the hail storms reported by print and electronic media. Hail stone size estimation has been attempted using build 9 algorithm of NEXRAD, USA and found to be working well for the tropical environment as well. The necessity of feedback from the public in timely reporting severe storms with precision and accuracy has been stressed for developing a database to fine tune the hail warning and hail stone size estimation algorithms.

Page(s): 435-447

Regional climate model simulations of the 2009 Indian summer monsoon

Sun, 29 Jul 2012 22:58:59 GMT

Title: Regional climate model simulations of the 2009 Indian summer monsoon

Authors: Bhate, Jyoti; Unnikrishnan, C K; Rajeevan, M

Abstract: Using a regional climate model, the 2009 Indian summer monsoon circulation and rainfall are simulated using observed sea surface temperatures as boundary conditions. For this purpose, the Weather Research and Forecasting (WRF) model (V 3.2) was used as a regional climate model. The model simulations were made for the period 1 May to 30 September with 1 May initial conditions with two domains of 45 km and 15 km and 51 vertical levels. The large scale monsoon circulation and rainfall patterns simulated by the model were examined along with the diurnal and intra-seasonal variations during the 2009 monsoon season using the TRMM 3G68 rainfall data. The results showed a wet bias in the model rainfall simulations over the Indian region with more rainfall in the model compared to the TRMM rainfall observations. This wet bias was attributed to stronger low level monsoon flow in the model simulations over the Indian region. Even though, the model simulations of atmospheric humidity are reasonably accurate, the positive bias in the low level vorticity contributed to the wet bias of rainfall over the Indian region. Model simulations showed errors in characterizing the diurnal variation of monsoon rainfall over the Indian region, especially in the observed phase angle (time of rainfall peak). The model simulations could not capture the observed early morning rainfall peak along the foothills of Himalayas caused by katabatic wind flow over the hills. Model simulations of amplitude of diurnal variation are, however, comparable with the observed amplitude derived from TRMM satellite data. Model simulations also showed encouraging results in simulating the intra-seasonal rainfall variations over the central Indian region and the monsoon onset phase over the Kerala coast.

Page(s): 488-500

Some characteristics of earth-space path propagation phenomena at a tropical location

Sun, 29 Jul 2012 22:58:59 GMT

Title: Some characteristics of earth-space path propagation phenomena at a tropical location

Authors: Maitra, Animesh; Adhikari, Arpita; Bhattacharya, Aniruddha

Abstract: Two significant propagation characteristics of Ku-band satellite signal, namely rain attenuation and depolarization have been studied over Kolkata (22°34'N, 88°29'E), a tropical location. The existing Simple Attenuation Model (SAM) for rain attenuation estimation has been improved by incorporating the effective rain rate at different rain rates. The impact of rain drop size distribution (DSD) on determining the extent of depolarization at the tropical location is investigated. The lognormal distribution has been found to be more accurate than the gamma distribution to model rain drop sizes in the present case. Rain DSD varies for different types of rain, even during different phases of rain event and is found to influence the cross-polar enhancement of Ku-band signal. The annual variation of cross-polar enhancement indicates the dominance of different types of rain during different seasons of the year.

Page(s): 481-487

Variability of raindrop size distribution and its impact on polarimetric rain rate estimators

Sun, 29 Jul 2012 22:58:59 GMT

Title: Variability of raindrop size distribution and its impact on polarimetric rain rate estimators

Authors: Jyothi, K Amar; Rao, T N

Abstract: Four years of raindrop size distribution (DSD) by Joss-Waldvogel (JW) Disdrometer measurements made at Gadanki (13.5°N, 79.2°E) have been used to derive relations for rainfall estimation from polarimetric radar variables. The polarimetric variables are estimated in X-band (at 9.368 GHz, the chosen frequency for the radar being developed at Gadanki) from the observed DSD measurements and scattering amplitudes derived using T-matrix scattering simulations. Three rainfall estimators, namely R-Z, R-K_DP and R-(Z_H, Z_DR), are derived for both stratiform and convective types of rain. Also, following the earlier reports, which highlighted the large variability of DSD between the seasons, the above relations are derived for three seasons (pre-monsoon, southwest monsoon and northeast monsoon). The scatter plots of R and Z show large scatter around the regression fits, even after separating the data into different seasons and types of rain, indicating the large and complex variability of DSD. Among all the relations, R-Z relation depends heavily on the DSD with its coefficients vary significantly between the seasons and types of rain. The other two relations show weak dependency on DSD, however, the coefficients are found to be distinctly different from those reported elsewhere. Both qualitative and quantitative evaluation analyses on a case study reveal that R-(Z_H,Z_DR) relation provides better R among the three relations.

Page(s): 471-480