ABSTRACT

The ability to correct for the influence of forest cover is crucial for retrieval of surface geophysical parameters such as snow cover and soil properties from microwave remote sensing. Existing correction approaches to brightness temperatures for northern boreal forest regions consider forest transmissivity constant during wintertime. However, due to biophysical protection mechanisms, below freezing air temperatures freeze the water content of northern tree species only gradually. As a consequence, the permittivity of many northern tree species decreases with the decrease of air temperature under sub-zero temperature conditions. This results in a monotonic increase of the tree vegetation transmissivity, as the permittivity contrast to the surrounding air decreases. The influence of this tree temperature-transmissivity relationship on the performance of the frequency difference passive microwave snow retrieval algorithms has not been considered. Using ground-based observations and an analytical model simulation based on Mätzler’s approach (1994), the influence of the temperature-transmissivity relationship on the snow retrieval algorithms, based on the spectral difference of two microwave channels, is characterized. A simple approximation approach is then developed to successfully characterize this influence (the RMSE between the analytical model simulation and the approximation approach estimation is below 0.3 K). The approximation is applied to spaceborne observations, and demonstrates the capacity to reduce the influence of the forest temperature-transmissivity relationship on passive microwave frequency difference brightness temperature.

摘要

修正森林覆盖影响的能力对于从微波遥感反演积雪和土壤特性等地表地球物理参数至关重要。现有的北方北方森林地区亮度温度校正方法考虑了冬季森林的透射率恒定。然而，由于生物物理保护机制，低于冰点的气温只会逐渐冻结北方树种的水分。因此，在零下温度条件下，许多北方树种的介电常数随着气温的降低而降低。这导致树木植被透射率单调增加，因为与周围空气的介电常数对比度降低。尚未考虑这种树的温度-透射率关系对频差被动微波雪反演算法性能的影响。使用地面观测和基于 Mätzler 方法 (1994) 的分析模型模拟，表征了温度-透射率关系对基于两个微波通道光谱差异的雪反演算法的影响。然后开发了一种简单的近似方法来成功表征这种影响（分析模型模拟和近似方法估计之间的 RMSE 低于 0.3 K）。该近似值应用于星载观测，