Assimilation of INSAT-3D imager water vapour clear sky brightness temperature in the NCMRWF’s assimilation and forecast system

Abstract

This paper describes the direct assimilation of water vapour (WV) clear sky brightness temperatures (CSBTs) from the INSAT-3D imager in the National Centre for Medium Range Weather Forecasting (NCMRWF) Unified Model (NCUM) assimilation and forecast system. INSAT-3D imager WV CSBTs show a systematic bias of 2-3 K compared to the data simulated from the model first guess fields in the pre-assimilation study. The bias in the INSAT-3D imager WV CSBTs is removed using a statistical bias correction prior to assimilation. The impact of INSAT-3D imager WV channel CSBTs is investigated through different approaches: (i) single observation experiments and (ii) global assimilation experiments using the hybrid-four-dimensional variational technique. Single observation experiments of channels of the same frequency from different instruments like the INSAT-3D imager and sounder, and the Meteosat visible and infrared imager (MVIRI) onboard Meteosat-7, show the INSAT-3D imager and MVIRI WV channels have a similar impact on the analysis increment. Global assimilation clearly shows the positive impact of the INSAT-3D imager WV CSBTs on the humidity and upper tropospheric wind fields, whereas the impact on the temperature field, particularly over the tropics, is neutral. Validation of model forecasted parameters with the in situ radio sonde observations also showed the positive impact of assimilation on the humidity and wind fields. INSAT-3D imager WV CSBTs have been assimilated operationally in NCUM since August 2018.

Quality Scoring of the Fengyun 4A Clear Sky Radiance Product

“…and the results are LUT scores for RMSE p (dots in Figure 6b). To score the FOVs with continuous RMSE p values, we exploited an exponential decay model to fit the relationship between RMSE p and score in a least squares manner (solid lines in Figure 6b) and derive parameter 'k' in (6). To validate the correctness of the RMSE p , we examined the consistency between the observed and predicted RMSEs in different score ranges for both channels.…”

“…The main direct impacts on the analysis were tropospheric humidity and wind increment, and positive impacts were revealed for almost all upper tropospheric variables in forecast verifications. Rani et al [6] assimilated the clearsky BTs of WV channels of the INSAT-3D imager into the assimilation and forecast system of the National Centre for Medium Range Weather Forecasting (NCMRWF), demonstrating a positive impact on the humidity and upper tropospheric wind fields, whereas the impact on the temperature field, particularly over the tropics, was neutral. Moreover, studies have shown that assimilating WV radiances would improve the forecasting of convective processes [7], coastal quantitative precipitation [8,9], and the overall tracking and intensity of tropical storms [10].…”