Regional Coupled Atmosphere-Ocean Assimilation System Based on NHM-LETKF

We have constructed a regional coupled atmosphere-ocean assimilation system using the local ensemble transform Kalman filter (LETKF) and a nonhydrostatic model (NHM) coupled with a multilayer ocean model and the third generation ocean wave model. Atmospheric observations (winds, air temperature and moisture) are obtained from Comprehensive Database for Assimilation (CDA) in the Japan Meteorological Agency. In addition, the assimilation system also controls sea surface temperature using the Group for High Resolution Sea Surface Temperature (GHRSST) Level 2P Gridded Global Subskin Sea Surface Temperature from the Advanced Scanning Microwave Radiometer - Earth Observing System (AMSR-E) on the National Aeronautics and Space Administration (NASA) Aqua Satellite. The performance of the air-sea coupled assimilation system was preliminarily verified with a case study of Typhoon Sinlaku in 2008. The analysis and prediction covered a ~3600 km x ~1900 km computational domain with a horizontal grid spacing of 15 km. The system had 40 vertical levels with variable intervals from 40 m for the near-surface layer to 1180 m for the uppermost layer. The system had maximum height approaching ~23 km. The analysis period was from 1200 UTC 1 to 1800 UTC 19 September in 2008 with cycles every 6 hours. The number of ensemble member was 50. A preliminary experiment showed that tropical cyclone intensity analysis was improved with more realistic horizontal distribution of sea surface temperature than prescribed distribution obtained from oceanic reanalysis data with a horizontal resolution of 0.5°. Figure shows the horizontal distribution of assimilated sea surface temperature (left) at 1200 UTC on 19 September 2008 and that of daily Microwave optimally interpolated sea surface temperature on the same day (right). A cold wake induced by Sinlaku was well assimilated in the coupled assimilation system.