Abstract
Abstract. The Dual Temperature Difference (DTD) model, introduced by Norman et al. (2000), uses a two source energy balance modelling scheme driven by remotely sensed observations of diurnal changes in land surface temperature (LST) to estimate surface energy fluxes. By using a time-differential temperature measurement as input, the approach reduces model sensitivity to errors in absolute temperature retrieval. The original formulation of the DTD required an early morning LST observation (approximately 1 h after sunrise) when surface fluxes are minimal, limiting application to data provided by geostationary satellites at sub-hourly temporal resolution. The DTD model has been applied primarily during the active growth phase of agricultural crops and rangeland vegetation grasses, and has not been rigorously evaluated during senescence or in forested ecosystems. In this paper we present modifications to the DTD model that enable applications using thermal observations from polar orbiting satellites, such as Terra and Aqua, with day and night overpass times over the area of interest. This allows the application of the DTD model in high latitude regions where large viewing angles preclude the use of geostationary satellites, and also exploits the higher spatial resolution provided by polar orbiting satellites. A method for estimating nocturnal surface fluxes and a scheme for estimating the fraction of green vegetation are developed and evaluated. Modification for green vegetation fraction leads to significantly improved estimation of the heat fluxes from the vegetation canopy during senescence and in forests. When the modified DTD model is run with LST measurements acquired with the Moderate Resolution Imaging Spectroradiometer (MODIS) on board the Terra and Aqua satellites, generally satisfactory agreement with field measurements is obtained for a number of ecosystems in Denmark and the United States. Finally, regional maps of energy fluxes are produced for the Danish Hydrological ObsErvatory (HOBE) in western Denmark, indicating realistic patterns based on land use.
Highlights
Over the past 35 yr, a wide variety of approaches have been developed to model the surface energy balance using satellite-derived observations of land surface temperature (LST) (Kalma et al, 2008), with ongoing work in a number of techniques such as the triangle method (de Tomas et al, 2012) or one source energy balance models (Boulet et al, 2012)
Modifications to the Dual Temperature Difference (DTD) model are presented that enable it to be used with LST observations from the Moderate Resolution Imaging Spectroradiometer (MODIS) sensor aboard the Terra and Aqua polar orbiting satellites, facilitating regional surface energy flux modelling over boreal regions
The effectiveness of modifying fg to improve the accuracy of modelled fluxes was tested at all the field sites with the DTD driven by field based measurements, with the exception of normalized difference vegetation index (NDVI) and enhanced vegetation index (EVI) used to compute fg, which were obtained from MODIS products
Summary
Over the past 35 yr, a wide variety of approaches have been developed to model the surface energy balance using satellite-derived observations of land surface temperature (LST) (Kalma et al, 2008), with ongoing work in a number of techniques such as the triangle method (de Tomas et al, 2012) or one source energy balance models (Boulet et al, 2012). The original model formulation requires an early morning LST observation (approximately 1 h after local sunrise) when fluxes are usually minimal This means that, like ALEXI, it is dependent on the high temporal resolution of geostationary satellite measurements, which is unsuitable for applications at higher latitudes, such as in northern Eurasia and northern North America, where the view zenith angle (VZA) from geostationary satellites is large, causing loss of spatial resolution and accuracy due to longer atmospheric path lengths. Modifications to the DTD model are presented that enable it to be used with LST observations from the Moderate Resolution Imaging Spectroradiometer (MODIS) sensor aboard the Terra and Aqua polar orbiting satellites, facilitating regional surface energy flux modelling over boreal regions.
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