Aquarius L-band scatterometer and radiometer observations over a Tibetan Plateau site

In this paper, the impact of freeze-thaw, soil moisture and vegetation on L-band backscatter and emission is studied using Aquarius scatterometer/radiometer measurements collected from August 2011 to May 2013 over the northeastern part of the Tibetan Plateau. The study area is the Maqu region that holds a regional-scale monitoring network consisting of twenty soil moisture/temperature stations, which is selected as one of the core international Calibration/Validation (Cal/Val) sites for NASA's Soil Moisture Active Passive (SMAP) mission. Comparisons of Aquarius scatterometer/radiometer measurements with soil moisture recorded by capacitance probes installed at a 5-cm soil depth illustrate that (i) L-band microwave observations are also sensitive to the amount of liquid water in soil below freezing point, and (ii) the sensitivity of Aquarius observations over the Maqu area dissipates above soil moisture contents of 0.3 m(3) m(-3). Further effects of vegetation become directly noticeable only within passive microwave observations at moisture levels larger than 0.4 m(3) m(-3). The impact of vegetation is studied in more detail through analysis of the Radar Vegetation Index (RVI). Although seasonal variability is captured, the dynamic range of the RVI is insufficient for a meaningful signal-to-noise. Further vegetation optical depth (tau) is estimated using the tau- concept by reconstructing the Microwave Polarization Difference Index (MPDI) derived from Aquarius radiometer data. Peaks in the r estimates are noted in the months January/February and July/August. Evidence suggests that the magnitude of tau is a measure for the frost depth when temperatures are below freezing point whereas the behavior of tau in the warm season is in line with the vegetation dynamics. (C) 2015 Elsevier B.V. All rights reserved.