Abstract
In ground-based microwave radiometer remote sensing, low-elevation-angle (−3°~3°) radiation data are often discarded because they are considered to be of little value and are often difficult to model due to the complicated mechanism. Based on the observed X-band horizontal polarization low elevation angle microwave radiation data and the meteorological data at the same time, this study investigated the generation mechanism of low elevation angle brightness temperature (LEATB) and its relationship with meteorological data, i.e., temperature, humidity, and wind speed, under low sea state. As a result, one could find that the LEATB was sensitive to the atmosphere at the elevation angle between 1° to 3°, and a diurnal variation of the LEATB reached up to 10 K. This study also found a linear relationship between the LEATB and sea surface wind speed under low sea state at an elevation range from −3° to 0°, i.e., the brightness temperature decreased as the wind speed increased, which was inconsistent with the observations at the elevation angle from −10° to −5°. The variation of the LEATB difference according to the change in the over-the-horizon detection capability (OTHDC) of the shipborne microwave radar was examined to identify the reason for this phenomenon theoretically. The results showed that the LEATB difference was significantly influenced by a change in the OTHDC. Further, this study examined a remote sensing method to extract the sea surface wind speed data from experimental LEATB data under low sea state. The results demonstrated that the X-band horizontal polarization LEATBs were useful to retrieve the sea surface wind speed data at a reasonable accuracy—the root mean square error of 0.02408 m/s. Overall, this study proved the promising potential of the LEATB data for retrieving temperature profiles, humidity profiles, sea surface winds, and the OTHDC.
Highlights
Remote sensing of microwave radiation for atmosphere and ocean is an important and active research field and has produced many achievements that have been widely used [1,2]
Sea surface upwelling radiation TB data at 6.925, 10.65, and 18.7 GHz were used to measure wind speeds over the sea surface by advanced microwave scanning radiometer-E (AMSR-E), and the analysis resulted in the estimated bias and root mean square errors (RMSEs) of −0.126 and 1.191 m/s, −0.094 and 1.152 m/s, and −0.085 and 1.338 m/s, respectively [5]
The wind speeds within super typhoon Nepartak have been retrieved using brightness temperatures at 6.9 and 10.7 GHz derived from the advanced microwave scanning radiometer 2 (AMSR2) sensor in the global change observation mission-water1 (GCOM-W1) satellite [8]
Summary
Remote sensing of microwave radiation for atmosphere and ocean is an important and active research field and has produced many achievements that have been widely used [1,2]. Ocean surface wind field can be measured by shore-based or shipborne high-frequency or X-band radars [11,12,13], ground-based microwave radiometers have been used to monitor the sea surface meteorological parameters. Ground-based microwave radiometers provide the multi-elevation-angle atmospheric radiation information through the elevation-angle-scanning measurement to improve the accuracy of the monitored atmospheric profile. A ground-based X-band VV polarized radar, located at point B, about 50 meters away from the radiometer at an altitude of 25 m ASL, was used to obtain the sea surface radar echo data in the same direction as the microwave radiometer. September 2018, September 2018, 12 October 2018, and 23 October 2018were the dates when the radar was turned on twice in a day while only once on 21 October 2018
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