The Impact of Multi-Satellite Data on the Initialization and Simulation of Hurricane Lili's (2002) Rapid Weakening Phase

Numerical experiments have been conducted to examine the impact of multi-satellite data on the initialization and forecast of the rapid weakening of Hurricane Lili (2002) from 0000 UTC to landfall in Louisiana on 1300 UTC 3 October. MM5 4DVAR sensitivity runs were conducted separately with QuikSCAT surface winds, GOES-8 cloud drift/water vapor winds, and Aqua MODIS temperature/dewpoint sounding data to investigate their individual impact on storm track and intensity. Assimilating QuikSCAT surface wind data improves the analysis outer-core surface winds, as well as the inner-core low-level temperature and moisture fields. Substantial adjustments of winds are noted on the periphery of the hurricane by assimilating GOES-8 satellite-derived upper-level winds. Both track forecasts initialized at 1200 UTC 2 October with 4DVAR of QuikSCAT and GOES-8 show improvement compared to those initialized with the model background. However, the inner-core wind analysis is still too weak, so the intensity prediction cannot be improved. Assimilating Aqua MODIS sounding data improves the outer-core thermodynamic features. In particular, the low-level moisture field west of the hurricane is drier than the background field. The Aqua MODIS data has a slight impact on the track forecast, but more importantly shows evidence of impacting the model intensity predicting by retarding the incorrect prediction of intensification. All three experiments also show that bogussing of an inner-core wind vortex is required to depict the storm's initial intensity. To properly investigate Lili's weakening, data assimilation experiments which incorporate hurricane bogussing, QuikSCAT winds, GOES-8 winds, and Aqua MODIS sounding data were performed. The 4DVAR satellite-bogus data assimilation is conducted in two consecutive 6-h windows preceding Lili's weakening. The hurricane's west side is drier than the first guess after including the Aqua MODIS sounding data in 4DVAR. Low-level dry air propagates towards the hurricane center from the southwest quadrant, resulting in an open eyewall at 0000 UTC 3 October, when the hurricane begins to rapidly weaken. The satellite-bogus data assimilation experiment matches the initial intensity better than the bogus-only run that is conducted using only 4DVAR vortex bogussing. Moreover, the satellite-bogus 4DVAR run weakens whereas the bogus-only 4DVAR keeps the intensity constant for the first 6 h. Overall, the satellite data – particularly the Aqua MODIS sounding information – makes an immediate impact on the hurricane intensity change beyond normal bogussing procedures. The track forecast with the satellite data is also more accurate than just using bogussing alone. This study suggests that dry air intrusion played an important role in Lili's rapid weakening. It also demonstrates the potential benefit of using satellite data in a 4DVAR context – particularly high-resolution soundings – on puzzling cases like Hurricane Lili (2002).