Sensitivity of Passive Microwave Observations to Soil Moisture and Vegetation Water Content: L-Band to W-Band

摘要

Ground-based multifrequency (L-band to W-band, 1.41–90 GHz) and multiangular (20 $^{circ}$–50$^{circ}$ ) bipolarized (V and H) microwave radiometer observations, acquired over a dense wheat field, are analyzed in order to assess the sensitivity of brightness temperatures $(T_{b})$ to land surface properties: surface soil moisture $(m_{v})$ and vegetation water content (VWC). For each frequency, a combination of microwave $T_{b}$ observed at either two contrasting incidence angles or two polarizations is used to retrieve $m_{v}$ and VWC, through regressed empirical logarithmic equations. The retrieval performance of the regression is used as an indicator of the sensitivity of the microwave signal to either $m_{v}$ or VWC. In general, L-band measurements are shown to be sensitive to both $m_{v}$ and VWC, with lowest root mean square errors (0.04 $hbox{m}^{3} cdot hbox{m}^{-3}$ and 0.52 $hbox{kg} cdot hbox{m}^{-2}$ , respectively) obtained at H polarization, 20$^{circ}$ and 50$^{circ}$ incidence angles. In spite of the dense vegetation, it is shown that $m_{v}$ influences the microwave observations -n-nfrom L-band to K-band (23.8 GHz). The highest sensitivity to soil moisture is observed at L-band in all configurations, while observations at higher frequencies, from C-band (5.05 GHz) to K-band, are only moderately influenced by $m_{v}$ at low incidence angles (e.g., 20 $^{circ}$). These frequencies are also shown to be very sensitive to VWC in all the configurations tested. The highest frequencies (Q- and W-bands) are shown to be moderately sensitive to VWC only. These results are used to analyze the response of W-band emissivities derived from the Advanced Microwave Sounding Unit instruments over northern France.