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Varma A.K.,Meteorology and Oceanography Group | Liu G.,Florida State University
Journal of Geophysical Research: Atmospheres | Year: 2010

Classification of rain type in satellite microwave observations is useful for various studies ranging from numerical weather prediction and precipitation climatology to satellite retrieval of rain amounts. In this study we have first examined the possibility of determining the distribution of convective/ stratiform rain within a typical microwave radiometric pixel size area represented by the Tropical Rainfall Measuring Mission Microwave Imager (TMI) and then formulated an empirical relation between the convective to stratiform ratio and observed brightness temperatures. Rain classification with satellite microwave observation is hampered by the small size of the rain events. It is found from the rain observations during July 2000 that a significant number of 53% convective and 28% stratiform rain fill less than one fourth of the TMI pixel size area. The nonlinear relationship between brightness temperature and rain rate, along with horizontal and vertical inhomogeneity of the rain type distribution within the pixel, makes it difficult to work out the exact proportion of convective to stratiform istribution within the pixel. Here an algorithm is proposed to determine rain type on the basis of regression with 10 functions of 19, 37, and 85 GHz channels into three broad convectivestratiform proportions. This algorithm is able to identify rain types in about 70% of the TMI pixels accurately. To broaden the utility of the proposed method, a procedure has been developed by which the method can be applied to any other microwave radiometers with similar channels to TMI. Using this procedure, a successful application of the algorithm to Special Sensor Microwave Imager observations is demonstrated. Copyright 2010 by the American Geophysical Union. Source

Jaiswal N.,Meteorology and Oceanography Group | Kishtawal C.M.,Meteorology and Oceanography Group
IEEE Geoscience and Remote Sensing Letters | Year: 2011

In this letter, a new method is discussed for the automatic determination of the center of tropical cyclones (TCs) by extracting the spiral features within it using the infrared (IR) images from geostationary satellites. Meteosat-5 IR images of two TCs viz., Mala (April 2429, 2006), and Sub Tropical Storm 4 (STS-4) (October 1518, 1999) have been analyzed using the image processing techniques, and the center of the TC is then estimated by fitting the spiral at different locations. The present method provides accurate estimates of the cyclone center for the images where the spiral patterns are well featured. However, the method leads to larger errors during formative or decaying phase of cyclone due to the absence of robust pattern in the images. The present method has a potential to be applied in a completely automated mode and can be used to replace the manual center determination which is being done traditionally. © 2010 IEEE. Source

Rakesh V.,CSIR - Central Electrochemical Research Institute | Singh R.,Meteorology and Oceanography Group | Joshi P.C.,Meteorology and Oceanography Group
Pure and Applied Geophysics | Year: 2011

This study examines the short-range forecast accuracy of the Pennsylvania State University-National Center for Atmospheric Research Mesoscale Model (MM5) as applied to the July 2006 episode of the Indian summer monsoon (ISM) and the model's sensitivity to the choice of different cumulus parameterization schemes (CPSs), namely Betts-Miller, Grell (GR) and Kain-Fritsch (KF). The results showed that MM5 day 1 (0-24 h prediction) and day 2 (24-48 h prediction) forecasts using all three CPSs overpredicted monsoon rainfall over the Indian landmass, with the larger overprediction seen in the day 2 forecasts. Among the CPSs, the rainfall distribution over the Indian landmass was better simulated in forecasts using the KF scheme. The KF scheme showed better skill in predicting the area of rainfall for most of the rainfall thresholds. The root mean square error (RMSE) in day 1 and day 2 rainfall forecasts using different CPSs showed that rainfall simulated using the KF scheme agreed better with the observed rainfall. As compared to other CPSs, simulation using the GR scheme showed larger RMSE in wind speed prediction at 850 and 200 hPa over the Indian landmass. MM5 24-h temperature forecasts at 850 hPa with all the CPSs showed a warm bias of the order of 1 K over the Indian landmass and the bias doubled in 48-h model forecasts. The mean error in temperature prediction at 850 hPa over the Indian region using the KF scheme was comparatively smaller for all the forecast intervals. The model with all the CPSs over predicted humidity at 850 hPa. The improved prediction by MM5 with the KF scheme is well complemented by the smaller error shown by the KF scheme in vertical distribution of heat and mean moist static energy in the lower troposphere. In this study, the KF scheme which explicitly resolve the downdrafts in the cloud column tended to produce more realistic precipitation forecasts as compared to other schemes which did not explicitly incorporate downdraft effects. This is an important result especially given that the area covered by monsoon-precipitating systems is largely from stratiform-type clouds which are associated with strong downdrafts in the lower levels. This result is useful for improving the treatment of cumulus convection in numerical models over the ISM region. © 2010 Springer Basel AG. Source

