Impact of SAPHIR radiances on the simulation of tropical cyclones over the Bay of Bengal using NCMRWF hybrid-4DVAR assim
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Impact of SAPHIR radiances on the simulation of tropical cyclones over the Bay of Bengal using NCMRWF hybrid-4DVAR assimilation and forecast system DEVANIL CHOUDHURY1,2, ANKUR GUPTA1,*, S INDIRA RANI1 and JOHN P GEORGE1 1
National Centre for Medium Range Weather Forecasting (NCMRWF), Ministry of Earth Sciences (MoES), A-50, Institutional Area, Sector 62, Noida 201 309, India. 2 Center for Monsoon System Research, Institute of Atmospheric Physics, Chinese Academy of Sciences, Beijing 100 029, People’s Republic of China. *Corresponding author. e-mail: [email protected] [email protected] MS received 12 March 2020; revised 4 June 2020; accepted 9 July 2020
Observing System Experiments (OSEs) were conducted to analyze the impact of assimilation of Megha-Tropique’s (MT) Sounder for Probing Vertical ProBles of Humidity (SAPHIR) radiances on the simulation of tracks and intensity of three tropical cyclones (Kyant, Vardah, and Maarutha) formed over the Bay of Bengal during 2016–2017 North Indian Ocean cyclone period. National Centre for Medium Range Weather Forecast (NCMRWF) UniBed Model (NCUM) Hybrid-4DVAR assimilation and forecast system was used for the OSEs. Assimilation of SAPHIR radiances produced an improvement of 9% and 12%, respectively, in the cyclones’ central sea level pressure (CSLP) and the maximum sustained wind (MSW), while an improvement of 38% was seen in the cyclone tracks within the forecast lead time of 120 hrs. Initial assessment shows that the improvement in the cyclone intensity is due to the assimilation of the unique surface peaking channel of SAPHIR (channel-6), whereas the improvement in the cyclone track is due to the assimilation remaining Bve channels of SAPHIR. Thus, the assimilation of SAPHIR radiances in the NCUM system showed improvement in both intensity and track of the cyclones over the Bay of Bengal; however, more cyclone cases over different ocean basins have to be analyzed to make a robust conclusion. This study speciBes the importance of similar microwave humidity instruments in the same frequency range for the detailed exploration of cyclone track and structure. Keywords. SAPHIR; hybrid-4DVAR; data assimilation; tropical cyclone.
1. Introduction Tropical cyclone (TC) forecasts over the North Indian Ocean are increasingly dependent on the Numerical Weather Prediction (NWP) models in recent years. The numerical forecasts of TCs in most ocean basins have improved considerably in
the last few decades, attributed mainly to an increase in model resolution and better representation of convection and microphysics (Gentry and Lackmann 2010) in addition to improvements in observing system and data assimilation methods. The simulation of TC’s tracks is highly sensitive to initial conditions (Singh and Mandal 2014;
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Greeshma et al. 2015), small perturbations in the initial conditions can lead to large errors in forecasted tracks. The importance of better representation of humidity proBle
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