Estimation of surface heat fluxes at field scale using surface layer versus mixed-layer atmospheric variables with radiometric temperature observations

摘要

Radiometric surface temperature observations T_k(#PHI#). nesr-surface meteorological/surface energy flux (METFLUX) and atmospheric boundary layer (ABL) data were collected during the Washita 94 Experiment conducted in the Little Washita Experimental Watershed near Chickasha. Oklahoma. The T_k(#PHI#) measurements were made from ground and aircraft platforms near the METFLUX stations located over vegetated sarfaces of varying amounts of cover and over bare soil. Continuous. half-hourly averaged ground-based T_k(#PHI#) measurements essentially at the point scale were calibrated with periodic ground transect and aircrafl-based T_k(#PHI#) observations at coarser resolutions so that the continuous T_k(#PHI#) measurements would be representative of surface temperatures at the field scale (i.e.. on the order of lO~4 m~2). The METFLUX data were collected nominally at 2 m above the surface, while ABL measurements were made in the lower 8-10 km of the atmosphere. The "local': wind speed. u, and air temperature. T_A. from the METFLUX stations, as well as the mixed-layer wind speed. U_M,. and potential temperature. (#THETA#_M were used in a two-source energy balance model for computing fluxes with continuous T_k(#PHI#) measurements from the various surfaces. Standard Monin-Obukhov surface layer similarity was used with the "local" n and T_A data from the METFLUX stations. Bulk similarity approaches were used with the U_M and #THETA#_M data referenced either to ABL height or the top of the surface layen This latter approach of using mixed-layer data to drive model computations for the different sites is similar to the so-called flux-aggregation schemes or methods proposed to account for subgrid variability in atmospheric models, such as the "tile" or "mosaic" approach. There was tess agreement between modeled and measured fluxes when using mixedlayer versus local meteorological variables data for driving the model, and the type of bulk formulation used (i.e.. whether local or regional surface roughness was used) also had a significant impact on the results Differences between the flux observations and model predictions using surface layer similarity with local u and T_M data were about 25ck on average, while using the bulk formulations with U_M and (#THETA#_M, differences averaged about 30%. This larger difference was caused by an increase in biases and scatter between modeled and measured fluxes for some sites. Therefore. computing spatially distributed local-scale fluxes with ABL observations of mixed-layer properties will probably yield less reliable flux predictions than using local meteorological data, if available. Given the uncertainty in flux observations is about 20cA. these estimates are still considered reasonable and moreover permit the mapping of spatially distributed surface fluxes at regional scales using a single observation of U_M and #THETA#_M with high resolution T_k(#PHI#) data. Such T_k(#PHI#) observations with a 90-m pixel resolution will be available from the Advanced Spacebornc Thermal Emission and Reflection Radiometer to be launched on NASA's Earth Observing System.