Heat flux parameterization for sparse and dense grasslands with the analytical land-atmosphere radiometer model (alarm)

Abstract

The dependence of radiometric surface temperature (\(\theta\)s) on view angle and the unclear definition of the aerodynamic temperature, which is the temperature that gives the correct sensible heat flux estimate at a specified roughness length, bring about a challenge in estimating sensible heat flux from \(\theta\)s. An analytical-land-atmosphere-radiometer model (ALARM) has been developed to convert \(\theta\)s taken at any zenith view angle to a clearly defined equivalent isothermal surface temperature, \(\theta\)i, at a defined scalar roughness length. ALARM is an analytical model based on K-theory that links the foliage temperature profile to the radiometric surface temperature and the temperature ‘felt’ by the turbulent lower atmosphere. ALARM has previously been applied with slightly different values of its parameters to several grassland sites of varying canopy density. Our objective in this study was to apply ALARM to these and to one additional dataset with a single parameterization. When compared to the reference (measured) values of sensible heat flux H, ALARM estimates of H had root mean square errors of about 35 W m-2. These results were comparable to those from two other simple canopy models also tested with these datasets.