为衡量全极化微波辐射计各通道对不同环境参数的探测能力,在构建全极化微波辐射传输正演模型的基础上,采用敏感性分析的方法,在固定背景场条件下,对全极化微波辐射计不同频率、各极化通道亮温对海面风速、海面风向、海面温度、大气水汽含量和云中液态水含量等重要环境参数的敏感度进行了分析和量化计算,对正演模型仿真亮温、星载全极化微波辐射计WindSat实测亮温相对于真实背景场参数、NCEP分析场资料和TAO/TRITON浮标实测海面风场数据的敏感性进行了分析。分析结果验证了全极化微波辐射计对海面风场的观测能力,衡量了上述重要环境参数对星载全极化微波辐射计各通道的单独影响程度,为中国自主研制全极化微波辐射计时通道指标设计、环境参数反演通道选取及算法提供支持。

Polarimetric microwave radiometer can provide sea surface wind vector products; this ability is a new development in spaceborne passive sensing. Sensitivity analysis of polarimetric microwave radiometer brightness temperature observations with respect to environmental factors may indicate the effect of environmental factors on polarimetric microwave radiometer brightness temperature observations. This finding may provide theoretical support for channel selection in the retrieval of important physical factors.
On the basis of the polarimetric microwave forward model, we adopted sensitivity analysis to analyze, calculate, and quantify the sensitivity of each channel from the polarimetric microwave radiometer with important environmental parameters, such as sea surface wind speed, sea surface wind direction, sea surface temperature, atmospheric water vapor content, and cloud liquid water content, under fixed background field conditions. We also analyzed the sensitivity of the brightness temperature, which is simulated by the forward model or measured by using spaceborne polarimetric microwave radiometer WindSat, with respect to important environment parameters under real background field conditions from WindSat environmental data record, NCEP analysis data field, and TAO/TRITON buoy data record.
Results include: (1) The brightness temperature of 6.8 GHz and 10.7 GHz vertical and horizontal polarization channel has good linearity relative to sea surface temperature, which can be used for sea surface temperature inversion. (2) The changes in the brightness temperature of 23.8 GHz vertical and horizontal polarization channel are the largest relative to the rate of atmospheric water vapor content changes, which is mainly used for atmospheric water vapor content retrieval. (3) Vertical and horizontal polarization channels at 37 GHz have relatively obvious characterization relative to cloud liquid water content, which could be used to cloud liquid water content inversion. (4) The brightness temperatures of vertical polarization and horizontal polarization channel have good linearity retrieval to wind speed at a low observation frequency, which is used for sea surface wind speed inversion. (5) When the wind direction changes, the brightness temperature channels of the third and fourth Stokes channels exhibit obvious inverted fluctuation characteristics, which can be used for sea surface wind direction inversion. (6) The fluctuation with wind direction is completely covered in the real background field at the vertical and horizontal polarization channels, but the third and fourth Stokes channel observations can still characterize wind direction changes. (7) The sensitivity of the third and fourth Stokes channels to other environmental parameters is much lower than the sensitivity to sea surface wind speed and wind direction, which cannot be used for inversion of other environmental parameters. (8) The third and fourth Stokes channels for sea surface wind speed are also highly sensitive. Under a high wind speed condition, the actual measured brightness temperature of the third and fourth Stokes channels is large, which indicates that the measurement includes strong wind vector signals. Sea surface wind retrieval accuracy is better under high wind conditions compared with that under low wind speed conditions. (9) In the 0°±30°, 180°±30° range, the brightness temperature measured value of the third and fourth Stokes channels is small, and the noise signal influence to wind is large. Thus, the retrieval accuracy of wind directions decreases in those ranges, and retrieval results appear fuzzy. (10) In the actual measurement, the wind direction signal from the third and fourth Stokes channels is subject to interference from other environmental factors to some extent, which makes retrieving the sea surface wind direction difficult.
The conclusions from sensitivity analysis could provide support to the independent development of China to design a polarimetric microwave radiation channel, select an environmental parameter inversion channel, and propose an environmental parameter inversion algorithm. Moreover, this study establishes a foundation to improve the accuracy of surface wind vector retrieval by removing adverse effects caused by atmospheric water vapor content and cloud liquid water content.