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Narkhedkar S.D.S.G.,Indian Institute of Tropical Meteorology | Sikka D.R.,Mausam Vihar
Mausam | Year: 2011

An attempt has been made to compare dynamically the recent two drought years, viz., 2002 and 2009, from energetics aspects. For that different energy terms, their generation and conversion among different terms have been computed during 1 st May - 30 th September for the above two years over a limited region between 65° E & 95° E, 5° N & 35° N. These computations are based on daily NCEP 2.5° × 2.5° data during 1 st May - 30 th September of the above two years. The study shows that although for both the years we had drought, the mean monsoon circulation itself was much weaker in 2009 as compared to 2002, in daily, monthly as well as in the seasonal scale. The weaker mean monsoon circulation, in daily scale, has been demonstrated by comparatively longer spells during June-September 2009 with negative value of conversion of zonal available potential energy to zonal kinetic energy [C(A z, K z)]. Large negative values of the monthly total as well as seasonal total of [C(A z, K z)] indicate weaker mean monsoon circulation in monthly and in the seasonal scale. The study also shows that in both years, the Seasonal total (June-September) of conversion of zonal available potential energy to eddy available potential energy [C(A z, A E)] was positive, but in 2009 its order of magnitude was 10 -4 J/kg/cm 2/sec where as in 2002 it was 10 -6 J/kg/cm 2/sec, indicating that the influence of mid-latitude westerly was much more in 2009, which may have attributed towards weaker mean monsoon circulation in 2009.

Nor'wester studies have a long history of climatological, synoptic and radar observations. These studies have been briefly mentioned and the field programs for the study of Nor'westers implemented in India Meteorological Department (IMD) from 1931-1941 have been touched upon. Indian atmospheric science community organized a multi-year STORM program during 2007-2010 to understand the formation of these severe local storms and also understand their dynamics through modeling. An attempt is made to use INSAT Infrared and Visible imageries to document the convective cells which developed over Eastern and North-East (NE) Indian states and adjoining countries of Bangladesh, Bhutan and Nepal for the year 2009. Also convective cells which organized themselves into Mesoscale Convective Complexes (MCCs) for the four years period 2007-2010 have been studied. It is found that by and large Eastern India (Jharkhand, Orissa, Sub Himalayan West Bengal and Bangladesh) is responsible for the initiation of convection. Development occurs as the cells propagate over the neighbouring areas of Bangladesh and NE India. Important observations with regard to initiation, maturity and dissipation etc. of the MCCs are provided. It is suggested that half hourly to hourly monitoring of convection can be accomplished by using INSAT imagery, along with multiple overlapping radar coverages, which could help in nowcasting of convective cells. Synoptic and thermodynamic forcing can help as broad guidance. The only effective way for effective warning is nowcasting using satellite and multiple radar coverage. © 2012 Government of India.

Narkhedkar S.G.,Indian Institute of Tropical Meteorology | Sikka D.R.,Mausam Vihar
Atmosfera | Year: 2012

An attempt has been made to make a composite energetics study for the three contrasting types south west monsoon season (SWMS) over India, viz. marginally normal SWMS (2000 and 2001 when seasonal total rainfall was very close to 90%), normal SWMS (2003, 2005, 2006, 2007 and 2008 when seasonal total rainfall was very close to 100% of its long period average) and deficient SWMS (2002, 2004 and 2009, when seasonal total rainfall was less than 90% of its long period average). For that, decadal average for the decade 2000-2009 and anomaly, based on above decadal average, for individual year of this decade, of different energy terms, their generation and conversion among different terms have been computed daily during 1 May-30 September in the recent decade (2000-2009) over a limited region between 65°E to 95°E, 5°N to 35°N. These computations are based on daily NCEP 2.5°x 2.5°data during 1 May-30 September of the above ten years. The composite of these anomalies have then been constructed for the deficient monsoon years (2002, 2004 and 2009), marginally normal monsoon years (2000 and 2001) and for the normal monsoon years (2003, 2005, 2006, 2007 and 2008). In the decadal average, a steady fall in C(A Z, K Z) and C(A E, K E) till August and steady rise in C(A Z, A E) till July are noticed, indicating a suppressed mean monsoon circulation due to the influence of mid latitude baroclinic westerly systems. Analysis of the composite anomaly of different energetics parameter indicates that the deficient SWM, in the decade under study, are characterized by weaker mean monsoon circulation, as compared to normal or marginally normal SWM, in daily, monthly and seasonal scale, due to anomalous influence of mid latitude baroclinic westerly systems.

