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Saha U.,University of Calcutta | Maitra A.,University of Calcutta | Midya S.K.,University of Calcutta | Das G.K.,Regional Meteorological Center
Atmospheric Research | Year: 2014

Thunderstorm, associated with strong convective activity in the tropics, is one of the most prominent weather phenomena in the atmosphere. A critical analysis is done on the nature of variation of the thunderstorm frequencies over an urban metropolitan location Kolkata (22°32'N, 88°20'E), India with the pre-monsoon and monsoon rainfall amounts during the period 1997-2008. The occurrences of severe thunderstorms are decreasing during the last decade, although the number of ordinary thunderstorms occurring in this period has an increasing trend. A decrease in Convective Available Potential Energy (CAPE), Vertical Wind Shear (VWSH), Deep Layer Shear (0-6. km AGL) and an increase in Lifted Index (LI) may be an indicator for the suppression of the severity of thunderstorms over the urban location. There is also a decreasing trend for the pre-monsoon rainfall and an increasing trend in the monsoon rainfall amounts over the region. A further study indicated a significant positive correlation for all the types of thunderstorm (severe, ordinary and total) events with the pre-monsoon rainfall amount which are mainly associated with the vigorous convective phenomenon. On the other hand, a significant anti-correlation is observed between the severe thunderstorm frequencies with the monsoon rainfall amount for the same period. The decrease in the severity of the thunderstorm events is accompanied by an increase of pre-monsoon cloud Liquid Water Content (LWC) with Integrated Water Vapor (IWV). Hence, there is an expected strong association of the thunderstorm frequency with the pre-monsoon and monsoon rainfall amounts at this tropical location. Possible explanations are presented. © 2013 Elsevier B.V.

Dalal S.,Jadavpur University | Lohar D.,Jadavpur University | Sarkar S.,Jadavpur University | Sadhukhan I.,P.A. College | Debnath G.C.,Regional Meteorological Center
Atmospheric Research | Year: 2012

Premonsoon thunderstorms, locally known as Nor'westers, were studied over the eastern part of India using routine observations and data acquired from STORM (Severe Thunderstorm Observation and Regional Modelling) program during the premonsoon season, i.e., March through May, of 2006-08. Doppler radar image analysis reveals that premonsoon convective activities on many occasions may be described as squall-type linear Mesoscale Convective Systems (MCSs) which are composed of three common organizational modes viz. Trailing Stratiform (TS), Leading Stratiform (LS) and Parallel Stratiform (PS). The most dominant and common mode of organization, in terms of frequency of occurrences, duration, mean speed and inter-conversion among the different modes, is the TS, contributing about 65% of the cases while LS and PS contribute only about 15% and 20% respectively. Examination of pre-storm environments indicates that line-perpendicular and line-parallel storm-relative winds possibly determine the modes of organization. Case studies, one from each class, were also carried out and the observed structures were found to be similar to that observed in warmer mid-latitudes with certain exceptions. Unlike mid-latitude MCSs, convective cells during the premonsoon season initiate over the region with the support of weak synoptic setting and in course of time, organize themselves to become an MCS under favorable mesoscale convective environment. However they are short-lived irrespective of the modes of organization. © 2011 Elsevier B.V..

Routray A.,Indian Institute of Technology Delhi | Mohanty U.C.,Indian Institute of Technology Delhi | Rizvi S.R.H.,U.S. National Center for Atmospheric Research | Niyogi D.,Purdue University | And 2 more authors.
Quarterly Journal of the Royal Meteorological Society | Year: 2010

This work is a first assessment of utilizing Doppler Weather Radar (DWR) radial velocity and reflectivity in a mesoscale model for prediction of Bay of Bengal monsoon depressions (MDs). The Weather Research Forecasting (WRF) modelling system-Advanced Research version (ARW) is customized and evaluated for the Indian monsoon region by generating domain-specific Background Error (BE) statistics and experiments involving two assimilation strategies (cold start and cycling). The monthly averaged 24 h forecast errors for wind, temperature and moisture profiles were analysed. From the statistical skill scores, it is concluded that the cycling mode assimilation enhanced the performance of the WRF three-dimensional variational data assimilation (3DVAR) system over the Indian region using conventional and non-conventional observations. DWR data from a coastal site were assimilated for simulation of two different summer MDs over India using the WRF-3DVAR analysis system. Three numerical experiments (control without any Global Telecommunication System (GTS) data, with GTS, and GTS as well as DWR) were performed for simulating these extreme weather events to study the impact of DWR data.The results show that even though MDs are large synoptic systems, assimilation of DWR data has a positive impact on the prediction of the location, propagation and development of rain bands associated with the MDs. All aspects of the MD simulations such as mean-sea-level pressure, winds, vertical structure and the track are significantly improved due to the DWR assimilation. Study results provide a positive proof of concept that the assimilation of the Indian DWR data within WRF can help improve the simulation of intense convective systems influencing the large-scale monsoonal flow. © 2010 Royal Meteorological Society.

