U C Mohanty
Articles written in Journal of Earth System Science
Volume 102 Issue 1 March 1993 pp 73-87
With an objective to understand the influence of surface marine meteorological parameters in relation to the extreme monsoon activity over the Indian sub-continent leading to flood/drought, a detailed analysis of the sea level pressure over the Southern Hemisphere and various surface meteorological parameters over the Indian seas is carried out. The present study using the long term data sets (Southern Hemispheric Sea Level Pressure Analysis; Comprehensive Ocean Atmospheric Data Set over the Indian Seas; Surface Station Climatology Data) clearly indicates that the sea surface temperature changes over the south eastern Pacific (El Ninõ/La Niña) have only a moderate impact (not exceeding 50% reliability) on the Indian summer monsoon activity. On the other hand, the sea level pressure anomaly (SOI) over Australia and the south Pacific has a reasonably high degree of significance (more than 70%) with the monsoon activity over India. However, these two parameters (SLP and SST) do not show any significant variability over the Indian seas in relation to the summer monsoon activity.
Over the Indian seas, the parameters which are mainly associated with the convective activity such as cloud cover, relative humidity and the surface wind were found to have a strong association with the extreme monsoon activity (flood/drought) and thus the net oceanic heat loss over the Indian seas provides a strong positive feed-back for the monsoon activity over India.
Volume 104 Issue 1 March 1995 pp 49-77
The present study describes an analysis of Asian summer monsoon forecasts with an operational general circulation model (GCM) of the European Centre for Medium Range Weather Forecasts (ECMWF), U.K. An attempt is made to examine the influence of improved treatment of physical processes on the reduction of systematic errors. As some of the major changes in the parameterization of physical processes, such as modification to the infrared radiation scheme, deep cumulus convection scheme, introduction of the shallow convection scheme etc., were introduced during 1985–88, a thorough systematic error analysis of the ECMWF monsoon forecasts is carried out for a period prior to the incorporation of such changes i.e. summer monsoon season (June–August) of 1984, and for the corresponding period after relevant changes were implemented (summer monsoon season of 1988).
Monsoon forecasts of the ECMWF demonstrate an increasing trend of forecast skill after the implementation of the major changes in parameterizations of radiation, convection and land-surface processes. Further, the upper level flow is found to be more predictable than that of the lower level and wind forecasts display a better skill than temperature. Apart from this, a notable increase in the magnitudes of persistence error statistics indicates that the monsoon circulation in the analysed fields became more intense with the introduction of changes in the operational forecasting system.
Although, considerable reduction in systematic errors of the Asian summer monsoon forecasts is observed (up to day-5) with the introduction of major changes in the treatment of physical processes, the nature of errors remain unchanged (by day-10). The forecast errors of temperature and moisture in the middle troposphere are also reduced due to the changes in treatment of longwave radiation. Moreover, the introduction of shallow convection helped it further by enhancing the vertical transports of heat and moisture from the lower troposphere. Though, the hydrological cycle in the operational forecasts appears to have enhanced with the major modifications and improvements to the physical parameterization schemes, certain regional peculiarities have developed in the simulated rainfall distribution over the monsoon region. Hence, this study suggests further attempts to improve the formulations of physical processes for further reduction of systematic forecast errors.
Volume 104 Issue 4 December 1995 pp 635-666
A number of sensitivity experiments have been conducted to investigate the influence of using synthetic data on cyclone forecasts by a global spectral model. Some well known vortices have been used and the generated wind and pressure profiles are compared. It is found that the Rankine vortex and Holland’s vortex show the best representation of cyclonic circulation. Hence these two vortices are used in the sensitivity studies to simulate two cyclones, one of May 1979 and the other of August 1979. For this purpose the FGGE level-III b data set, produced at ECM WF, UK is used. Synthetic temperature and humidity data are also introduced to make the cyclones more realistic.
