Momentum transport of wave zero during March: A possible predictor for the Indian summer monsoon

Analysis of monthly momentum transport of zonal waves at 850 hPa for the period 1979 to 1993, between ‡S and ‡N for January to April, using zonal (u) and meridional (v) components of wind taken from the ECMWF reanalysis field, shows a positive correlation (.1% level of significance) between the Indian summer monsoon rainfall (June through September) and the momentum transport of wave zero TM(0) over latitudinal belt between 25‡S and 5‡N (LB) during March. Northward (Southward) TM(0) observed in March over LB subsequently leads to a good (drought) monsoon season over India which is found to be true even when the year is marked with the El-Nino event. Similarly a strong westerly zone in the Indian Ocean during March, indicates a good monsoon season for the country, even if the year is marked with El-Nino. The study thus suggests two predictors, TM(0) over LB and the strength of westerly zone in the Indian Ocean during March.     

Circulation characteristics of a monsoon depression during BOBMEX-99 using high-resolution analysis

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.     

Equatorial modes observed in atmospheric variables

Wavenumber-frequency spectral analysis of different atmospheric variables has been carried out using 25 years of data. The area considered is the tropical belt 25°S–25°N. A combined FFT-wavelet analysis method has been used for this purpose. Variables considered are outgoing long-wave radiation (OLR), 850 hPa divergence, zonal and meridional winds at 850, 500 and 200 hPa levels, sea level pressure and 850 hPa geopotential height. It is shown that the spectra of different variables have some common properties, but each variable also has few features different from the rest. While Kelvin mode is prominent in OLR and zonal winds, it is not clearly observed in pressure and geopotential height fields; the latter two have a dominant wavenumber zero mode not seen in other variables except in meridional wind at 200 hPa and 850 hPa divergences. Different dominant modes in the tropics show significant variations on sub-seasonal time scales.     

Utility of DMSP-SSM/I for integrated water vapour over the Indian seas

Recent algorithms for Special Sensor Microwave/Imager (DMSP-SSM/I) satellite data are used for estimating integrated water vapour over the Indian seas. Integrated water vapour obtained from these algorithms is compared with that derived from radiosonde observations at Minicoy and Port Blair islands. Algorithm-3 of Schlussel and Emery (1990) performed best. On the basis of this algorithm, distribution of integrated water vapour is determined during the monsoon depression (22nd–27th July, 1992) that formed over the Bay of Bengal.     

Impact of assimilation of ATOVS temperature and humidity and SSM/I total precipitable water on the simulation of a monsoon depression

The present study explored the effect of assimilation of Advanced TIROS Vertical Sounder (ATOVS) temperature and humidity profiles and Spectral sensor microwave imager (SSM/I) total precipitable water (TPW) on the simulation of a monsoon depression which formed over the Arabian Sea during September 2005 using the Weather Research and Forecast model. The three-dimensional variational (3DVAR) data assimilation technique has been employed for the purpose of assimilation of satellite observations. Statistical scores like “equitable threat score,” “bias score,” “forecast impact,” and “improvement parameter” have been used to examine the impact of the above-mentioned satellite observations on the numerical simulation of a monsoon depression. The diagnostics of this study include verification of the vertical structure of depression, in terms of temperature anomaly profiles and relative vorticity profiles with observations/analysis. Additional diagnostics of the study include the analysis of the heat budget and moisture budget. Such budget studies have been performed to provide information on the role of cumulus convection associated with the depression. The results of this study show direct and good evidence of the impact of the assimilation of the satellite observations using 3DVAR on the dynamical and thermodynamical features of a monsoon depression along with the effect of inclusion of satellite observation on the spatial pattern of the simulated precipitation associated with the depression. The “forecast impact” parameter calculated for the wind speed provides good evidence of the positive impact of the assimilation of ATOVS temperature and humidity profiles and SSM/I TPW on the model simulation, with the assimilation of the ATOVS profiles showing better impact in terms of a more positive value of the “forecast impact” parameter. The results of the study also indicate the improvement of the forecast skill in terms of “equitable threat score” and “bias score” due to the assimilation of satellite observation.     

