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Upscaling Flux Observations from Local to Continental Scales Using Thermal Remote Sensing

^a USDA-ARS, Hydrology and Remote Sensing Lab., Bldg. 007, BARC West, 10300 Baltimore Ave., Beltsville, MD 20705
^b Dep. of Soil Science, Univ. of Wisconsin, Madison, WI 53706

View larger version (21K): [in this window] [in a new window]   Fig. 1. Schematic diagram representing the (a) ALEXI and (b) DisALEXI modeling schemes, coupling atmospheric boundary layer (ABL) and two-source surface energy balance (TSEB) submodels. The models compute fluxes of sensible heat (H) from the soil and canopy (subscripts c and s) along gradients in temperature (T), and regulated by transport resistances RA (aerodynamic), RX (bulk leaf boundary layer) and RS (soil surface boundary layer). DisALEXI uses the air temperature predicted by ALEXI near the blending height (TA) to disaggregate 5-km ALEXI fluxes, given vegetation cover [f(i)] and directional surface radiometric temperature [TRAD(i)] information derived from high-resolution remote-sensing imagery at look angles i. See Norman et al. (2003) for further details.

View larger version (146K): [in this window] [in a new window]   Fig. 2. Disaggregated maps of latent heating (30-m resolution) in 5-km ALEXI grid cells around eddy-correlation (EC) towers in the OASIS network. Crosses indicate the location of the towers, with no more than one tower per grid cell. Axes are labeled as distance in meters.

View larger version (51K): [in this window] [in a new window]   Fig. 3. Disaggregated latent heating estimates (24-m resolution) on 2 July 1997 over the El Reno study area during SGP97, projected to UTM Zone 14 coordinates. Crosses indicate locations of four flux towers, while the white line shows the El Reno (ER) track flown by the Canadian Twin Otter aircraft. Black lines represent the scale of the 5-km ALEXI grid. In this case, all towers were located in the same ALEXI grid cell.

View larger version (75K): [in this window] [in a new window]   Fig. 4. Disaggregated latent heating estimates (60-m resolution) on 1 July 2002 over the Walnut Creek study area during SMACEX, projected to UTM Zone 15 coordinates. Crosses indicate locations of 12 flux towers, while the white lines demarcate the Walnut Creek Twin Otter aircraft transects. Black lines represent the scale of the 5-km ALEXI grid. The towers were located in approximately eight ALEXI grid cells.

View larger version (28K): [in this window] [in a new window]   Fig. 5. Comparison of tower measurements of sensible heat (H), latent energy (E), net radiation (RN), and soil heat flux (G) from the OASIS (30-min averages), SGP97 (hourly averages), and SMACEX (30-min averages) experiments with instantaneous flux predictions from the ALEXI and DisALEXI models (see Table 1). The ALEXI fluxes represent a 5-km average corresponding to the model grid cell containing the tower site, while the DisALEXI fluxes have been integrated across the flux tower footprint. Open H and E symbols indicate uncorrected measurements, while gray-filled symbols represent fluxes corrected for energy budget closure by conserving the Bowen ratio. Root mean square difference (RMSD) values were computed for all four flux components combined, using closure-corrected H and E. Latent heat fluxes associated with the bare soil site in SGP97 are highlighted with an open box.

View larger version (81K): [in this window] [in a new window]   Fig. 6. Comparison of aircraft latent heat profiles acquired over the El Reno transect (SGP97) on 2 July 1997 with model predictions from DisALEXI. Uncorrected and closure-corrected aircraft fluxes as a function of position (Easting UTM, Zone 14) along transect, with DisALEXI estimates integrated (a) across the aircraft source area footprint and (b) across a swath directly beneath the aircraft. Middle panel shows DisALEXI map of latent heating with aircraft transect (upper dotted line) and maxima of footprint function (lower dotted line) indicated. White lines represent axis of footprint function (in the upwind direction).

View larger version (85K): [in this window] [in a new window]   Fig. 7. Comparison of aircraft sensible heat profiles acquired over El Reno transect (SGP97) on 2 July 1997 with model predictions from DisALEXI. Uncorrected and closure-corrected aircraft fluxes as a function of position (Easting UTM, Zone 14) along transect, with DisALEXI estimates integrated (a) across the aircraft source area footprint and (b) across a swath directly beneath the aircraft. Middle panel shows DisALEXI map of latent heating with aircraft transect (upper dotted line) and maxima of footprint function (lower dotted line) indicated. White lines represent axis of footprint function (in upwind direction).

View larger version (14K): [in this window] [in a new window]   Fig. 8. Time evolution of tower and aircraft Bowen ratio (BR) measurements and MODIS leaf area index (LAI) estimates during the Soil Moisture–Atmospheric Coupling Experiment, averaged across the Walnut Creek study area. Upper panel shows daily precipitation measured at seven rain gauges in the watershed.

View larger version (94K): [in this window] [in a new window]   Fig. 9. Multiscale evapotranspiration (ET) maps for 1 July 2002 produced with ALEXI and DisALEXI using surface temperature data from aircraft (30-m resolution), Landsat (60 m), GOES Imager (5 km), and GOES Sounder (10 km). The continental-scale ET map is a 14-d composite of clear-sky model estimates.