With the use of Holland’s vortex the system is found to move faster than with the Rankine vortex. Also, the tracks of the cyclones simulated with Rankine vortex are found to be on the left side of the observed track while that of Holland’s vortex is on the right side of the observed track. However, substantial filling up of the systems are noticed with introduction of diabatic initialization of the mass and velocity fields and the forecasts of both the vortices behave differently. It is suggested that proper selection of synthetic vortex, initialization scheme and resolution of the model are very important for better forecast of cyclones.
Volume 109 Issue 2 June 2000 pp 293-303
An attempt has been made to study the marine boundary layer characteristics over Bay of Bengal using BOBMEX (Bay of Bengal and Monsoon Experiment) pilot experiment data sets, which was conducted between 23rd October and 12th November 1998 on board ORV Sagar Kanya. A one-dimensional multilevel atmospheric boundary layer with TKE-ε closure scheme is employed to study the marine boundary layer characteristics. In this study two synoptic situations are chosen: one represents an active convection case and the other a suppressed convection. In the present article the marine boundary layer characteristics such as temporal evolution of turbulent kinetic energy, height of the boundary layer and the airsea exchange processes such as sensible and latent heat fluxes, drag coefficient for momentum are simulated during both active and suppressed convection. Marine boundary layer height is estimated from the vertical profiles of potential temperature using the stability criterion. The model simulations are compared with the available observations.
Volume 109 Issue 2 June 2000 pp 305-314
The present study is based on the observed features of the MBL (Marine Boundary Layer) during the Bay of Bengal and Monsoon Experiment (BOBMEX) — Pilot phase. Conserved Variable Analysis (CVA) of the conserved variables such as potential temperature, virtual potential temperature, equivalent potential temperature, saturation equivalent potential temperature and specific humidity were carried out at every point of upper air observation obtained on board ORV Sagar Kanya. The values are estimated up to a maximum of 4 km to cover the boundary layer. The Marine Boundary Layer Height is estimated from the conserved thermodynamic profiles. During the disturbed period when the convective activity is observed, the deeper boundary layers show double mixing line structures. An attempt is also made to study the oceanic heat budget using empirical models. The estimated short-wave radiation flux compared well with the observations.
Volume 111 Issue 3 September 2002 pp 351-364
Oceansat-1 was successfully launched by India in 1999, with two payloads, namely Multi-frequency Scanning Microwave Radiometer (MSMR) and Ocean Color Monitor (OCM) to study the biological and physical parameters of the ocean. The MSMR sensor is configured as an eight-channel radiometer using four frequencies with dual polarization. The MSMR data at 75 km resolution from the Oceansat-I have been assimilated in the National Centre for Medium Range Weather Forecasting (NCMRWF) data assimilation forecast system. The operational analysis and forecast system at NCMRWF is based on a T80L18 global spectral model and Spectral Statistical Interpolation (SSI) scheme for data analysis. The impact of the MSMR data is seen globally, however it is significant over the oceanic region where conventional data are rare. The dry-nature of the control analyses have been removed by utilizing the MSMR data. Therefore, the total precipitable water data from MSMR has been identified as a very crucial parameter in this study. The impact of surface wind speed from MSMR is to increase easterlies over the tropical Indian Ocean. Shifting of the positions of westerly troughs and ridges in the south Indian Ocean has contributed to reduction of temperature to around 30‡S.
Volume 111 Issue 3 September 2002 pp 365-378
In this study, the possible linkage between summer monsoon rainfall over India and surface meteorological fields (basic fields and heat budget components) over monsoon region (30‡E-120‡E, 30‡S30‡N) during the pre-monsoon month of May and summer monsoon season (June to September) are examined. For this purpose, monthly surface meteorological fields anomaly are analyzed for 42 years (1958-1999) using reanalysis data of NCEP/NCAR (National Center for Environmental Prediction/National Center for Atmospheric Research). The statistical significance of the anomaly (difference) between the surplus and deficient monsoon years in the surface meteorological fields are also examined by Student’s t-test at 95% confidence level.