Mohapatra M.,Mausam Bhavan | Sikka D.R.,Mausam Vihar | Bandyopadhyay B.K.,Mausam Bhavan | Tyagi A.,MoES
Mausam | Year: 2013

A program has been evolved for improvement in prediction of track and intensity of tropical cyclones over the Bay of Bengal resulting in the Forecast Demonstration Project (FDP). FDP program is aimed to demonstrate the ability of various Numerical Weather Prediction (NWP) models to assess the genesis, intensification and movement of cyclones over the north Indian ocean with enhanced observations over the data sparse region and to incorporate modification into the models which could be specific to the Bay of Bengal based on the in-situ measurements. FDP Program is scheduled in three phases, viz., (i) Pre-pilot phase (15 Oct - 30 Nov 2008, 2009, (ii) Pilot phase (15 Oct - 30 Nov, 2010-2012) and (iii) Final phase (15 Oct - 30 Nov, 2013-14). India is planning to take up aircraft probing of cyclones over the Bay of Bengal during 15 Oct - 30 Nov, 2013-14 with hired aircraft and dropsonde experiments. To accomplish the above objective, the initiative was carried out with priorities on (i) observational upgradation, (ii) modernization of cyclone analysis and prediction system, (iii) cyclone analysis and forecasting procedure, (iv) warning products generation, presentation & dissemination, (v) confidence building measures and capacity building. Various strategies were adopted for improvement of observation, analysis and prediction of cyclone. Several national institutions participated for joint observational, communicational & NWP activities during the pre-pilot and pilot phases of FDP campaign during 2008-11. The comparison of observational systems before and after FDP indicates a significant improvement in terms of Radar, Automatic Weather Station (AWS), high wind speed recorders over the region. It has resulted in reduction in monitoring and forecasting errors. The performance of NWP models have increased along with the introduction of NWP platforms like IMD GFS, WRF, HWRF and ensemble prediction system (EPS). Salient features of achievements along with the problems and prospects of this project are presented and discussed in this paper. With repeated attempts, the aircraft probing of cyclones could not be possible till now. It is a major challenge for the future campaign during 2013-14.

Yesubabu V.,University of Pune | Islam S.,University of Pune | Sikka D.R.,Mausam Vihar | Kaginalkar A.,University of Pune | Kashid S.,University of Pune
Natural Hazards | Year: 2014

Prediction of heavy rainfall events due to severe convective storms in terms of their spatial and temporal scales is a challenging task for an operational forecaster. The present study is about a record-breaking heavy rainfall event observed in Pune (18°31′N, 73°55′E) on October 4, 2010. The day witnessed highest 24-h accumulated precipitation of 181.3 mm and caused flash floods in the city. The WRF model-based real-time weather system, operating daily at Centre for Development of Advanced Computing using PARAM Yuva supercomputer showed the signature of this convective event 4-h before, but failed to capture the actual peak rainfall and its location with reference to the city's observational network. To investigate further, five numerical experiments were conducted to check the impact of assimilation of observations in the WRF model forecast. First, a control experiment was conducted with initialization using National Centre for Environmental Prediction (NCEP)'s Global Forecast System 0.5° data, while surface observational data from NCEP Prepbufr system were assimilated in the second experiment (VARSFC). In the third experiment (VARAMV), NCEP Prepbufr atmospheric motion vectors were assimilated. Fourth experiment (VARPRO) was assimilated with conventional soundings data, and all the available NCEP Prepbufr observations were assimilated in the fifth experiment (VARALL). Model runs were compared with observations from automated weather stations (AWS), synoptic charts of Indian Meteorological Department (IMD). Comparison of 24-h accumulated rainfall with IMD AWS 24-h gridded data showed that the fifth experiment (VARALL) produced better picture of heavy rainfall, maximum up to 251 mm/day toward the southern side, 31 km away from Pune's IMD observatory. It was noticed that the effect of soundings observations experiment (VARPRO) caused heavy precipitation of 210 mm toward the southern side 49 km away from Pune. The wind analysis at 850 and 200 hPa indicated that the surface and atmospheric motion vector observations (VARAMV) helped in shifting its peak rainfall toward Pune, IMD observatory by 18 km, though VARALL over-predicted rainfall by 60 mm than the observed. © 2013 Springer Science+Business Media Dordrecht.

Dimri A.P.,Jawaharlal Nehru University | Niyogi D.,Purdue University | Barros A.P.,Duke University | Ridley J.,Office Hadley Center | And 3 more authors.
Reviews of Geophysics | Year: 2015

