Research
Approach
One of our research topics centers on utilizing a coupled hydrologic/radiobrightness model to provide “best estimates” of footprint-scale mean volumetric soil moisture and TB at C and X bands with associated variance and confidence limits. This information provides quantitative validation of C and X band TB and soil moisture derived from the Advanced Microwave Scanning Radiometer for the Earth Observing System, or AMSR-E. Modeling is conducted at a high spatial and temporal resolution relative to AMSR observations. In so doing, we are able to evaluate a.) the errors associated with using limited GSM data or point-scale measurements of soil moisture from network stations to estimate footprint-scale mean soil moisture, b.) the errors associated with asynchronous sampling times, and c.) the relationship between surface moisture (~1 cm) and profile moisture. These analyses are necessary to characterize the accuracy of the AMSR data products at footprint scale. By applying the model in simulation mode we will assess the limits of roughness/soil/vegetation parameter values beyond which AMSR soil moisture retrievals are not possible. Comparison between these simulations and AMSR-derived TB will identify areas where subpixel-scale heterogeneity warrants the use of effective parameter values or regions where AMSR C and X band soil moisture retrievals are not feasible.
Our hydrologic model produces soil moisture and temperature profiles that are utilized by the radiative transfer model to estimate TB at C and X bands. Appropriate parameters for the radiobrightness model are derived by tuning the model to simulate TB observations by the Polarimetric Scanning Radiometer (PSR). We run the model by assimilating soil moisture data from the various networks of in situ automated instruments, soil moisture derived from GSM data, and TB derived from aircraft-based microwave instruments during the Soil Moisture Experiments in 2002 (SMEX02). We aggregate the high-resolution model-derived C and X band TB to the AMSR footprint scale. We also run the model at the AMSR footprint scale using footprint mean parameters and variables. Because numerous uncertainties exist in defining the parameters and variables required for soil moisture modeling, model runs are being conducted using an ensemble of input data to derive the best estimate of the mean and variance of spatially-distributed profile soil moisture and TB at C and X bands. The two model-derived TB data sets are compared with the AMSR-observed C and X band TB. The results of the ensemble runs will be analyzed and interpreted to address the AMSR validation issues.
Accomplishments From The Past Year
We ran our hydrologic model over the SMEX02 domain to produce soil moisture and temperature profiles that will be utilized in our radiative transfer model. In an attempt to better understand the contrast in brightness temperatures for different land cover types in the SMEX02 domain, we developed an algorithm to estimate field scale mean microwave brightness temperatures from aircraft data. This algorithm also has enormous potential for deconvolving AMSR-E data at the EASE grid scale to assess the extent to which the simplistic linear average resampling method of converting footprint values to EASE grid values is homogenizing the data set. Our independent field sampling program during SMEX02 combined with our modeling efforts also allowed us to investigate aspects of the relationship between the near-surface moisture profile, TB and retrieved effective moisture. We also supported the Soil Moisture Experiments in 2003 (SMEX03) experiment taking a leadership role in the planning and execution of sampling activities in the northern Alabama study area.
Acknowledgements:
This research was supported by NASA through grant no. 291-07-75-90 to Universities Space Research Association and grant no. NCCW-0084 to Alabama A&M University, Center for Hydrology, Soil Climatology and Remote Sensing.