Dash M.K.,Indian Institute of Technology Kharagpur | Pandey P.C.,Indian Institute of Technology Bhubaneswar | Vyas N.K.,Meteorology and Oceanography Group | Turner J.,British Antarctic Survey
International Journal of Climatology | Year: 2013

The linkages between the El Niño Southern Oscillation (ENSO) and the sea ice extent (SIE) in the Weddell Sea (South Atlantic) and the Bellingshausen-Amundsen Sea (South-Eastern Pacific) sectors of the Southern Ocean have been studied for the period 1979-2005 using crosswavelet analysis. The analysis showed that the relationship between the tropical expression of ENSO and the SIE in these two areas are different before and after 1992. Further, we investigated the structure and strength of the regional Ferrel cell (RFC) during El Niño and La Niña episodes using composite latitude-pressure cross-sections of wind anomalies for these two periods. Contrasting features were observed in the structure and strength of the RFC before and after 1992 in both, the South Atlantic and the South-Eastern Pacific. These modulations in the RFC control the heat transport from the tropics to high latitudes and hence the extent of sea ice in both the regions. We propose that the modulation of the RFC is responsible for the phase shift in the tropical-polar teleconnection. © 2012 Royal Meteorological Society. Source

Singh R.,Meteorology and Oceanography Group | Pal P.K.,Meteorology and Oceanography Group | Joshi P.C.,Meteorology and Oceanography Group
Journal of Geophysical Research: Atmospheres | Year: 2010

The very high resolution radiometer (VHRR) channel 3 (at 6.2 μm) on the Indian Geostationary satellite Kalpana is sensitive to mid-upper tropospheric water vapor. This paper describes the assimilation of Kalpana VHRR channel 3 clear-sky radiances (hereafter water vapor (WV) radiances) in the Weather Research and Forecasting (WRF) three-dimensional variational assimilation (3D-Var) system. The Kalpana WV radiances (in terms of blackbody equivalent brightness temperatures) are used in combination with a fast radiative transfer model and 3D-Var assimilation system in the WRF model. Extensive preassimilation monitoring of the WV radiances has been carried out, showing a diurnal bias of approximately 1 K in the Kalpana WV radiances compared to radiances simulated from the WRF model first-guess fields. The control (without Kalpana WV radiances) as well as experimental (which assimilated bias corrected Kalpana WV radiances) runs were made for 24 h starting at 0000 UTC during July 2008. The assimilation experiments for July 2008 (22 cases) demonstrated a positive impact of the assimilated Kalpana WV radiances on both the analysis state as well as subsequently short-term (6-24 h) forecasts. At 0 h (analysis), the agreement of mid-upper tropospheric moisture sensitive channels (channels 11 and 12) in NOAA/HIRS, which is an independent observation, is improved (improvement is 19% for channel 11 and 17% for channel 12) with the assimilation of Kalpana WV radiances compared to control experiment. In the radiance assimilation experiment, at 6 h (24 h), the root mean square differences between model equivalent Kalpana WV radiances and Kalpana observed WV radiances showed an improvement of 20% (1.7%) relative to the control experiment. Compared with National Center for Environmental Prediction analysis, assimilation of Kalpana WV radiances shows positive impact on the mid-upper tropospheric moisture and a neutral impact on the temperature and wind forecasts. Compared with atmospheric infrared sounder retrieved and radiosonde observed thermodynamic profiles, Kalpana radiances show positive impact on the mid-upper tropospheric moisture and temperature and a mixed (negative/positive) impact on the lower and upper tropospheric moisture and temperature forecasts. The comparison of model predicted rainfall with TRMM measurements indicates that Kalpana radiances impacted the rainfall positively. Copyright 2010 by the American Geophysical Union. Source

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