The interaction of large scale and mesoscale environment leading to formation of intense thunderstorms over Kolkata part I: Doppler radar and satellite observations

The weather systems that predominantly affect the eastern and northeastern parts of India during the pre-monsoon summer months (March, April and May) are severe thunderstorms, known as Nor'westers. The storms derive their names from the fact that they frequently strike cities and towns in the southern part of West Bengal in the afternoon from the north-west direction while traveling far from its place of genesis over the Bihar plateau. The storms are devastating in nature particularly due to strong (gusty) winds, heavy rains and hails associated with it. Although these storms are well known for its power of causing damages, studies on them are relatively few due to their small size and sparse network of observations. To address this important issue, the evolution of two Nor'westers of 12 March and 22 May 2003 over Kolkata is studied in detail in this paper using hourly Doppler weather radar (DWR) observations and high resolution Meteosat-5 imageries. In addition, supporting meteorological reports are used to find the large scale conditions that influence the moisture convergence and vertical wind shear. The genesis of both the storms is found to be over Bihar-Jharkhand region and beyond the range of the DWR. The satellite observations are found to be useful in identifying the location and initiation of the storms. The movements of the storms are captured by the DWR estimated vertical cross-section of reflectivities. The Doppler estimate shows that the 12 March storm had a vertical extent of about 10-12 km at the time of maturity and that of 22 May reaching up to 18 km signifying deep convection associated with these events. The genesis, maturity and dissipation are well brought out by the hourly DWR and satellite imageries. The DWR observations suggest that the systems move at a speed of 20-25m/s. The DWR estimated precipitation shows a detailed spatial distribution around Kolkata with several localized zones of heavy rain and this is found to be well supported by the nearby station observations. This study establishes that DWR observations along with hourly satellite imageries are able to capture the evolution of Nor'westers. The study also shows that the composite DWR-satellite information is a reliable tool for nowcasting the location, time and path of movement of Nor'westers. Based on these observations, a conceptual model of the Nor'wester is proposed. © Printed in India

Investigation of features of May, 2001 tropical cyclone over the Arabian Sea through IRS-P4 and other satellite data

In this paper, utility of satellite derived atmospheric motion vectors and geophysical parameters is brought out to discern appropriate signals for improving short-range forecasts in respect of development/dissipation of tropical cyclones over the Indian region. Results of a particular case study of May, 2001 cyclone, which formed in the Arabian Sea are reported. Analysis of wind field with input of modified cloud motion vectors and water vapour wind vectors is performed utilizing Optimum Interpolation (OI) technique at 850 and 200 hPa for finding dynamical changes such as vorticity, convergence and divergence for the complete life period of this cyclone. Simultaneously, variations in geophysical parameters obtained from IRS-P4 and TRMM satellites in ascending and descending nodes are compared with dynamical variations for discerning some positive signals to improve short range forecasts over the Indian region. The enhancement of cyclonic vorticity at 200 hPa over larger area surrounding center of cyclone was observed from 26 to 28 May 2001 which gave a positive signal for dissipation of storm

Tropical Cyclone Hazard to Mumbai in the Recent Historical Climate

The hazard to the city of Mumbai, India, from a possible severe tropical cyclone under the recent historical climate is considered. The authors first determine, based on a review of primary sources, that the Bombay Cyclone of 1882, documented in a number of print and Internet sources and claimed to have caused 100 000 or more deaths, did not occur. Two different tropical cyclone hazard models, both of which generate large numbers of synthetic cyclones using environmental data—here taken from reanalyses in the satellite era—as input, are then used to quantify the hazard, in conjunction with historical observations. Both models indicate that a severe cyclone landfall at or near Mumbai is possible, though unlikely in any given year. Return periods for wind speeds exceeding 100 kt (1 kt = 0.5144 m s−1) (the threshold for category 3 in the Saffir–Simpson hurricane wind scale) at Mumbai itself are estimated to be in the range of thousands to greater than 10 000 years, while the return period for a storm with maximum wind speed of 100 kt or greater passing within 150 km of Mumbai (possibly close enough to generate a substantial storm surge at the city) is estimated to be around 500 years. Return periods for winds exceeding 65 kt (hurricane intensity on the Saffir–Simpson hurricane wind scale) are estimated to be around 200 years at Mumbai itself, and 50–90 years within 150 km. Climate change is not explicitly considered in this study, but the hazard to the city is likely to be increasing because of sea level rise as well as changes in storm climatology