Significant negative anomalies of mean sea level pressure are observed over India, Arabian Sea and Arabian Peninsular in the pre-monsoon month of May and monsoon season. Significant positive anomalies in the zonal and meridional wind (at 2 m) in the month of May are observed in the west Arabian Sea off Somali coast and for monsoon season it is in the central Arabian Sea that extends up to Somalia. Significant positive anomalies of the surface temperature and air temperature (at 2 m) in the month of May are observed over north India and adjoining Pakistan and Afghanistan region. During monsoon season this region is replaced by significant negative anomalies. In the month of May, significant positive anomalies of cloud amount are observed over Somali coast, north Bay of Bengal and adjoining West Bengal and Bangladesh. During monsoon season, cloud amount shows positive anomalies over NW India and north Arabian Sea.
There is overall reduction in the incoming shortwave radiation flux during surplus monsoon years. A higher magnitude of latent heat flux is also found in surplus monsoon years for the month of May as well as the monsoon season. The significant positive anomaly of latent heat flux in May, observed over southwest Arabian Sea, may be considered as an advance indicator of the possible behavior of the subsequent monsoon season. The distribution of net heat flux is predominantly negative over eastern Arabian Sea, Bay of Bengal and Indian Ocean. Anomaly between the two extreme monsoon years in post 1980 (i.e., 1988 and 1987) shows that shortwave flux, latent heat flux and net heat flux indicate reversal in sign, particularly in south Indian Ocean. Variations of the heat budget components over four smaller sectors of Indian seas, namely Arabian Sea, Bay of Bengal and west Indian Ocean and east Indian Ocean show that a small sector of Arabian Sea is most dominant during May and other sectors showing reversal in sign of latent heat flux during monsoon season.
Volume 112 Issue 1 March 2003 pp 79-93
The present study is carried out to examine the performance of a regional atmospheric model in forecasting tropical cyclones over the Bay of Bengal and its sensitivity to horizontal resolution. Two cyclones, which formed over the Bay of Bengal during the years 1995 and 1997, are simulated using a regional weather prediction model with two horizontal resolutions of 165 km and 55 km. The model is found to perform reasonably well towards simulation of the storms. The structure, intensity and track of the cyclones are found to be better simulated by finer resolution of the model as compared to the coarse resolution. Rainfall amount and its distribution are also found to be sensitive to the model horizontal resolution. Other important fields, viz., vertical velocity, horizontal divergence and horizontal moisture flux are also found to be sensitive to model horizontal resolution and are better simulated by the model with finer horizontal grids.
Volume 112 Issue 1 March 2003 pp 95-111
In this study, we present the mean seasonal features of the Indian summer monsoon circulation in the National Centre for Medium Range Weather Forecasting (NCMRWF) global data assimilation and forecast system. The large-scale budgets of heat and moisture are examined in the analyzed and model atmosphere. The daily operational analyses and forecasts (day 1 through day 5) produced for the summer seasons comprising June, July and August of 1995 and 1993 have been considered for the purpose. The principal aim of the study is two-fold. Primarily, to comprehend the influence of the systematic errors over the Indian summer monsoon, secondarily, to analyze the performance of the model in capturing the interseasonal variability.
The heat and moisture balances show reduction in the influx of heat and moisture in the model forecasts compared to the analyzed atmosphere over the monsoon domain. Consequently, the diabatic heating also indicates reducing trend with increase in the forecast period. In effect, the strength of Indian summer monsoon, which essentially depends on these parameters, weakens considerably in the model forecasts. Despite producing feeble monsoon circulation, the model captures interseasonal variability realistically. Although, 1995 and 1993 are fairly normal monsoon seasons, the former received more rainfall compared to the latter in certain pockets of the monsoon domain. This is clearly indicated by the analyzed and model atmosphere in terms of energetics.
Volume 112 Issue 2 June 2003 pp 147-163
Convective activity is one of the major processes in the atmosphere influencing the local and large-scale weather in the tropics. The latent heat released by the cumulus cloud is known to drive monsoon circulation, which on the other hand supplies the moisture that maintains the cumulus clouds. An investigation is carried out on the convective structure of the atmosphere during active and suppressed periods of convection using data sets obtained from the Bay of Bengal and Monsoon Experiment (BOBMEX). The cumulus convection though being a small-scale phenomenon, still influences its embedding environment by interaction through various scales. This study shows the variation in the kinematic and convective parameters during the transition from suppressed to active periods of convection. Convergence in the lower levels and strong upward vertical velocity, significant during active convection are associated with the formation of monsoon depressions. The apparent heat source due to latent heat release and the vertical transport of the eddy heat by cumulus convection, and the apparent moisture sink due to net condensation and vertical divergence of the eddy transport of moisture, are estimated through residuals of the thermodynamic equation and examined in relation to monsoon activity during BOBMEX.
