Abstract
The Soil Moisture Active Passive (SMAP) mission was designed to provide a global mapping of soil moisture (SM) measured by L-band passive and active microwave sensors. In this study, we evaluate the newly released SMAP enhanced SM products over the Tibetan Plateau by performing comparisons among SMAP standard products, in-situ observations and Community Land Model (CLM) simulations driven by high-resolution meteorological forcing. At local scales, the enhanced SMAP products, the standard products and CLM simulations all generally compare well with the in-situ observations. The SMAP products show stronger correlations (0.64–0.88) but slightly larger unbiased root mean square errors (ubRMSE, ~0.06) relative to the CLM simulations (0.58–0.79 and 0.037–0.047, for correlation and ubRMSE, respectively). At the regional scale, both SMAP products show similar spatial distributions of SM on the TP (Tibetan Plateau), although, as expected, the enhanced product provides more fine details. The SMAP enhanced product is in good agreement with model simulations with respect to temporal and spatial variations in SM over most of the TP. Regions with low correlation between SMAP enhanced products and model simulations are mainly located in the northwestern TP and regions of complex topography, where meteorological stations are sparse and non-existent or elevation is highly variable. In such remote regions, CLM simulations may be problematic due to inaccurate land cover maps and/or uncertainties in meteorological forcing. The independent, high-resolution observations provided by SMAP could help to constrain the model simulation and, ultimately, improve the skill of models in these problematic regions.
Highlights
Soil moisture (SM) is an essential variable for the understanding, modeling and forecasting of weather and climate [1,2,3], the monitoring and early warning of floods and droughts [4,5], and the estimation of crop yield [6,7]
This study provides the first comprehensive assessment of Soil Moisture Active Passive (SMAP) 9-km enhanced Level 3 passive SM product over the Tibetan Plateau, by comparing it against ground observations in two SM and temperature monitoring networks, the SMAP 36-km Level 3 passive SM product and Community Land Model (CLM) simulations
Comparison of the standard and enhanced SMAP SM products shows that the enhanced product includes similar spatial variation to those in the standard one, but with a greater ability to produce fine details in the spatial distribution of SM
Summary
Soil moisture (SM) is an essential variable for the understanding, modeling and forecasting of weather and climate [1,2,3], the monitoring and early warning of floods and droughts [4,5], and the estimation of crop yield [6,7]. The National Aeronautics and Space Administration’s (NASA) Soil Moisture Active Passive (SMAP) mission [8] was launched on 31 January 2015, and it has been providing high resolution global maps of SM and freeze–thaw states since 31 March 2015 (passive sensor) and 13 April 2015 (active sensor). The SMAP satellite carries an L-band radar (centered at 1.26 GHz) and an L-band radiometer (centered at 1.41 GHz) that provide backscatter information at a 3-km spatial resolution and brightness temperature observations at 36-km resolution. Despite the loss of the radar, SMAP continues to provide high-resolution SM observations to the extent possible by using two approaches [11]. The first approach combines the current SMAP coarse-resolution passive observations with high-resolution radar observations from other satellites in orbit [12,13]. The enhanced SMAP L3_SM_P_E product, which is the focus of this paper, is a daily global composite of the enhanced SMAP L2_SM_P_E product
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