Typhoon Simulations with Assimilated GPS Occultation Refractivity

¹Ching-Yuang Huang, ²Ying-Hwa Kuo, ³Shu-Hua Chen and ² Francois Vandenberghe

¹Department of Atmospheric Sciences, National Central University, Taiwan

²National Center for Atmospheric Research, Boulder, USA

³Department of Land, Air, and Water Resources, University of California, Davis, USA Abstract

The mesoscale model (MM5) with three-dimensional variational (3DVAR) assimilation is utilized to investigate influences of GPS occultation refractivity on simulations of typhoons past Taiwan.  The GPS refractivity data are taken from currently onboard CHAMP and SAC-C satellites, which provide several profiles in the simulated oceanic region.  Moisture increments from ingested GPS refractivity exhibit a maximum magnitude of about 2 g kg^-1 associated with much less temperature increments.  The differences between the computed and observed refractivities are only about 10 units at the middle levels with an influential radius of several hundreds of km.  For the no-GPS run, the simulated Typhoon Nari in 2001 consistently moves southwestward toward Taiwan but then exhibits a further leftward track along northwestern Taiwan after landfall. With assimilated GPS refractivity, the simulated track is closer to the west coast.  Both rainfall intensity and track at later stages are also improved for the GPS run.  The simulated rainfalls for Nakri in 2002 for the GPS and no-GPS runs in general are similar, but for the former a more pronounced low is produced to southeast of Taiwan and then results in clearer localized precipitation in northeast Taiwan as observed.  Both Nari and Nakri runs with GPS data show improved prediction on 24-h accumulated rainfall as supported by higher threat scores and smaller rms errors against observations on the island.  A cycling 3DVAR is also explored in Nari simulations to investigate the impact of complementary QuikSCAT near-surface wind at the oceanic swath of the cyclone path.  It was found that inclusion of this surface wind results in an improved track with less lagging.