Interannual variability of onset of the summer monsoon over India and its prediction

In this article, the interannual variability of certain dynamic and thermodynamic characteristics of various sectors in the Asian summer monsoon domain was examined during the onset phase over the south Indian peninsula (Kerala Coast). Daily average (0000 and 1200 UTC) reanalysis data sets of the National Centre for Environmental Prediction/National Centre for Atmospheric Research (NCEP/NCAR) for the period 1948–1999 were used. Based on 52 years onset date of the Indian summer monsoon, we categorized the pre-onset, onset, and post-onset periods (each an average of 5 days) to investigate the interannual variability of significant budget terms over the Arabian Sea, Bay of Bengal, and the Indian peninsula. A higher difference was noticed in low-level kinetic energy (850 hPa) and the vertically integrated generation of kinetic energy over the Arabian Sea from the pre-onset, onset, and post-onset periods. Also, significant changes were noticed in the net tropospheric moisture and diabatic heating over the Arabian Sea and Indian peninsula from the pre-onset to the post-onset period. It appears that attaining the magnitude of 40 m² s⁻² and then a sharp rise in kinetic energy at 850 hPa is an appropriate time to declare the onset of the summer monsoon over India. In addition to a sufficient level of net tropospheric moisture (40 mm), a minimum strength of low-level flow is needed to trigger convective activity over the Arabian Sea and the Bay of Bengal. An attempt was also made to develop a location-specific prediction of onset dates of the summer monsoon over India based on energetics and basic meteorological parameters using multivariate statistical techniques. The regression technique was developed with the data of May and June for 42 years (1948–1989) and validated with 10 years NCEP reanalysis from 1990 to 1999. It was found that the predicted onset dates from the regression model are fairly in agreement with the observed onset dates obtained from the Indian Meteorology Department.     

High resolution numerical weather prediction over the Indian subcontinent

In this study, the Florida State University Global Spectral Model (FSUGSM), in association with a high-resolution nested regional spectral model (FSUNRSM), is used for short-range weather forecasts over the Indian domain. Three-day forecasts for each day of August 1998 were performed using different versions of the FSUGSM and FSUNRSM and were compared with the observed fields (analysis) obtained from the European Center for Medium Range Weather Forecasts (ECMWF). The impact of physical initialization (a procedure that assimilates observed rain rates into the model atmosphere through a set of reverse algorithms) on rainfall forecasts was examined in detail. A very high nowcasting skill for precipitation is obtained through the use of high-resolution physical initialization applied at the regional model level. Higher skills in wind and precipitation forecasts over the Indian summer monsoon region are achieved using this version of the regional model with physical initialization. A relatively new concept, called the ‘multimodel/multianalysis superensemble’ is described in this paper and is applied for the wind and precipitation forecasts over the Indian subcontinent. Large improvement in forecast skills of wind at 850 hPa level over the Indian subcontinent is shown possible through the use of the multimodel superensemble. The multianalysis superensemble approach that uses the latest satellite data from the Tropical Rainfall Measuring Mission (TRMM) and the Defense Meteorological Satellite Program (DMSP) has shown significant improvement in the skills of precipitation forecasts over the Indian monsoon region.     

Meteorological fields variability over the Indian seas in pre and summer monsoon months during extreme monsoon seasons

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.     

Retrieval of vertical wind profiles during monsoon from satellite observed winds over the Indian ocean using complex EOF analysis

The aim of this paper is to study the feasibility of deriving vertical wind profiles from current satellite observations. With this aim, we carried out complex empirical orthogonal function (CEOF) analysis of a large number of radiosonde observations of wind profiles over the Indian Ocean during the monsoon months. It has been found that the first two CEOFs explain 67% of the total variance in wind fields. While the first principal component is well correlated with the winds at 850 mb (r = 0:80), the second one is highly correlated with winds at 200 mb (r = 0:89). This analysis formed the basis of a retrieval algorithm which ensures the retrieval of vertical profiles of winds using satellite tracked cloud motion vector winds. Under the assumption that accurate measurements of wind are available at the above mentioned levels, the r.m.s. error of retrieval of each component of wind is estimated to range between 2 ms^-1 and 6 ms^-1 at different levels, which is much less than the natural variance of winds at these levels. For a better visualization of retrieval, we have provided retrieved and true wind profiles side by side for four typical synoptic conditions during the monsoon season.     

