Validation of SMOS Brightness Temperatures During the HOBE Airborne Campaign, Western Denmark

Abstract

The Soil Moisture and Ocean Salinity Mission (SMOS) delivers global surface soil moisture fields at high temporal resolution which is of high relevance for water management and climate predictions. For data validation, an airborne campaign with the L-band radiometer EMIRAD-2 and concurrent ground sampling was carried out within one SMOS pixel in the Skjern River Catchment, Denmark. By means of this dataset the objective of this study is a step-wise comparison of brightness temperatures from point via air-to spaceborne (SMOS) scale. From soil moisture samples brightness temperatures were estimated through the L-band Microwave Emission of the Biosphere (L-MEB) model with land-cover specific model settings. A simple uncertainty assessment by means of a set of model runs with parameters varied within a most likely interval resulted in a considerable range of brightness temperatures. Under certain constellations the ground data was in good agreement with EMIRAD. Likewise was the latter in accordance with SMOS data. The comparison of more sophisticated upscaling by means of air-and spaceborne weighting functions and a simple mean showed no significant change. Small temporal variability in soil moisture conditions and RFI-prone SMOS data throughout the campaign limited the extent of the validation work. Further attempts over longer time frames are planned by means of soil moisture network data as well as studies on the impacts of organic layers under natural vegetation and higher open water fractions at surrounding grid nodes.

Analysis of SMOS brightness temperature and vegetation optical depth data with coupled land surface and radiative transfer models in Southern Germany

“…Examples are Albergel et al (2011), Montzka et al (2011), Parrens et al (2012 and Bircher et al (2012). Albergel et al (2011) and Parrens et al (2012) have shown that there is still potential to improve soil moisture retrievals from SMOS brightness temperatures in southern France.…”

“…They used calibrated statistical relationships based on reference soil moisture values and additional information like leaf area index (LAI) simulated by a land surface model to produce better soil moisture estimates. Bircher et al (2012) have compared SMOS L1c and airborne brightness temperatures with modelled brightness temperatures using in situ data as input on different spatial scales on one day in Denmark. They developed an improved L-MEB parameterisation for local conditions.…”

“…They developed an improved L-MEB parameterisation for local conditions. It has been reported, for example, by Bircher et al (2012) and Panciera et al (2009) that it is necessary to optimise the parameterisation under local conditions to obtain best results. Bircher et al (2012) and Hornbuckle et al (2011) report that brightness temperatures at certain angles may be more reliable than at others.…”

Soil Moisture Data for the Validation of Permafrost Models Using Direct and Indirect Measurement Approaches at Three Alpine Sites

“…Due to their coarse resolution and the complex retrieval algorithms for the signals in steep topographic terrains, they are unable to capture the true soil moisture in regions with pronounced topography that changes over small spatial scale. Furthermore, the in situ soil moisture measurements necessary for the validation of remote sensing products (e.g., Albergel et al, 2011;Bircher et al, 2012;Rautiainen et al, 2012;Magagi et al, 2013) are also sparse in these areas, since measurements in mountainous terrains are costly to implement and were so far not in the focus of climatic studies. Although, the influence of soil moisture on the thermal regime of frozen ground was confirmed in many modeling studies (e.g., Hinkel et al, 2001;Boike et al, 2008;Westermann et al, 2009;Scherler et al, 2010) soil moisture measurements remain restricted to uncoordinated and project based installations (e.g., Hilbich et al, 2011;Rist and Phillips, 2005).…”

Soil Moisture Retrieval Based on GPS Signal Strength Attenuation

“…As reference, in situ time domain reflectometry (TDR) or frequency domain (FD) probes and gravimetric measurements are used as well as model approaches [28][29][30][31][32][33][34]. A comprehensive overview on in situ SM measurements from many different networks around the globe can be obtained in the International Soil Moisture Network (ISMN) [35].…”