Abstract
Precipitable water vapor (PWV) is one of the most variable components of the atmosphere in both space and time. In this study, a passive microwave-based retrieval algorithm for PWV over land without land surface temperature (LST) data was developed. To build the algorithm, two assumptions exist: (1) land surface emissivities (LSE) at two adjacent frequencies are equal and (2) there are simple parameterizations that relate transmittance, atmospheric effective radiating temperature, and PWV. Error analyses were performed using radiosonde sounding observations from Zhangye, China, and CE318 measurements of Dalanzadgad (43°34′37′′N, 104°25′8′′E) and Singapore (1°17′52′′N, 103°46′48′′E) sites from Aerosol Robotic Network (AERONET), respectively. In Zhangye, the algorithm had a Root Mean Square Error (RMSE) of 4.39 mm and a bias of 0.36 mm on cloud-free days, while on cloudy days there was an RMSE of 4.84 mm and a bias of 0.52 mm because of the effect of liquid water in clouds. The validations in Dalanzadgad and Singapore sites showed that the retrieval algorithm had an RMSE of 4.73 mm and a bias of 0.84 mm and the bigger errors appeared when the water vapor was very dry or very moist.
Highlights
Precipitable water vapor (PWV) is an important atmospheric component that influences many atmospheric processes [1]
It is apparent that there is an Root Mean Square Error (RMSE) of 0.60 mm and a bias of 0.019 mm between the newly estimated PWV and the originally calculated values and that no land surface emissivities (LSE) error exists
Over 99% of the errors are under 2 mm, which indicates that the PWV model is not very sensitive to errors in the LSE
Summary
Precipitable water vapor (PWV) is an important atmospheric component that influences many atmospheric processes [1]. It is crucial for studies of climate change and global warming because water vapor is the most abundant greenhouse gas [2]. Sun photometers operating in the strong water vapor absorption band have been used to determine PWV, but they are limited to be clear of clouds on the path to the Sun [13,14,15,16]. Dual-frequency, ground-based microwave radiometers have been used for more than 30 years to derive both PWV and cloud liquid water; they use frequency bands around the water vapor absorption line at 22.235 GHz
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