Retrieval of sea surface velocities using sequential Ocean Colour Monitor (OCM) data

The Indian remote sensing satellite, IRS-P4 (Oceansat-I) launched on May 26th, 1999 carried two sensors on board, i.e., the Ocean Colour Monitor (OCM) and the Multi-frequency Scanning Microwave Radiometer (MSMR) dedicated for oceanographic research. Sequential data of IRS-P4 OCM has been analysed over parts of both east and west coast of India and a methodology to retrieve sea surface current velocities has been applied. The method is based on matching suspended sediment dispersion patterns, in sequential two time lapsed images. The pattern matching is performed on a pair of atmospherically corrected and geo-referenced sequential images by Maximum Cross-Correlation (MCC) technique. The MCC technique involves computing matrices of cross-correlation coefficients and identifying correlation peaks. The movement of the pattern can be calculated knowing the displacement of windows required to match patterns in successive images. The technique provides actual flow during a specified period by integrating both tidal and wind influences. The current velocities retrieved were compared with synchronous data collected along the east coast during the GSI cruise ST-133 of R.V. Samudra Kaustubh in January 2000. The current data were measured using the ocean current meter supplied by the Environmental Measurement and CONtrol (EMCON), Kochi available with the Geological Survey of India, Marine Wing. This current meter can measure direction and magnitude with an accuracy of ±5‡ and 2% respectively. The measurement accuracies with coefficient of determination (R ² ) of 0.99, for both magnitude (cm.s^-1) and direction (deg.) were achieved.     

The effect of a surface data assimilation technique and the traditional four-dimensional data assimilation on the simulation of a monsoon depression over India using a mesoscale model

The objective of this study is to investigate the impact of a surface data assimilation (SDA) technique, together with the traditional four-dimensional data assimilation (FDDA), on the simulation of a monsoon depression that formed over India during the field phase of the 1999 Bay of Bengal Monsoon Experiment (BOBMEX). The SDA uses the analyzed surface data to continuously assimilate the surface layer temperature as well as the water vapor mixing ratio in the mesoscale model. The depression for the greater part of this study was offshore and since successful application of the SDA would require surface information, a method of estimating surface temperature and surface humidity using NOAA-TOVS satellites was used. Three sets of numerical experiments were performed using a coupled mesoscale model. The first set, called CONTROL, uses the NCEP (National Center for Environmental Prediction) reanalysis for the initial and lateral boundary conditions in the MM5 simulation. The second and the third sets implemented the SDA of temperature and moisture together with the traditional FDDA scheme available in the MM5 model. The second set of MM5 simulation implemented the SDA scheme only over the land areas, and the third set extended the SDA technique over land as well as sea. Both the second and third sets of the MM5 simulation used the NOAA-TOVS and QuikSCAT satellite and conventional upper air and surface meteorological data to provide an improved analysis. The results of the three sets of MM5 simulations are compared with one another and with the analysis and the BOBMEX 1999 buoy, ship, and radiosonde observations. The predicted sea level pressure of both the model runs with assimilation resembles the analysis closely and also captures the large-scale structure of the monsoon depression well. The central sea level pressures of the depression for both the model runs with assimilation were 2–4 hPa lower than the CONTROL. The results of both the model runs with assimilation indicate a larger spatial area as well as increased rainfall amounts over the coastal regions after landfall compared with the CONTROL. The impact of FDDA and SDA, the latter over land, resulted in reduced errors of the following: 1.45 K in temperature, 0.39 m s⁻¹ in wind speed, and 14° in wind direction compared with the BOBMEX buoy observation, and 1.43 m s⁻¹ in wind speed, 43° in wind direction, and 0.75% in relative humidity compared with the CONTROL. The impact of SDA over land and sea compared with SDA over land only showed a further marginal reduction of errors: 0.23 K in air temperature (BOBMEX buoy) and 1.33 m s⁻¹ in wind speed simulations.     

Surface and upper air meteorological features during onset phase of 2003 monsoon

The second campaign of the Arabian Sea Monsoon Experiment (ARMEX-II) was conducted in two phases viz., March–April and May–June 2003. In the present work, the buoy and ocean research vessel data collected during the second phase of ARMEX-II have been analysed to bring out the characteristic features of monsoon onset. The results have shown that the thermodynamical features such as build up of lower tropospheric instability and increased height of zero degree isotherm occurred about a week before the monsoon onset over Kerala and adjoining southeast Arabian Sea. There was a sharp fall in the temperature difference between 850 and 500 hPa, and the height of zero degree isotherm about 2–3 days before the monsoon onset. The flux of sensible heat was positive (sea to air) over south Arabian Sea during the onset phase. Over the Bay of Bengal higher negative (air to sea) values of sensible flux prevailed before the monsoon onset which became less negative with the advance of monsoon over that region. The pre-onset period was characterized by large sea surface temperature (SST) gradient over the Arabian Sea with rapid decrease towards north of the warm pool region. The buoy observations have shown that SST remained close to 30.5°C in the warm pool region during the pre-onset period in 2003 but only 2–3 degrees away (north of this region) SSTs were as low as 28.5–29°C. An interesting aspect of sea level pressure (SLP) variability over the Indian seas during the onset phase of summer monsoon 2003 was undoubtedly, the highest SLP in the warm pool region inspite of very high SSTs.     

