Studies of 19.5 GHz sky radiometric temperature: Measurements and applications

Abstract

Using a single‐frequency ground‐based microwave radiometer with steerable horn antenna at frequency 19.5 GHz, we measure sky radiometric temperature over the Taiwan area during the period May 1997 to September 1999. Statistics indicate that for more than 80% of the time, in the absence of surface precipitation, the brightness temperature is above 27 K. Different elevation angle observations show that the median brightness temperature increases with decreasing elevation angle from about 30 K at zenith to about 128 K at 80° off‐zenith angle. The opacity (or gaseous attenuation) is also estimated from measured brightness temperature. We found that the elevation angle variation of the median value of opacity follows exactly the secant law, while that of opacity with other probability does not. The mean radiated temperature Tm over the Taiwan area is also calculated using 5 years of radiosonde data, showing that Tm is a function of elevation angle and its value at 10° elevation angle is greater than that at zenith by 0.5–1.3%. A method of retrieving integrated precipitable water vapor content (IPWV) from the observed brightness temperature measured by a single‐frequency radiometer at 19.5 GHz is developed and examined. Comparing the retrieved and measured IPWVs shows a good agreement between them, validating the method proposed in this article. The correlation between 19.5 GHz radiometric opacity and 52 MHz wind profiler backscatter from clear‐air sky is also analyzed. The result shows that the correlation will be negative if the vertical distribution of the wind profiler backscatter is characterized by salient layered structures. Therefore great caution should be taken when radiometric measurements are incorporated with wind profiler backscatter to retrieve the humidity profile.