We would like to express our gratitude to Dr. T. J. Schmugge at USDA Hydrology Laboratory;
Beltsville for providing us the TIMS data and P. Bessemoulin of CNRM for the radiosoundings. We would also like to think Dr. A. Gillespie and other anonymous reviewers for very helpful and stimulating comments. Dr. Z.-L. Li wishes to thank Professor M. P. Stoll at LSIIT and the CNRS for allowing him to leave Strasbourg in 1998 and Dr. Wan at UCSB for providing him the financial support during his stay at UCSB. Dr. Wan is supported by the NASA Contract NAS5-31370;