Showing papers by "National Centre for Medium Range Weather Forecasting published in 2003"

Daily Rainfall for the Indian Monsoon Region from Merged Satellite and Rain Gauge Values: Large-Scale Analysis from Real-Time Data

Abstract: A system for objectively producing daily large-scale analysis of rainfall for the Indian region has been developed and tested by using only available real-time rain gauge data and quantitative precipitation estimates from INSAT-1D IR data. The system uses a successive correction method to produce the analysis on a regular latitude–longitude grid. Quantitative precipitation estimates from the Indian National Satellite System (INSAT) operational geostationary satellite, INSAT-1D, IR data are used as the initial guess in the objective analysis method. Accumulated 24-h (daily) rainfall analyses are prepared each day by merging satellite and rain gauge data. The characteristics of the output from this analysis system have been examined by comparing the accumulated monthly observed rainfall with other available independent widely used datasets from the Global Precipitation Climatology Project (GPCP) and Climate Prediction Center Merged Analysis of Precipitation (CMAP) analyses. The monthly data prepare...

Indian Ocean surface winds from NCMRWF analysis as compared to QuikSCAT and moored buoy winds

Abstract: The quality of the surface wind analysis at the National Centre for Medium Range Weather Forecasts (NCMRWF), New Delhi over the tropical Indian Ocean and its improvement in 2001 are examined by comparing it within situ buoy measurements and satellite derived surface winds from NASA QuikSCAT satellite (QSCT) during 1999, 2000 and 2001. The NCMRWF surface winds suffered from easterly bias of 1.0–1.5 ms-1 in the equatorial Indian Ocean (IO) and northerly bias of 2.0–3.0 ms-1 in the south equatorial IO during 1999 and 2000 compared to QSCT winds. The amplitude of daily variability was also underestimated compared to that in QSCT. In particular, the amplitude of daily variability of NCMRWF winds in the eastern equatorial IO was only about 60% of that of QSCT during 1999 and 2000. The NCMRWF surface winds during 2001 have significantly improved with the bias of the mean analyzed winds considerably reduced everywhere bringing it to within 0.5 ms-1 of QSCT winds in the equatorial IO. The amplitude and phase of daily and intraseasonal variability are very close to that in QSCT almost everywhere during 2001. It is shown that the weakness in the surface wind analysis during 1999 and 2000 and its improvement in 2001 are related to the weakness in simulation of precipitation by the forecast model in the equatorial IO and its improvement in 2001.

Forecasting Maximum and Minimum Temperatures by Statistical Interpretation of Numerical Weather Prediction Model Output

Abstract: The inability of a general circulation model (GCM) to predict the surface weather parameters accurately necessitates statistical interpretation of numerical weather prediction (NWP) model output. Here a system for forecasting maximum and minimum temperatures has been developed and implemented for 12 locations in India based on the perfect prog method (PPM) approach. The analyzed data from the ECMWF for a period of 6 yr (1985–90) are used to develop PPM model equations. Daily forecasts for maximum and minimum temperatures are then obtained from these equations by using T-80 model output. In order to assess the skill and quality of the temperature forecasts, an attempt has been made to verify them by employing the conditional and marginal distribution of forecasts and observations using the data of four monsoon seasons from 1997 through 2000.

Analyses of orissa super cyclone using TRMM (TMI), DMSP (SSM/I) and OCEANSAT-I (MSMR) derived data

Abstract: The super cyclone in October 1999 was the most intense tropical cyclone in the last century in Orissa, a coastal state in India. This state was battered for more than two days by strong winds and intense rain killing thousands of people. The main objective of this study is to examine the impact of total precipitable water content (TPWC) and surface wind speed data from sensors on board the Tropical Rainfall Measuring Mission (TRMM), Defense Meteorological Satellite Project (DMSP), and Indian Remote Sensing Satellite (OceanSat-I) satellites on the data assimilation system at NCMRWF, New Delhi during the Orissa cyclone period. Comparison of various assimilation experiments suggests that the utilization of TRMM Microwave Imager (TMI) data in the assimilation produced the best analyses. However, in all the forecasts, the storm was predicted to weaken and did not have a reasonably good track. Assimilation experiments with the other two satellite data showed the cyclone track much to the south of the observed t...

Assimilation of special observations taken during the INDOEX and its impact on the global analysis-forecast system

