Use of AIRS/AMSU retrieved soundings to improve prediction of Gulf moisture return

The Atmospheric InfraRed Sounder (AIRS) is a cross-track scanning infrared spectrometer/radiometer on board the Aqua satellite of the Earth Observing System (EOS). Combined with the Advanced Microwave Sounding Unit (AMSU), it is capable of retrieving high resolution temperature and moisture profiles which may be utilized in model forecasts. The ability of the satellite to obtain data over the Gulf of Mexico promises to improve forecasts of return moisture flow from the Gulf. This may have a significant impact on forecasts of precipitation in the southern plains to the southeast United States. This research uses the ARPS Data Analysis System (ADAS) to assimilate the sounding data in a high-resolution nonhydrostatic model as a first step before using the direct radiance data.

In the case of April 9, 2005, the NAM (Eta) model underpredicted the moisture return along the Gulf coast of Texas as manifested in the Corpus Christi, TX sounding from 00 UTC April 10. The AIRS retrievals may provide a useful supplement to terrestrial data in this case since there are a limited number of routine observations in the Gulf of Mexico. The AIRS retrievals were assimilated into the ARPS model, and a 5-hour forecast was produced. Archived NAM forecasts with a 40km resolution were used as the background field. Preliminary results show that the AIRS data had a small effect on the analysis. The AIRS data increased the moisture content in the western part of the swath, but decreased it in the northeastern section. The zone of strongest moisture return may have been present just outside the data coverage swath for this pass. Different combinations of AIRS quality control flags were used to determine what the impact would be on the forecast analysis. There was very little variation in the five hour forecast of the 850mb moisture field between the analyses with the different quality control flags. The exception is in the case where the strictest quality control was used and the background field becomes more dominant. The initial 850mb analysis with the AIRS data showed a maximum moisture increase near 1.3 g/kg in areas of moistening, and a maximum moisture decrease of 2.5 g/kg in areas of drying. The 5-hour forecast shows a magnitude of moisture increase on the order of 0.8 g/kg, and a magnitude of moisture decrease around 1.2 g/kg. In ongoing work we are running the model over a longer forecast period to determine the influence of the AIRS data on the precipitation forecast for April 10, 2005, and results will be reported in the extended abstract and at the conference.

Fig 1 Below. Specific humidity analysis at 850 hPa (left frame) and difference from background NAM (Eta) 7-h forecast (right). Squares show locations of AIRS retrieval soundings.