Remote Sensing of Precipitable Water Vapor and Cloud Liquid Water Content Over the Ocean by Ground-Based Dual-Wavelength Radiometer

The microwave radiation transfer in a cloudy atmosphere at wavelength 1.35cm and 0.85cm was calculated by using conventional radiosonde data at Guangzhou, China (23°N, 113.3°E) and the Yap Island, a trust territory of the U. S. A. (9.6°N, 138°E). Regressive relationships were established among microwave brightness temperature (Tb _λ ), microwave attenuation (τ _λ ), atmospheric precipitable water vapor (Q), and liquid water content of clouds (L). A physical-statistical method is modified to retrieve the Q and L from Tb _λ and τ _λ based on these relationships.From December 1985 to January 1986, we jointed the “Survey over Western Pacific Tropical Area,” a part of a research project entitled “Ocean-Atmosphere Interaction over Western Pacific Area and Its Relation to Interannual Variation of Eastern Asia Climate” sponsored by the Chinese Academy of Sciences. The unique data of Q and L over tropical and subtropical areas were obtained by measurements of the dual-wavelength radiometers. Some statistical results and cases are given.In addition, a model calculation indicates the possibility of determining the precipitable water vapor in a clear atmosphere by using 0.85 cm radiometer only. The measurements over the ocean confirm the theoretical estimation. Based on this, a criterion of identifying cloudy or clear days by using dual-wavelength radiometers is suggested.