Assimilation of Thermodynamic Variables from Satellite Measurements in Regional NWP for Tropical Cyclone Forecast Improvement

Reliable forecasts of tropical cyclones (TCs) such as Isaac and Sandy which made landfall on the continental United States (CONUS) during 2012 are critical for decision making and better preparation. Obtaining good TC intensity forecasts remains one of the most challenging aspects. Observations of atmospheric thermodynamic variables in the TC environment as well as in the inner core are very important to the prediction of the storm evolution and landfall impacts. Satellite sounders such as AIRS, IASI, CrIS, AMSU and ATMS, provide valuable thermodynamic information over oceans. In particular, the AMSU-A temperature information has been demonstrated to be very useful for hurricane forecast improvements, while the positive impacts of moisture information have yet to be fully realized, even in global NWP. Most global NWP centers are using direct assimilation of radiances, based on the successes at both NCEP and the ECMWF. However, due to the challenge of assimilating moisture information, it is still worthwhile to study the assimilation of 300 – 700 hPa layer precipitable water (LPW) which has the best moisture information from advanced IR sounders. The 300 – 700 hPa LPW has been demonstrated to have better accuracy and precision than GFS forecasts when radiosondes, ground-based microwave radiometer and ground-based GPS-Met are used as references. The forward operator for LPW has be developed and included in the GSI for assimilating LPW. The LPW assimilation is compared with radiance assimilation in Hurricane Sandy (2012) forecast experiments. In the first experiment, radiances and derived LPW from one sounder (AIRS) will be examined, while in the second experiment, multiple advanced IR sounder (AIRS, CrIS, IASI) radiances and derived LPW are tested. The IR sounder thermodynamic variable assimilation studies will include assimilation of all conventional and AMSU/ATMS radiances. To verify the impacts of assimilating thermodynamic variables, the hurricane track, minimum sea level pressure (SLP) and maximum wind speed observations from the National Hurricane Center (NHC) are used as references for comparisons. The forecast 24 hour accumulated precipitation is verified against the NOAA CPC Morphing Technique (CMORPH) high resolution precipitation analysis over the ocean. Both the frequency bias and the equitable threat score (ETS) are computed to show the impacts of thermodynamic variables on hurricane precipitation forecasts. The GOES-13 Imager brightness temperature measurements are also compared with that of simulated temperatures from the forecasts, indicating that the mesoscale features around the hurricane Sandy can be well captured by assimilating moisture information from satellite.