AIRS Thermodynamic Profile Assimilation Leads To Improved Precipitation Forecasts

NASA’s Atmospheric Infrared Sounder (AIRS) has the capability of producing near-radiosonde quality vertical soundings with a vertical resolution much greater than previous sounders. SPoRT has successfully assimilated temperature and moisture soundings retrieved from AIRS into the WRF 3DVAR analysis system (WRF-Var) and assessed using 37 case study days from winter 2007. The assimilated data produce a 3-dimensional analysis that is then used to initialize a WRF forecast on a regional, 12-km domain.

One particular case in which AIRS has excelled is a February 13, 2007 extreme rainfall case in Eastern Texas prompted by a cold front pushing eastward across the Great Plains. On February 12, clear skies over Eastern Texas and the Western Gulf of Mexico allowed for the observation and assimilation of many high-quality AIRS profiles at around 0900Z. The figure below shows the location of the AIRS profiles. The colored pixels indicate the pressure above which the highest quality AIRS profiles are assimilated (e.g. black pixels denote profiles that have quality data all the way to the surface). Note that there is a large swath of AIRS profile data located over the Western Gulf of Mexico.

The WRF model was initialized at 0000 UTC on February 12, data were assimilated at 0900 UTC, and these AIRS-enhanced initial conditions were used to finish the forecast out to 24 hours. The image below depicts three model metrics from the 24-hour forecast (valid at 0000 UTC on February 13): 6-hour cumulative precipitation (top row), 1000 hPa dew point temperature (middle row), and convective available potential energy (CAPE). The far right column is the verifying “ground truth” analysis. For 6-hour cumulative precipitation, the truth field is gridded Stage IV precipitation fields, which combine radar and rain gauge data. For the dew point and CAPE fields, the North American Mesoscale (NAM) model analysis valid at 0000 UTC on 2/13 is used. The left column represents the Control (no AIRS) forecast; the middle column the AIRS-enhanced forecast.

With the inclusion of AIRS temperature and moisture profiles (upper middle), the intensity and location of the 6-hour cumulative precipitation maximum in the line of storms crossing Eastern Texas is represented much better than in the Control (upper left) compared to the Stage IV precipitation data (upper right). This convective precipitation is improved with the inclusion of AIRS due to improved forecasts of CAPE and lower-level moisture, which lead to greater instability in the model in that region. These improvements are validated in the comparisons to the NAM analysis. The advance of the 18-20 degree Celsius lower level dew points off the coast of Texas is depicted better in the AIRS-enhanced model run (middle) compared to the Control (middle left). Similarly, the AIRS-enhanced CAPE (lower middle) shows some of the 1800 J/kg CAPE values approaching the Texas coast that appear in the NAM analysis of CAPE (lower right). The Control run (lower left) has no CAPE values in that region above 1400 J/kg.

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