Abstract: PARA LA DETERMINACION PRECISA DE LA ESTRUCTURA TRIDIMENSIONAL DE LA CIRCULACION ATMOSFERICA ES MUY IMPORTANTE ASIMILAR CADA OBSERVACION DE TODAS LAS FUENTES DE VARIABLES, ESPECIALMENTE SOBRE LAS REGIONES OCEANICAS ESCASAS DE DATOS. DURANTE LA FASE INTENSIVA DE CAMPO DE 1999 DEL EXPERIMENTO DEL OCEANO INDICO (INDOEX-IFP 99), SE HICIERON VARIAS OBSERVACIONES PARTICULARES SOBRE EL OCEANO INDICO QUE DIERON LA OPORTUNIDAD DE ESTUDIARLO MAS EXHAUSTIVAMENTE. EN ESTE ESTUDIO, ESTAS OBSERVACIONES PARTICULARES SE HAN ASIMILADO, JUNTO CON OTRAS OBSERVACIONES CONVENCIONALES Y NO CONVENCIONALES RECIBIDAS EN EL CENTRO NACIONAL PARA EL PRONOSTICO METEOROLOGICO DE RANGO MEDIO (NCMRWF), A TRAVES DEL SISTEMA GLOBAL DE TELECOMUNICACIONES EN SUS SISTEMA DE ANALISIS GLOBAL DE PRONOSTICO. EL PROPOSITO DE ESTE ESTUDIO ES DETERMINAR EL IMPACTO DE LOS DATOS ADICIONALES SOBRE EL SISTEMA GLOBAL DE ASIMILACION DE DATOS DEL NCMRWF, ASI COMO GENERAR MEJORES CONDICIONES INICIALES PARA SU USO FUTURO EN ESTUDIOS DE MODELADO. SE HA ESTUDIADO EL IMPACTO DE LOS GRUPOS DE DATOS EN ANALISIS Y PRONOSTICOS SUBSECUENTES DE 5 DIAS EN LOS CAMPOS MEDIOS MENSUALES. EL CAMPO MEDIO DE VIENTO MUESTRA EL FORTALECIMIENTO DE LAS TENDENCIAS DE LOS VIENTOS EN LOS NIVELES BAJOS Y EL FORTALECIMIENTO DEL VIENTO ALREDEDOR DEL ANTICICLO SUBTROPICAL EN LOS NIVELES ALTOS. SE HA ENCONTRADO QUE EL IMPACTO ES MAYOR SOBRE LAS REGIONES OCEANICAS ESCASAS DE DATOS Y EN LOS NIVELES ALTOS EN COMPARACION CON LOS NIVELES BAJOS. SE HA ESTUDIADO EL IMPACTO DE ESTOS GRUPOS DE DATOS SOBRE DIFERENTES CONDICIONES INICIALES ASI COMO SOBRE EL PRONOSTICO. SE HAN OBSERVADO IMPACTOS SIGNIFICATIVOS SOBRE AQUELLAS REGIONES OCEANICAS DONDE PERSISTEN SISTEMAS SINOPTICOS BIEN DEFINIDOS.

Impact of a consistent height reassignment method for INSAT derived cloud motion vectors

Abstract: ¶Cloud Motion Vector wind (CMV) data is one of the major products derived from current day operational geostationary satellites and continues to be a useful source of information for global data assimilation systems. The CMV fields result basically from the apparent cloud motion estimated through a sequence of very high resolution radiometer (VHRR) images and the corresponding cloud level is inferred from observed brightness temperatures (BT) in the infrared window channel. The main problem in CMV production is associated with the assignment of correct height to the cloud tracked motions in view of the known limitations associated with the cloud emissivity characteristics and estimation of BTs. Despite these inherent uncertainties satellite derived CMVs are widely used to augment the conventional upper air wind data especially over the oceanic regions. In this study, an attempt is made to improve the quality of INSAT derived CMVs through an appropriate height re-assignment by employing first guess fields (short range forecasts) of a global forecast model. Two experimental height re-assignment methods (EXP-I: Taking first-guess height field into consideration; EXP-II: Taking first-guess height and wind fields into consideration) for correcting assigned heights of CMVs are employed to examine the consistency of the fields. An objective assessment of these reassignment methods is carried out through verification of CMVs against collocated Radiosonde/Rawinsonde (TEMP) and Pilot Balloon (PILOT) wind reports received through Global Telecommunication System (GTS) at New Delhi for a period of 1–21 November, 1996. The collocation statistics clearly show a relative improvement in the quality of CMVs EXP-II. The assimilation-forecast cycle is repeated by subjecting directly derived CMVs to EXP-II method for whole of the above 21-day period and the impact of improved CMVs is examined in terms of objective skill scores (RMSE, Anomaly Correlation Coefficient). The impact of the improved CMVs in the synoptic scales is further examined by conducting assimilation experiments for two typical cases of tropical circulation systems formed over the Bay of Bengal during December 1996 and May 1997, respectively.

The Indian summer monsoon as revealed by the NCMRWF system

Abstract: 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.

A study on the structure of the convective atmosphere over the Bay of Bengal during BOBMEX-99

Abstract: 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.

Observed daily large-scale rainfall patterns during BOBMEX-1999

Abstract: A daily rainfall dataset and the corresponding rainfall maps have been produced by objective analysis of rainfall data. The satellite estimate of rainfall and the raingauge values are merged to form the final analysis. Associated with epochs of monsoon these rainfall maps are able to show the rainfall activities over India and the Bay of Bengal region during the BOBMEX period. The intra-seasonal variations of rainfall during BOBMEX are also seen using these data. This dataset over the oceanic region compares well with other available popular datasets like GPCP and CMAP. Over land this dataset brings out the features of monsoon in more detail due to the availability of more local raingauge stations.