Lala Roy S.,Regional Meteorological Center | Chakravarty N.,Positional Astronomy Center
Mausam | Year: 2015

In the present work, statistical index are formed using the effective meteorological parameters for predicting the rainfall day and heavy rainfall day of Kolkata (22.53° N, 88.33° E) India during June to October utilizing the data of 25 years (1980-2005). Linear Discriminant Analysis (LDA), which is a multivariate statistical technique has been utilized to 22 selected meteorological parameters to find out the statistical index that has been utilized to predict the rainfall day and the heavy rainfall day for the period June to October for the year 2006-2008 and then validate with the actual rainfall day and heavy rainfall day of these years. It was found that it yielded 60.34% and 73.36% correct prediction for No rainfall days and rainfall days respectively during the period in the next three years (2006-2008). It yielded 90.98% and 84.21% correct prediction for No heavy rainfall days and Heavy rainfall days respectively during the period in the next three years (2006-2008). For all 22 parameters the efficient skill scores namely, True Skill Score (TSS), Heidke Skill Score (HSS), Critical Success Index (CSI) are computed for both rainfall prediction and heavy rainfall prediction. The investigation revealed that LDA technique is more efficient in prediction of heavy rainfall where there were sharp contrast between the parameters of the two groups, namely no heavy rainfall days and heavy rainfall days. © 2015, India Meteorological Department. All Rights Reserved.

Geetha B.,Regional Meteorological Center | Raj Y.E.,Regional Meteorological Center
Mausam | Year: 2015

The dates of onset and withdrawal of northeast monsoon (NEM) over Coastal Tamil Nadu (CTN) are determined for the 140 year period of 1871-2010 using daily rainfall data of 25 coastal stations of Tamil Nadu and South Andhra Pradesh. For the period 1901-2000, these dates are actually re-determined as they had earlier been determined by the second author using a coarse dataset of daily rainfall data of 4-6 coastal stations of Tamil Nadu. The methodology adopted is the same as the one adopted in the earlier study. The re-determined dates are compared with the ones determined earlier. For the period 1901-2000, the mean date of NEM onset and its standard deviation remain unchanged at 20 October and 8 days respectively, but, the mean date of NEM withdrawal is extended by three days, from 27 December to 30 December. For the periods 1871-1900 and 2001-2010, the mean dates of NEM onset are 17 October and 18 October respectively and the mean date of NEM withdrawal is the same for both the periods at 23 December. In the decadal scale, the mean date of NEM onset over CTN varies between 16-25 October and that of withdrawal, between 19 December to 9 January. Time series analysis of dates of NEM onset and withdrawal indicate that there is no significant trend in both the series. Also, there is no significant correlation between the two series and hence, for a given year, the dates of NEM onset and withdrawal are independent of each other. However, there are indications of existence of significant cross / lag / cross-lag correlations within and amongst the two series. Analysis of dates of NEM withdrawal on a sub-regional scale indicates that withdrawal of NEM from CTN occurs in a phased manner, with the withdrawal from the northern parts (mean date: 17 December) occurring about two weeks prior to the withdrawal from the central and southern parts of CTN (mean date : 31 December). © 2015 India Meteorological Department, All rights reserved.

Datta R.,Regional Meteorological Center
Mausam | Year: 2013

It is seen that in the Bay of Bengal or in the Gulf, most of the time the atmospheric phenomena, like, cyclone, hurricane etc. move towards right to its motion. To study such occurrences; we have considered fluid dynamics of ocean coupled with atmospheric motion. In the present study we have considered the eye of the cyclonic system that consist of fluid dynamical source and fluid dynamical sink at a small distance apart, and thus, constitute the fluid dynamical doublet of the object system. Then the fluid dynamical doublet of the object system and its image system has been considered with respect to a firm wall (here the sea shore). The fluid dynamical equation of complex potential with respect to the object system, the image system and the stream velocity have been undertaken. The complex potential of the object doublet, image doublet and the stream velocity have been considered. The velocity vector, consequently the pressure has been retrieve with the help of Bernoulli's equation of fluid motion. Then the minimum /maximum pressure on the wall that is on the sea shore has been calculated analytically. Thus, it is found that on the basis of some prevailing conditions existing wind and energy the cyclone or hurricane move towards the sea coast or to the right of its motion.

