Impact of Assimilating Environmental Satellite Observations on Tropical Storm Position and Intensity Analyses and Forecasts

Accurate estimates of the position and intensity of tropical cyclones have been difficult to obtain, especially for weak vortices during cyclogenesis. This is partly due to the lack of in-situ observations as well as appropriate data assimilation techniques for the latent heat driven tropical atmosphere. Ensemble data assimilation techniques use short-range ensemble forecasts to estimate flow dependent multivariate forecast error covariances. Cross covariances of moisture with winds and temperature can improve data assimilation in tropics. This study explores the impact of assimilating satellite observations of the environment surrounding developing tropical cyclones with an ensemble filter. As a first example, GPS radio occultation (RO) refractivity profiles are assimilated in NCAR's WRF/DART ensemble system during the genesis of Hurricane Ernesto (2008). Analyses and forecasts of the initial storm vortex are enhanced by assimilating the RO data available 1-2 days before. The RO data increase the analysis moisture content and cyclonic circulation in the upstream easterly wave. This enhanced cyclonic circulation moves into the storm genesis area and results in an improved analysis of the initial vortex. This suggests that satellite observations in the environment of tropical cyclones have potential to improve the vortex initialization through ensemble data assimilation. The impact of assimilating environmental satellite microwave and IR soundings is also examined.