L-band remote sensing increases sampled levels of global soil moisture - air temperature coupling strength

Authors: DONG, J. - US Department Of Agriculture (USDA); Crow, Wade

Submitted to: Remote Sensing of Environment
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 10/16/2018
Publication Date: 10/29/2018
Citation: Dong, J., Crow, W.T. 2018. L-band remote sensing increases sampled levels of global soil moisture - air temperature coupling strength. Remote Sensing of Environment. 220:51-58. https://doi.org/10.1016/j.rse.2018.10.024.
DOI: https://doi.org/10.1016/j.rse.2018.10.024

Interpretive Summary: In agricultural regions, summer heatwaves can significantly reduce agricultural crop yields. Recent research has shown that these events can be intensified by interactions between the land surface and lower atmosphere. This suggests that monitoring land surface conditions (particularly soil moisture) can help us better predict the occurrence and severity of heatwaves. Because large-scale observations of soil moisture are difficult to obtain, remotely sensed surface soil moisture retrievals should (in theory) contribute significantly to these efforts. However, to date, research with older remote sensing products has concluded that satellite-based observations sample soil moisture over too superficial a vertical soil layer to be of practical value for these efforts. This paper updates this research using newer soil moisture products generated by the NASA Soil Moisture Active/Passive (SMAP) mission and demonstrates that the failure of older remote sensing products is attributable not to their limited vertical sampling but rather inadquate retrieval precision. In fact, higher-precision SMAP surface soil moisture products are of clear value for efforts to measure and monitor land-atmosphere interactions impacting summer climate in the central United States. This insight will eventually be used to improve coupled land/atmosphere models that form the basis of climate projections over important agricultural regions of the United States.

Technical Abstract: Due to their shallow penetration of the soil column, remote-sensing (RS) soil moisture retrievals are often considered ill-suited for measuring the strength of soil moisture – air temperature coupling. Consequently, soil moisture proxies derived from antecedent rainfall considerations are commonly applied in their place. However, the suitably of satellite RS products has not yet been examined for newer soil moisture products derived from L-band microwave radiometry. This study compares correlations between monthly soil moisture and the monthly number of summertime hot days (NHD) for the case of three separate RS-based soil moisture products and a fourth soil moisture proxy derived from the standard precipitation index (SPI). Compared with SPI-based estimates, C- and X-band RS soil moisture products demonstrate a significantly (at p = 0.05 [-] confidence) weaker correlation with NHD. However, 2010-2018 L-band Soil Moisture and Ocean Salinity (SMOS) based soil moisture-NHD correlation are generally comparable to the SPI case. Furthermore, utilizing higher-precision 2015-2018 soil moisture products from the L-band Soil Moisture Active and Passive (SMAP) mission further strengthens soil moisture-NHD correlation and outperforms SPI over global hot-spot regions (significant at p = 0.05 [-] confidence). Combined with the general equivalence of monthly surface and root zone soil moisture anomalies, these results imply that the signal-to-noise ratio (SNR, i.e. the relative size of soil moisture signal and random observation error variances) of RS-based surface soil moisture product, instead of their vertical measurement depth, is the key limiting factor determining their ability to quantify land surface - atmosphere interaction strengths. Based on this, we argue that L-band soil moisture products have reached a sufficient level of SNR to be of value for the study of land surface - atmosphere coupling.