Volume 112 Issue 2 June 2003 pp 165-184
The skill and efficiency of a numerical model mostly varies with the quality of initial values, accuracy on parameterization of physical processes and horizontal and vertical resolution of the model. Commonly used low-resolution reanalyses are hardly able to capture the prominent features associated with organized convective processes in a monsoon depression. The objective is to prepare improved high-resolution analysis by the use of MM5 modelling system developed by the Pennsylvania State University/National Center for Atmospheric Research (PSU/NCAR). It requires the objective comparison of high and low-resolution analysis datasets in assessing the specific convective features of a monsoon depression. For this purpose, reanalysis datasets of NCAR/NCEP (National Center for Atmospheric Research/National Centers for Environmental Prediction) at a horizontal resolution of 2.5‡ (latitude/longitude) have been used as first guess in the objective analysis scheme. The additional asynoptic datasets obtained during BOBMEX-99 are utilized within the assimilation process. Cloud Motion Wind (CMW) data of METEOSAT satellite and SSM/I surface wind data are included for the improvement of derived analysis. The multiquadric (MQD) interpolation technique is selected and applied for meteorological objective analysis at a horizontal resolution of 30 km. After a successful inclusion of additional data, the resulting reanalysis is able to produce the structure of convective organization as well as prominent synoptic features associated with monsoon depression. Comparison and error verifications have been done with the help of available upper-air station data. The objective verification reveals the efficiency of the analysis scheme.
Volume 112 Issue 2 June 2003 pp 185-204
The characteristic features of the marine boundary layer (MBL) over the Bay of Bengal during the southwest monsoon and the factors influencing it are investigated. The Bay of Bengal and Monsoon Experiment (BOBMEX) carried out during July–August 1999 is the first observational experiment under the Indian Climate Research Programme (ICRP). A very high-resolution data in the vertical was obtained during this experiment, which was used to study the MBL characteristics off the east coast of India in the north and south Bay of Bengal. Spells of active and suppressed convection over the Bay were observed, of which, three representative convective episodes were considered for the study. For this purpose a one-dimensional multi-level PBL model with a TKE-ε closure scheme was used. The soundings, viz., the vertical profiles of temperature, humidity, zonal and meridional component of wind, obtained onboard ORV Sagar Kanya and from coastal stations along the east coast are used for the study. The temporal evolution of turbulent kinetic energy, marine boundary layer height (MBLH), sensible and latent heat fluxes and drag coefficient of momentum are simulated for different epochs of monsoon and monsoon depressions during BOBMEX-99.The model also generates the vertical profiles of potential temperature, specific humidity, zonal and meridional wind. These simulated values compared reasonably well with the observations available from BOBMEX.
Volume 113 Issue 3 September 2004 pp 281-298
The study delineates the vorticity and angular momentum balances of Asian summer monsoon during the evolution and established phases. It also elucidates the differences between these balances in the National Centre for Environmental Prediction/National Centre for Atmospheric Research (NCEP/NCAR) reanalysis and the National Centre for Medium Range Weather Forecasts (NCMRWF) analysis fields. The NCEP/NCAR reanalysis for a 40 year period (1958-97) and the NCMRWF analysis for a three year (1994-96) period are made use of for the purpose. The time mean summer monsoon circulation is bifurcated into stable mean and transient eddy components and the mean component is elucidated.
The generation of vorticity due to stretching of isobars balances most of the vorticity transported out of the monsoon domain during the evolution period. However, during the established period, the transportation by the relative and planetary vorticity components exceeds the generation due to stretching. The effective balancing mechanism is provided by vorticity generation due to sub-grid scale processes. The flux convergence of omega and relative momenta over the monsoon domain is effectively balanced by pressure torque during the evolution and established phases. Nevertheless, the balance is stronger during the established period due to the increase in the strength of circulation.
