Improved analysis of the tropical cyclone outer wind structure using IR satellite wind retrievals in a global numerical weather prediction model

Operational estimates of tropical cyclone (TC) structure (e.g., maximum surface wind speed, central pressure, position and motion), often referred to as ‘TC vitals,' are used in global numerical weather prediction (NWP) models in a variety of ways. A common approach is to assimilate the TC central surface pressure as a conventional observation. The main purpose of assimilating the central pressure is to relocate the model background TC (typically a 6-h forecast) to the observed position and thereby improve the assimilation of other observations in the vicinity of the storm. However, for TCs with poor data coverage in the motion-critical 100-500 km annulus (Fiorino and Elsberry 1989) the pressure observation alone cannot realistically constrain the TC vortex. Thus, information on the outer wind structure is needed to improve the vortex analysis and to support the central pressure observation.

This study utilizes retrievals of the symmetric wind field based on geostationary satellite infrared (IR) imagery from the Colorado State University (CSU) Cooperative Institute for Research in the Atmosphere (CIRA). The IR WinD (IRWD) retrievals are based on a statistical relationship between patterns in IR imagery and wind observations from aircraft reconnaissance; and the IRWD give information on the lower-tropospheric (850 & 700 hPa) TC vortex in the motion-critical annulus. We generate ‘superobs' from the high-density IRWD retrievals by calculating the mean values at spatial scales appropriate for the model and data assimilation system and for this study use a scale of 90 km. The IRWD superobs were first tested for Hurricane Celia (04E) and Hurricane Darby (05E) in the eastern North Pacific -- two TCs that formed in late June 2010, because the Global Forecast System NWP model (GFS) failed to assimilate the central surface pressure and to accurately forecast the TC intensity. The superobs for the 04E and 05E cases were shown to be consistent with the wind radii in the TC vitals and when applied to the Ensemble Kalman Filter (EnKF) data assimilation for the GFS improved both the global NWP model TC track and intensity forecast. In addition to the test cases, we will show results for other storms and basins during the 2010 northern Hemisphere season.