Assimilation of Kalpana very high resolution radiometer water vapor channel radiances into a mesoscale model

Abstract

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.