SENSITIVITY STUDIES OF TOVS RETRIEVALS WITH 3I AND ITPP RETRIEVAL ALGORITHMS - APPLICATION TO THE RESOLUTION OF MESOSCALE PHENOMENA IN THE ANTARCTIC

Retrievals from TIROS-N Operational Vertical Sounder (TOVS) data with the International TOVS Processing Package (ITPP version 3.3) and the Improved Initialisation Inversion (3I version 2) algorithms are investigated. The comparative study comprises the retrieval mechanism from the first guess to the final result for collocated radiosonde/satellite data as well as the problem of horizontal resolution for simulated Antarctic cold air pools. For the latter problem, TOVS brightness temperatures are computed for simulated NOAA-10 overpasses in the Weddell Sea region of Antarctica by the ITPP and the 3R (Rapid Radiance Recognition) forward models for retrievals with ITPP and 3I, respectively. The main results of this study are: Comparisons of ITPP and 3R forward models with collocated radiosonde data for cloudless Antarctic profiles yield differences less than 2 K except for HIRS-channel 1 and water vapour channels. For typical examples of Antarctic profiles for the coastal region, results of 3I retrievals lie closer to the collocated radiosonde profiles than ITPP retrievals even for cases with surface inversions. Simulations of cold air pools for cloud-free conditions show that cold air pools with a strong temperature anomaly and with diameters of 800 km and greater are well reproduced by ITPP and 3I. For smaller cold air pools with a moderate temperature perturbation, the resolution limits are reached if the diameter is smaller than 400 and 200 km for ITPP and 3I, respectively. ITPP retrievals have smaller horizontal gradients and higher values for geopotential thicknesses in the center of the cold air pools. The better performance of the 3I retrievals for smaller cold air pools is a consequence of the better first guess of the TIGR dataset compared with the regression first guess of the ITPP.