Impact of assimilating SSM/I rainfall rates on numerical prediction of winter cyclones

• Published in: Monthly weather review 1994, Vol.122 (1), p.151-164
• Main Authors: CHANG, S. W, HOLT, T. R
• Format: Article
• Language: English
• Subjects: Earth, ocean, space; Exact sciences and technology; External geophysics; Meteorology; Weather analysis and prediction
• ISSN: 0027-0644; 1520-0493
• DOI: 10.1175/1520-0493(1994)122<0151:ioasrr>2.0.co;2

A series of observing system simulation experiments (OSSE) and real data assimilation experiments were conducted to assess the impact of assimilating Special Sensor Microwave/Imager (SSM/I)--estimated rainfall rates on limited-area model predictions of intense winter cyclones. For the OSSE, the slow-moving, fronto- and cyclogenesis along the east coast of United States during the second intensive observation period (IOP 2) of the Genesis of Atlantic Lows Experiment (GALE) (26-28 January 1986) was selected as the test case. The perfect ``observed'' rainfall rates were obtained by an integration of a version of the Naval Research Laboratory (NRL) limited-area model, whereas the ``forecast'' was generated by a degraded version of the NRL model. A number of OSSEs were conducted in which the ``observed'' rainfall rates were assimilated into the ``forecast'' model. Rainfall rates of various data frequencies, different vertical heating profiles, various assimilation windows, and prescribed systematic errors were assimilated to test the sensitivity of the impact. It was found that assimilation of rainfall rates, in general, improves the forecast in terms of sea level pressure S1 scores when either the ``observed'' or model-determined vertical heating profiles were used. The improvement was insensitive to the error in rainfall magnitude estimates but was sensitive to errors in geographic locations of the precipitation. More frequent observations (additional sensors in orbits) had positive but gradually diminishing benefits. Real SSM/I-measured rainfall rates were assimilated for the rapid-moving, intense marine cyclone of IOP 4 of the Experiment on Rapidly Intensifying Cyclones over the Atlantic (ERICA) (4-5 January 1989), which started from an initial offshore disturbance with a minimum pressure of 998 mb at 0000 UTC 4 January and developed into a very intense storm of 937 mb 24 h later. The NRL model simulated a well-behaved but less intense cyclogenesis episode based on the RAFS (Regional Analysis and Forecast System) initial analysis, reaching a minimum sea level pressure of 952 mb at 24 h. The first SSM/I aboard a DMSP (Defense Meteorological Satellite Program) satellite flew over the marine cyclone at 0000, 0930, and 2200 UTC 4 January and measured rainfall rates over portions of the warm and cold fronts associated with the cyclone. The SSM/I rainfall rates at 0000 and 0930 UTC were assimilated into the model as latent heating functions in plus or minus 3-h w