Meteorological objective analysis using multiquadric interpolation scheme over India and adjoining region

This paper is concerned with the development of the multiquadric interpolation scheme to produce gridded fields of meteorological variables. The results of the application of this method to real data is compared with analysis from Gandin's Optimum Interpolation scheme. Like the optimum interpolation scheme, which uses covariance functions as the basis functions, the multiquadric scheme uses hyperboloid radial basis function to fit the scattered data to a uniform grid. This scheme produces superior analysis compared to optimum interpolation analysis

Impact of revised simplified Arakawa-Schubert scheme on the simulation of mean and diurnal variability associated with active and break phases of Indian Summer Monsoon using CFSv2

The impact of revised simplified Arakawa-Schubert (RSAS) convective parameterization scheme in Climate Forecast System (CFS) version 2 (CFSv2) on the simulation of active and break phases of Indian summer monsoon (ISM) has been investigated. The results revealed that RSAS showed better fidelity in simulating monsoon features from diurnal to daily scales during active and break periods as compared to SAS simulation. Prominent improvement can be noted in simulating diurnal phase of precipitation in RSAS over central India (CI) and equatorial Indian Ocean (EIO) region during active periods. The spatial distribution of precipitation largely improved in RSAS simulation during active and break episodes. CFSv2 with SAS simulation has noticeable dry bias over CI and wet bias over EIO region which appeared to be largely reduced in RSAS simulation during both phases of the intraseasonal oscillation (ISO). During active periods, RSAS simulates more realistic probability distribution function (PDF) in good agreement with the observation. The relative improvement has been identified in outgoing longwave radiation, monsoon circulations, and vertical velocities in RSAS over SAS simulation. The improvement of rainfall distribution appears to be contributed by proper simulation of convective rainfall in RSAS. CFSv2 with RSAS simulation is able to simulate observed diurnal cycle of rainfall over CI. It correctly reproduces the time of maximum rainfall over CI. It is found that the improved feedback between moisture and convective processes in RSAS may be attributed to its improved simulation. Besides improvement, RSAS could not reproduce proper tropospheric temperature, cloud hydrometeors over ISM domain which shows the scope for future development

Meteorological objective analysis using three-dimensional numerical variational technique

In this paper an objective analysis scheme using a three-dimensional numerical variational technique has been developed over India and the surrounding region based on the assumption of quasi-geostrophic and thermal wind conditions. Analyses of height, wind and temperature fields at different pressure levels have been made for the depression case which is formed over the Bay of Bengal during the period 26 tb 31 July 1991. The analysis scheme is able to capture the system and to bring out the importance of upper tropospheric warming before the formation of the monsoon depression

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The National Centre for Environmental Prediction (NCEP) Climate Forecast System (CFS) is being used for operational monsoon prediction over the Indian region. Recent studies indicate that the moist convective process in CFS is one of the major sources of uncertainty in monsoon predictions. In this study, the existing simple cloud microphysics of CFS is replaced by the six-class Weather Research Forecasting (WRF) single moment (WSM6) microphysical scheme. Additionally, a revised convective parameterization is employed to improve the performance of the model in simulating the boreal summer mean climate and intraseasonal variability over the Indian summer monsoon (ISM) region. The revised version of the model (CFSCR) exhibits a potential to improve shortcomings in the seasonal mean precipitation distribution relative to the standard CFS (CTRL), especially over the ISM region. Consistently, notable improvements are also evident in other observed ISM characteristics. These improvements are found to be associated with a better simulation of spatial and vertical distributions of cloud hydrometeors in CFSCR. A reasonable representation of the subgrid-scale convective parameterization along with cloud hydrometeors helps to improve the convective and large-scale precipitation distribution in the model. As a consequence, the simulated low-frequency boreal summer intraseasonal oscillation (BSISO) exhibits realistic propagation and the observed northwest-southeast rainband is well reproduced in CFSCR. Additionally, both the high and low-frequency BSISOs are better captured in CFSCR. The improvement of low and high-frequency BSISOs in CFSCR is shown to be related to a realistic phase relationship of clouds

Renal functional and hemodynamic (blood volume, renal blood flow and glomerular filtration rate) study in protein calorie malnutrition

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