An IASI channel selection for UM 1D-Var and its impact on precipitation forecast over East Asia

Abstract

Firstly we attempted to understand the analysis performance and associated error characteristics of Unified Model (UM) analysis, aimed at improving initial conditions for better data assimilation system. In doing so, comparison was made between UM analyses at the Met Office and the Korea Meteorological Administration (KMA) against collocated Global Climate Observing System (GCOS) Reference Upper-Air Network (GRUAN) radiosonde observations for January–December 2012. The European Centre for Medium-Range Weather Forecasts (ECMWF) reanalysis data were also compared. Although temperature fields in the three analyses are in good agreement with the radiosonde observations, humidity forecasts appear to be less reliable, suggesting that improvement of humidity field is needed in the UM data assimilation system.. We presumed that the UM analysis and its forecast can be improved by running Infrared Atmospheric Sounding Interferometer (IASI) data assimilation with IASI channels different from current operationally used 183 channels. We do this by re-selecting new channels from operationally available 314 channel of the European Organisation for the Exploitation of Meteorological Satellites (EUMETSAT). In selecting channels, we calculated the impact of the individually added channel on the improvement in the analysis outputs from a one-dimensional variational analysis (1D-Var) for the UM data assimilation system at the Met Office, using a newly defined channel score index (CSI) as a figure of merit. And then 200 channels were orderly selected by counting individual channel's CSI contribution. Compared with the operationally used 183 channels for the UM at the Met Office, the new set shares 149 channels while other 51 channels are new. The impact of newly selected 200 channels was assessed via experimental trial runs using the UM global assimilation system. The new channels had an overall neutral impact in terms of improvement in forecasts, as compared with results from the operational channels. However, upper-tropospheric moist biases shown in the control run with operational channels were significantly reduced in the experimental trial with the newly selected channels. The reduction of moist biases was mainly due to the additional water vapor channels, which are sensitive to the upper-tropospheric water vapor. In addition, experiments have been conducted for assessing how new channels might improve the precipitation forecast over East Asia where distinct warm type heavy rainfall prevails. Experiments indicate that warm-type heavy rainfall appears to be better forecasted with a new set of IASI channels.