黑龙江省两次温带气旋暴雪过程对比分析

利用多种资料对黑龙江省两次由江淮气旋和蒙古气旋合并引发的暴雪过程的水汽、热动力条件和中尺度特征进行了对比分析。结果表明：（1）两次暴雪过程都发生在北支槽和短波槽合并、北支槽北部有冷涡的背景下,850 hPa上低涡合并促使江淮气旋和蒙古气旋合并;气旋合并后,低空急流为降雪提供了充足水汽,强暖平流使气旋爆发性发展,导致降雪加强。（2）两次降雪过程都表现出逗点云系的合并发展,"1211 "暴雪过程中高层形成涡旋偏西,700 hPa低涡东部偏南风引导气旋北上西折,低空急流和地形共同作用使暖湿空气强烈辐合上升,产生对流云,暴雪发生在A类逗点云系的头部,降雪强度大,范围广;"1412"暴雪过程高空槽低涡位置偏东,700 hPa低涡东部西南风始终引导气旋向东北方向移动,近地面层具有冷垫,暴雪主要发生在B类气旋逗点云系头部西侧中低云团中,降雪范围和强度较"1211"过程小。（3）低层（0.3 km）冷空气侵入和中高层（5.5 km）转为偏北风对判断降雪开始和结束有很好的表征意义。（4）冷涡前部强高压脊使冷涡移动缓慢,从而延长了降水的持续时间,气旋移动路径与高压脊伸展方向密切相关。     

Salient features of Andhra Pradesh cyclonic storm in the Bay of Bengal during September 1997

During 23–30 September 1997, a rare cyclonic storm has developed close to the Andhra coast, and it has later travelled parallel to coastline northward and finally crossed the land at Chittagong (22°N, 91°E) on 27 September. While translating along the east coast of India, it has produced heavy to very heavy rainfall on the coastal stations causing devastating floods. In this study, we made an attempt to understand the salient causes of this unique cyclone movement. We have analyzed daily fields of wind and relative humidity for 850, 700, 500 hPa and mean daily OLR data to understand the plausible reasons for its movement. The buoy data deployed by National Institute of Ocean Technology, Chennai, Viz. DS5 (15°N, 81°E), DS4 (19°N, 88°E) and SW7 (20°N, 86°E) were analyzed to understand the ocean–atmosphere interaction processes in the west Bay of Bengal during formation of the system. Analysis of OLR over the cyclonic storm region has revealed that the heavy rainfall areas coincide with low OLR (120–180 W m^?2). The persistent southward movement of 500 hPa ridge on the eastern wedge of the system along with the steering current at 200 hPa has helped in maintaining the movement of the system parallel to the east coast of India during its life cycle.     

Impact of vegetation on the simulation of seasonal monsoon rainfall over the Indian subcontinent using a regional model

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) versus that of USGS (United States Geological Survey) vegetation dataset on the simulation of the Indian summer monsoon. The present study has been conducted for five monsoon seasons (1998–2002), giving emphasis over the two contrasting southwest monsoon seasons of 1998 (normal) and 2002 (deficient). 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 verification 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.     

Remote sensing of atmospheric water vapour from Bhaskara IISAMIR data and its comparison withNOAA-7 water vapour data

Applying the method of ‘statistical linear regression’, atomspheric water vapour over oceanic areas has been estimated from the 19GHz and 22 GHz data of the satellite microwave radiometer (SAMIR) system onboard the Bhaskara II satellite. In the absence of any simultaneousin situ measurements on water vapour over ocean, theSAMIR-derived water vapour data have been compared with like data from theNOAA-7 satellite. It is suggested that a positive bias seen in theSAMIR data could be due to calibration errors in the basic data. In view of the observed bias, the original regression equation is modified and then used to obtain water vapour distributions over ocean for winter and south-west monsoon seasons usingSAMIR data of several orbits.     