Both the NCMRWF and NCEP fields indicate the mean features related to vorticity and angular momentum budgets realistically. Apart from the oceanic bias (strong circulation over oceans rather than continents), the summer monsoon circulation indicated by the NCEP is feeble compared to NCMRWF. The significant terms in the large-scale budgets of vorticity and angular momentum enunciate this aspect
Volume 114 Issue 1 February 2005 pp 17-36
Orissa is one of the most flood prone states of India. The floods in Orissa mostly occur during monsoon season due to very heavy rainfall caused by synoptic scale monsoon disturbances. Hence a study is undertaken to find out the characteristic features of very heavy rainfall (24 hours rainfall ≥125 mm) over Orissa during summer monsoon season (June–September) by analysing 20 years (1980–1999) daily rainfall data of different stations in Orissa. The principal objective of this study is to find out the role of synoptic scale monsoon disturbances in spatial and temporal variability of very heavy rainfall over Orissa.
Most of the very heavy rainfall events occur in July and August. The region, extending from central part of coastal Orissa in the southeast towards Sambalpur district in the northwest, experiences higher frequency and higher intensity of very heavy rainfall with less interannual variability. It is due to the fact that most of the causative synoptic disturbances like low pressure systems (LPS) develop over northwest (NW) Bay of Bengal with minimum interannual variation and the monsoon trough extends in west-northwesterly direction from the centre of the system. The very heavy rainfall occurs more frequently with less interannual variability on the western side of Eastern Ghat during all the months and the season except September. It occurs more frequently with less interannual variability on the eastern side of Eastern Ghat during September. The NW Bay followed by Gangetic West Bengal/Orissa is the most favourable region of LPS to cause very heavy rainfall over different parts of Orissa except eastern side of Eastern Ghat. The NW Bay and west central (WC) Bay are equally favourable regions of LPS to cause very heavy rainfall over eastern side of Eastern Ghat. The frequency of very heavy rainfall does not show any significant trend in recent years over Orissa except some places in north-east Orissa which exhibit significant rising trend in all the monsoon months and the season as a whole.
Volume 117 Issue 6 December 2008 pp 897-909
Doppler radar derived wind speed and direction proﬁles showed a well developed sea breeze circulation over the Chennai, India region on 28 June, 2003. Rainfall totals in excess of 100 mm resulted from convection along the sea breeze front. Inland propagation of the sea breeze front was observed in radar reﬂectivity imagery. High-resolution MM5 simulations were used to investigate the inﬂuence of Chennai urban land use on sea breeze initiated convection and precipitation. A comparison of observed and simulated 10 m wind speed and direction over Chennai showed that the model was able to simulate the timing and strength of the sea breeze. Urban effects are shown to increase the near surface air temperature over Chennai by 3.0 K during the early morning hours. The larger surface temperature gradient along the coast due to urban effects increased onshore ﬂow by 4.0m s−1. Model sensitivity study revealed that precipitation totals were enhanced by 25 mm over a large region 150 km west of Chennai due to urban effects. Deﬁciency in model physics related to night-time forecasts are addressed.
Volume 118 Issue 5 October 2009 pp 413-440
The change in the type of vegetation fraction can induce major changes in the local effects such as local evaporation,surface radiation,etc.,that in turn induces changes in the model simulated outputs.The present study deals with the effects of vegetation in climate modeling over the Indian region using the MM5 mesoscale model.The main objective of the present study is to investigate the impact of vegetation dataset derived from SPOT satellite by ISRO (Indian Space Research Organization)
The study reveals mixed results on the impact of vegetation datasets generated by ISRO and USGS on the simulations of the monsoon.Results indicate that the ISRO data has a positive impact on the simulations of the monsoon over northeastern India and along the western coast.The MM5- USGS has greater tendency of overestimation of rainfall.It has higher standard deviation indicating that it induces a dispersive effect on the rainfall simulation.Among the ﬁve years of study,it is seen that the RMSE of July and JJAS (June –July –August –September)for All India Rainfall is mostly lower for MM5-ISRO.Also,the bias of July and JJAS rainfall is mostly closer to unity for MM5-ISRO.The wind ﬁelds at 850 hPa and 200 hPa are also better simulated by MM5 using ISRO vegetation.The synoptic features like Somali jet and Tibetan anticyclone are simulated closer to the veri ﬁcation analysis by ISRO vegetation.The 2 m air temperature is also better simulated by ISRO vegetation over the northeastern India,showing greater spatial variability over the region. However,the JJAS total rainfall over north India and Deccan coast is better simulated using the USGS vegetation.Sensible heat ﬂux over north-west India is also better simulated by MM5-USGS.