Cyclonic storms associated with the midlatitude Subtropical Westerly Jet (SWJ), referred to as Western Disturbances (WDs), play a critical role in the meteorology of the Indian subcontinent. WDs embedded in the southward propagating SWJ produce extreme precipitation over northern India and are further enhanced over the Himalayas due to orographic land-atmosphere interactions. During December, January, and February, WD snowfall is the dominant precipitation input to establish and sustain regional snowpack, replenishing regional water resources. Spring melt is the major source of runoff to northern Indian rivers and can be linked to important hydrologic processes from aquifer recharge to flashfloods. Understanding the dynamical structure, evolution-decay, and interaction of WDs with the Himalayas is therefore necessary to improve knowledge which has wide ranging socioeconomic implications beyond short-term disaster response including cold season agricultural activities, management of water resources, and development of vulnerability-adaptive measures. In addition, WD wintertime precipitation provides critical mass input to existing glaciers and modulates the albedo characteristics of the Himalayas and Tibetan Plateau, affecting large-scale circulation and the onset of the succeeding Indian Summer Monsoon. Assessing the impacts of climate variability and change on the Indian subcontinent requires fundamental understanding of the dynamics of WDs. In particular, projected changes in the structure of the SWJ will influence evolution-decay processes of the WDs and impact Himalayan regional water availability. This review synthesizes past research on WDs with a perspective to provide a comprehensive assessment of the state of knowledge to assist both researchers and policymakers, and context for future research. © 2015. American Geophysical Union. All Rights Reserved.

The paper is devoted to examine the ability of a high-resolution National Center for Environmental Prediction (NCEP) T170/L42 Atmospheric General Circulation Model (AGCM), for exploring its utility for long-range dynamical prediction of seasonal Indian summer monsoon rainfall (ISMR) based on 5-members ensemble for the hindcast mode 20-year (1985-2004) period with observed global sea surface temperatures (SSTs) as boundary condition and 6-year (2005-2010) period in the forecast-mode with NCEP Coupled Forecast System (CFS) SSTs as boundary condition. ISMR simulations are examined on five day (pentad) rainfall average basis. It is shown that the model simulated ISMR, based on 5-members ensemble average basis had limited skill in simulating extreme ISMR seasons (drought/excess ISMR). However, if the ensemble averaging is restricted to similar ensemble members either in the overall run of pentad-wise below (B) and above (A) normal rainfall events, as determined by the departure for the threshold value given by coefficient of variability (CV) for the respective pentads based on IMD observed climatology, or during the season as a whole on the basis of percentage anomaly of ISMR from the seasonal climatology, the foreshadowing of drought/excess monsoon seasons improved considerably. Our strategy of improving dynamical seasonal prediction of ISMR was based on the premise that the intra-seasonal variability (ISV) and intra-annual variability (IAV) are intimately connected and characterized by large scale perturbations westward moving (10-20 day) and northward moving (30-60 day) modes of monsoon ISV during the summer monsoon season. As such the cumulative excess of B events in the simulated season would correspond to drought season and vice-versa. The paper also examines El Niño-Monsoon connections of the simulated ISMR series and they appear to have improved considerably in the proposed methodology. This strategy was particularly found to improve for foreshadowing of droughts. Based on results of the study a strategy is proposed for using the matched signal for simulated ISMR based on excess B over A events and vice-versa for drought or excess ISMR category. The probability distribution for the forecast seasonal ISMR on category basis is also proposed to be based on the relative ratio of similar ensemble members and total ensembles on percentage basis. The paper also discusses that extreme monsoon season are produced by the El Niño-Southern Oscillation (ENSO) and the Indian Ocean Dipole (IOD) modes in a combined manner and hence stresses to improve prediction of IOD mode in ocean-atmosphere coupled model just as it has happened for the prediction ENSO mode six to nine months in advance.

Sikka D.R.,Mausam Vihar | Tomar C.S.,Lodi Road | Laskar S.I.,Lodi Road | Goyal S.,Lodi Road | Tyagi A.,Ministry of Earth Science
Mausam | Year: 2013

Indian atmosphere-ocean science community organised a field programme known as Continental Tropical Convergence Zone (CTCZ) during 2011 to understand the synoptic and sub-seasonal fluctuations of summer monsoon over India. One of the objectives of the programme was to understand the evolution of the large scale circulation in relation to synoptic & intra-seasonal fluctuations of the monsoon. The paper addresses the large scale fluctuations of Monsoon-2011. The Monsoon-2011 performed very close to the normal rainfall of the season. However, on the sub-seasonal scale its performance was good during June, became deficient during July and it recovered from the beginning of August to the end of September. The early part of the season was accompanied by near neutral La-Nina conditions. However, sea surface temperatures (SSTs) in the Equatorial Eastern Pacific Ocean shifted to below normal (La-Nina conditions) from August to the end of September. As the Gangetic Plain was swept by marine origin air stream in the lower troposphere, the aerosol load remained much below the normal during the entire season. The paper discusses different aspect of synoptic, sub-seasonal fluctuations of monsoon in relation to remote forcing of the SSTs in the Pacific and the local forcing of the Indian Ocean Dipole (IOD). Also the performance of the operational numerical weather prediction (NWP) models: European Centre for Medium-Range Weather Forecasts (ECMWF), Global Forecast System (GFS) and Weather Research and Forecasting (WRF) is discussed. Several areas of further research using CTCZ data are also suggested based on the performance of the Monsoon-2011.

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