Soil Moisture Experiments (SMEX) 2002-2004
Measuring Soil Moisture
Soil moisture has a significant impact on human safety and our economy. It affects land-use and agricultural planning and the cost, quantity, and quality of food. Accurate weather forecasts require the rate of transfer of soil moisture to the atmosphere, whether by evaporation or plant transpiration. Trends in soil moisture are also reflected in climate and regional weather, and affect drought and fire danger levels. Detecting excessive soil moisture is needed for flood warning and mitigation.
To extend sparse observations of soil moisture at ground stations to national and global coverage requires microwave radiometers aboard satellites. A new instrument to be used by NOAA for weather and climate forecasting applications is the Japanese Advanced Microwave Scanning Radiometer E (AMSR-E), recently launched on the NASA Aqua satellite. AMSR-E images the Earth's microwave radiation in C- and X-bands, both of which are sensitive to soil moisture and vegetation cover, but with differing sensitivities that permit separation of soil moisture and vegetation information.
Optimal use of AMSR-E data by NOAA and other agencies requires an understanding of how to use the C- and X-band data for all types of soil and vegetation conditions. Accordingly, NOAA/ETL works with the USDA and NASA during the Soil Moisture Experiments (SMEX) campaigns to make critical airborne measurements needed to interpret AMSR-E data.
The 2004 Soil Moisture Experiment/North American Monsoon Experiment (SMEX04/NAME) campaign builds on preceding experiments (SGP99, SMEX02 and SMEX03) by focusing specifically on topography, vegetation and strengthening the soil moisture components of NAME. One of the main objectives of NAME is to improve prediction of warm season precipitation which is highly dependent on convection, which, in turn, is controlled, at least in part, by soil moisture and surface temperature. Therefore, an accurate characterization of spatial and temporal variability of soil moisture is critical to NAME in three ways; the spatial and temporal patterns of soil moisture estimated from remote sensing can be used for initialization and/or updating of the boundary conditions for the land surface component of land-atmosphere models, the spatial and temporal patterns of soil moisture can be used for validation of land surface model outputs, and to discern the relationship between soil moisture and warm season precipitation and associated feedback mechanisms.
The principal airborne system used during SMEX04/NAME was the NOAA/ETL Polarimetric Scanning Radiometer (PSR) system observing in the C- and X-band range. In addition to the several C- and X-band polarimetric channels the previous build of the instrument (PSR/CX) was modified to include a broadband frequency-swept spectrometer for interference mitigation studies (PSR/CXI). The Naval Research Laboratory (NRL) P-3 #674 aircraft was the platform for PSR/CXI.
The goal of the PSR/CXI was to acquire images with high spatial resolution using airborne passive microwave channels similar to those on AMSR-E and future L-band soil moisture missions, along with strategically located and designed automated and manual surface observations to verify and extend the satellite soil moisture observations over regions with sparse vegetation. Two 50 x 75 km regional study sites were established in Arizona (AZ) and Sonora, Mexico (SO) for the campaign. Measurements were timed to coincide with satellite data acquisition from the Aqua AMSR-E, Coriolis Windsat, TRMM TMI, DMSP SSM/I, Terra MODIS and ASTER, and Envisat ASAR. By the end of the campaign, a total of 11 and 11 mappings of the AZ and SO grids were obtained, respectively, over a 23 day period.
SMEX03 was designed to provide data to further develop and validate algorithms to accurately retrieve soil moisture from current satellite radiometers such as AMSR-E as well as future microwave sensors such as the NPOESS Conical Microwave Imager and Sounder (CMIS). SMEX03 includes several study regions (Oklahoma, Alabama, and Georgia) to provide a diversity of soil types, moisture levels, and vegetation cover.
SMEX03 uses as its principal airborne instrument the ETL Polarimetric Scanning Radiometer (PSR) system with C- and X-bands (PSR/CX). PSR/CX provides unique conically-scanned airborne microwave imagery with full polarization capability and high spatial resolution. The system also detects and rejects the anthropogenic radio interference that has been shown to corrupt many AMSR-E measurements. The small pixel size of PSR/CX (~0.5 km compared with ~75 km for AMSR-E) provides an important bridge across the large range of sampling scales from that of the SMEX03 ground stations to the AMSR-E satellite imagery.
The PSR/CX system was first demonstrated in Iowa during summer 2002 joint NASA-NOAA-USDA "Soil Moisture Experiment" (SMEX02). NASA sought to augment sparse probe-based measurements of soil moisture with global measurements from the Advanced Microwave Scanning Radiometer (AMSR-E) sensor on its Aqua satellite, and worked with ETL during SMEX02 to validate the AMSR-E sensor products. Additional science goals of SMEX02 included extension of the AMSR-E observations and algorithms to more challenging vegetation conditions typical of agricultural regions, and integration of land surface and atmospheric boundary layer measurements to better understand moisture feedback mechanisms.