Volume 120 Issue 5 October 2011 pp 795-805
The northeast (NE) monsoon season (October, November and December) is the major period of rainfall activity over south peninsular India. This study is mainly focused on the prediction of northeast monsoon rainfall using lead-1 products (forecasts for the season issued in beginning of September) of seven general circulation models (GCMs). An examination of the performances of these GCMs during hindcast runs (1982–2008) indicates that these models are not able to simulate the observed interannual variability of rainfall. Inaccurate response of the models to sea surface temperatures may be one of the probable reasons for the poor performance of these models to predict seasonal mean rainfall anomalies over the study domain. An attempt has been made to improve the accuracy of predicted rainfall using three different multi-model ensemble (MME) schemes,
Volume 121 Issue 2 April 2012 pp 297-316
Thunderstorm, resulting from vigorous convective activity, is one of the most spectacular weather phenomena in the atmosphere. A common feature of the weather during the pre-monsoon season over the Indo-Gangetic Plain and northeast India is the outburst of severe local convective storms, commonly known as ‘Nor’westers’(as they move from northwest to southeast). The severe thunderstorms associated with thunder, squall lines, lightning and hail cause extensive losses in agricultural, damage to structure and also loss of life. In this paper, sensitivity experiments have been conducted with the Non-hydrostatic Mesoscale Model (NMM) to test the impact of three microphysical schemes in capturing the severe thunderstorm event occurred over Kolkata on 15 May 2009. The results show that the WRF-NMM model with Ferrier microphysical scheme appears to reproduce the cloud and precipitation processes more realistically than other schemes. Also, we have made an attempt to diagnose four severe thunderstorms that occurred during pre-monsoon seasons of 2006, 2007 and 2008 through the simulated radar reflectivity fields from NMM model with Ferrier microphysics scheme and validated the model results with Kolkata Doppler Weather Radar (DWR) observations. Composite radar reflectivity simulated by WRF-NMM model clearly shows the severe thunderstorm movement as observed by DWR imageries, but failed to capture the intensity as in observations. The results of these analyses demonstrated the capability of high resolution WRF-NMM model in the simulation of severe thunderstorm events and determined that the 3 km model improve upon current abilities when it comes to simulating severe thunderstorms over east Indian region.
Volume 124 Issue 7 October 2015 pp 1531-1544
The heavy rainfall event during 14–17 June 2013 in Uttarakhand and more specifically, its occurrence around the Kedarnath region on 16 June 2013 with devastating floods and massive landslides ruined thousands of lives and properties. Increasing levels of water in two main rivers of the State, namely Alaknanda and Mandakini, resulted in the collapse of bridges, damaging and washing away of property worth many crores. In the present study, the advanced research version of the Weather Research and Forecasting (WRF) system (hereafter used as WRF model) is used to simulate this heavy rainfall event. The synoptic analysis at different locations such as Rudraprayag, Uttarkashi, Chamoli, Pauri, Tehri, Dehradun and adjoining districts suggested that the rainfall is about 200% more than normal. The rainfall associated with this event is well captured with the model simulation. The rainfall simulated by WRF model is in the range of 320–400 mm over Kedarnath during the actual occurrence of the event, which is in reasonably good agreement with the observed value of rainfall (325 mm) collected by Wadia Institute of Himalayan Geology.