Amplification of the Impact of Assimilating ATOVS Radiances on Simulated Surface Air Temperatures over Canterbury by the Southern Alps, New Zealand

Abstract

he New Zealand Limited Area Model is used to investigate the impact of assimilating NOAA-15 and -16 Advanced Television and Infrared Observation Satellite (TIROS) Operational Vertical Sounder (ATOVS) radiances on surface air temperature over Canterbury, New Zealand, for two föehn cases in January 2004. For both cases, the simulated westerly-northwesterly wind crossing the Southern Alps and descending in the lee (i.e., a föehn) was stronger with ATOVS data (pass 2) than without ATOVS data (pass 1). Also, for one case, the timing of the passage of a cold front over Canterbury was more accurately forecast in pass 2. The associated differences in the potential height ?H and winds ?V over South Island between pass 1 and pass 2 for both cases developed from small differences in the initial conditions. It is suggested the dynamical forcing of the Southern Alps contributes to the amplification of ?H and ?V. The enhanced ?V led to stronger adiabatic descent in the lee (or a stronger föehn) with stronger adiabatic warming and surface diabatic heating in pass 2. Additionally, the later passage of the cold front in pass 2 during one case allowed a longer period of heating of the surface air ahead of the cold front. As a result, large well-organized differences in surface air temperature between pass 1 and pass 2 (?T of 4?10 K) occurred over Canterbury. Thus, the Southern Alps acted to amplify the impact of assimilating ATOVS radiances on simulated surface air temperature over Canterbury under föehn conditions. Verification with surface observations at five climate stations over Canterbury showed a positive impact of ATOVS radiance assimilation for the two cases.