First steps towards an all-sky assimilation framework for tropical cyclone event over Bay of Bengal region: Evaluation and assessment of GMI radiances

Mangla, Rohit ; J, Indu ; Chambon, Philippe ; Mahfouf, Jean-François

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<p align=justify>The present study designs a data assimilation framework for all-sky GMI radiances in the Weather Research Forecast (WRF) model for the prediction of tropical cyclone events over the Bay of Bengal (BOB) region. Experiments are performed using the RTTOV-SCATT radiative transfer model for three cyclone events (Hudhud (2014), Vardah (2016), and Kyant (2016)) for 19 V, 23 V, 37 V, 89 V, 166 V, 183±3, and 183±7 V channels. Results show that observed and simulated brightness temperatures (Tb) agree well for low-frequency channels (<89 GHz). However, significant discrepancies do exist when simulating low Tb at high frequency channels. Therefore, a set of 26 simulations is considered using snow particle shapes from single scattering property databases. A new methodology is devised in order to compare the 26 simulations with observations; taking into account the uncertainties on hydrometeor radiative properties. Results reveal that model simulations are characterized by an underestimation of occurrences within several Tb ranges (~200-300 K). The probability distribution function (PDF) of normalized observed-minus first guess is close to a Gaussian shape after applying a dedicated model of observation errors. The potential to integrate the GMI sensor data within a WRF data assimilation system is thus demonstrated.</p>
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