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

摘要

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.