On the water and energy cycles in the Tropics

The water and energy cycles are major elements of the Earth climate. These cycles are especially active in the intertropical belt where satellites provide the most suitable observational platform. The history of Earth observations of the water cycle and of the radiation budget viewed from space reveals that the fundamental questions from the early times are still relevant for today's research. The last 2 decades have seen a number of milestones regarding the documentation of rainfall, mesoscale convective systems (MCS), water vapour and radiation at the top of the atmosphere (TOA). Beyond dedicated missions that provided enhanced characterizations of some elements of the atmospheric water cycle and field campaigns that allowed the gathering of validation data, the advent of the long record of meteorological satellites lead to new questioning on the homogenisation of the data time series, etc. The use of this record to document the tropical climate brought new results of the distribution of humidity and reinforced the understanding of some robust features of the African monsoon. Challenges for the immediate future concerns the deepening of the understanding of the role of cloud systems in the monsoon circulation, the downscaling of the documentation of the water and energy cycle at the scale of these cloud systems, the research of better adequation between the users and the satellite estimate of rainfall and finally a much needed methodological effort to build exploitable time series for the estimation of climatic trends in the water and energy cycle in the Tropics. The required observations to address these challenges are rapidly presented with emphasis on the upcoming Megha-Tropiques (MT) mission.     

Thermodynamic structure of the marine atmosphere over the region 80–87°E along 13°N during August (phase II) BOBMEX-99

Thermodynamic structure of the marine atmosphere in the region between 80 and 87‡E along 13‡N over the Bay of Bengal was studied using 13 high resolution radiosonde profiles from surface-400 hPa collected onboard ORV Sagar Kanya during the period 27th–30th August, during BOBMEX-99. Saturation point concept, mixing line analysis and conserved variable diagrams have been used to identify boundary layer characteristics such as air mass movement and stability of the atmosphere. The results showed relatively dry air near the ocean surface between 1000 and 950 hPa. This feature is confirmed by the conserved teta_v structure in this layer. Further, teta_v seldom showed any inversions in this region. The teta_e and teta_es profiles showed persistent low cloud layers between 900 and 700 hPa. The conserved variable diagrams (teta_e-q) showed the existence of double mixing line structures approximately at 950 and 700 hPa levels.     

Assimilation of Indian Doppler Weather Radar observations for simulation of mesoscale features of a land-falling cyclone

In this paper, impact of Indian Doppler Weather Radar (DWR) data, i.e., reflectivity (Z), radial velocity (Vr) data individually and in combination has been examined for simulation of mesoscale features of a land-falling cyclone with Advance Regional Prediction System (ARPS) Model at 9-km horizontal resolution. The radial velocity and reflectivity observations from DWR station, Chennai (lat. 13.0°N and long. 80.0°E), are assimilated using the ARPS Data Assimilation System (ADAS) and cloud analysis scheme of the model. The case selected for this study is the Bay of Bengal tropical cyclone NISHA of 27–28 November 2008. The study shows that the ARPS model with the assimilation of radial wind and reflectivity observations of DWR, Chennai, could simulate mesoscale characteristics, such as number of cells, spiral rain band structure, location of the center and strengthening of the lower tropospheric winds associated with the land-falling cyclone NISHA. The evolution of 850 hPa wind field super-imposed vorticity reveals that the forecast is improved in terms of the magnitude and direction of lower tropospheric wind, time, and location of cyclone in the experiment when both radial wind and reflectivity observations are used. With the assimilation of both radial wind and reflectivity observations, model could reproduce the rainfall pattern in a more realistic way. The results of this study are found to be very promising toward improving the short-range mesoscale forecasts.     

Observations of mid-troposhheric circulation from water vapour radiances of polar orbiting satellites

Water vapour tracers can provide useful information on winds at ≈ 500mb by observing the 6·7μ radiances. This fills the data gap in the cloud motion winds provided by conventional meteorological geostationary satellites. There is no geostationary satellite at present over the Indian Ocean with 6·7μ imaging capability to provide mid-tropospheric winds. The potentials of 6·7μ radiances, available from polar orbiting satellites, for mid-tropospheric circulation features have been examined in this study. Tiros-N satellite data of May 1979 and ECMWF level-IIIb wind data were analysed to relate the radiances with the streamlines. We find that the radiances of 6·7μ from orbiting satellites agree well with the wind field.