Balachandran S.,Regional Meteorological Center
Mausam | Year: 2013

Analysis of dynamical conditions in respect of formation and growth of tropical cyclone Khai Muk over the Bay of Bengal during 11-14 November 2008 is discussed with focus on barotropic energy conversion at lower level. It is observed that the extension of subtropical easterlies from the Western Pacific in to central Bay of Bengal to the north and equatorial westerlies to the south of ITCZ during the above period constituted a large scale horizontal shear flow. This led to generation of cyclonic shear vorticity and initiation of disturbance which later developed in to tropical cyclone Khai Muk. The basic zonal flow in the lower troposphere was barotropically unstable as depicted by change of sign of meridional distribution of absolute vorticity which provided the kinetic energy for the growth of eddy. There existed positive correlation between mean zonal flow and the meridional gradient of absolute vorticity which favoured increase of eddy kinetic energy through barotropic eddy-mean flow interactions. Energetic analysis indicated that areas of high rate of positive change of eddy kinetic energy coincided with positive areas of barotropic conversion and the magnitude of barotropic conversion matched with local rate of change of eddy kinetic energy around the area of vortex generation. Barotropic energy conversion by meridional shear of basic zonal flow was an important energy source for the formation and initial growth of tropical cyclone Khai Muk.

Asokan R.,Regional Meteorological Center | Amudha B.,Regional Meteorological Center
Mausam | Year: 2015

Temperature trends in the lower atmosphere are reported to be increasing due to natural and anthropogenic causes. The seasonal and annual variations in temperature in the northern and southern hemispheres over the Indian longitudes between 40° E and 120° E have been analysed using NCEP/NCAR reanalysis data set for the period 1948-2011. The analysis has been carried out for the satellite era 1979-2011 and for the whole period to bring out any plausible change in trends in temperature. It is observed that the annual trends indicate warming of the troposphere in both hemispheres whereas seasonal trends show mixed response obviously due to climatic forcing and land-ocean temperature contrasts. The variation in tropopause height is lesser over tropics of northern hemisphere than the southern hemisphere. The mean tropopause temperature is more in the satellite era. © 2015, India Meteorological Department. All rights reserved.

Deka R.L.,Indian Institute of Technology Guwahati | Mahanta C.,Indian Institute of Technology Guwahati | Pathak H.,Regional Meteorological Center | Nath K.K.,Assam Agricultural University | Das S.,Regional Meteorological Center
Theoretical and Applied Climatology | Year: 2013

The historical rainfall data of 110 years (1901-2010) of the two neighbouring basins of Brahmaputra and Barak of Assam, Northeast India were analyzed for monthly, seasonal and annual trends. The Mann-Kendall test and Sen's slope model were used to identify the trends and estimate the magnitude of change, respectively. The analysis revealed significant decreasing trend of rainfall during monsoon and post-monsoon seasons in the Barak basin during 1901-2010. Annual as well as monsoon rainfall during the recent 30-year normal period decreased significantly in both the basins. The last decade (2001-10) was the leanest decade in both the basins. Recent rainfall fluctuations with larger amplitudes indicate greater degree of uncertainties of heavy floods or short spell drought events. © 2012 Springer-Verlag Wien.

The surface pressure defect ΔP and the surface maximum wind speed Vm of a tropical cyclone which are two important measures for the intensity of a tropical cyclone are related by the well known relation Vm =K√ ΔP where K is the proportionality constant. Based on composites of observations of Vm and ΔP within the cyclone field, the empirical values of K have been derived in a large number of studies. The value of K thus derived has been found to vary in the range 10.5-16.0 when Vm and ΔP are measured in knots and hPa respectively. In this study the problem of estimating the value of K has been approached and treated from an entirely different angle. A general idealised pressure model derived using the concept of Pearson's Distributions in Statistical Distribution Theory has been initially assumed. Based on a few logical and acceptable assumptions such as - validity of cyclostrophic balance near the centre of the cyclone, existence of radius of maximum pressure gradient, convergence of integral defining cumulative surface pressure drop, relative vorticity to remain positive up to some distance from the centre, absolute vorticity to remain always positive within the cyclone field - the ranges of variables defining the general pressure model, and hence for K, have been derived without in any way relying upon actual observations of Vm or ΔP. After incorporating frictional forces, environmental flow and force due to translation speed of a tropical cyclone, the final value of K has been derived as 11.0. This theoretically estimated K value compared very well with the empirically derived values but was slightly on the lower side. Apparently most of the empirical K values derived in other studies generated overestimated Vm values for given ΔP and this aspect has been discussed.

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