Abstract
Abstract. Semi-arid forests are found to sustain a massive sensible heat flux in spite of having a low surface to air temperature difference by lowering the aerodynamic resistance to heat transfer (rH) – a property called the canopy convector effect (CCE). In this work large-eddy simulations are used to demonstrate that the CCE appears more generally in canopy turbulence. It is indeed a generic feature of canopy turbulence: rH of a canopy is found to reduce with increasing unstable stratification, which effectively increases the aerodynamic roughness for the same physical roughness of the canopy. This relation offers a sufficient condition to construct a general description of the CCE. In addition, we review existing parameterizations for rH from the evapotranspiration literature and test to what extent they are able to capture the CCE, thereby exploring the possibility of an improved parameterization.
Highlights
Understanding the role of turbulence in interactions between vegetation canopies and the atmosphere is crucial for interpreting momentum and scalar fluxes above vegetation
The canopy aerodynamic resistance is a concept borrowed from the evapotranspiration literature where it represents the resistance between the idealized “big-leaf” and the atmosphere for heat or vapor transfer (Monteith, 1973; Foken et al, 1995; Alves et al, 1998; Monteith and Unsworth, 2007)
The first one is to investigate whether the existing parameterizations exhibit the canopy convector effect and the second one is to identify the uncertainties associated with these different parameterizations since they are applied in different climate models often under conditions of thermal stratification
Summary
Understanding the role of turbulence in interactions between vegetation canopies and the atmosphere is crucial for interpreting momentum and scalar fluxes above vegetation This is relevant for a number of practical applications, such as regional and global weather and climate modeling, energy balance closure studies, and development of forest management strategies. In the Yatir forest, the entire net solar radiation flux (up to 800 W m−2) is equilibrated by a massive sensible heat flux (H ) of similar magnitude Note that this high H cannot be explained by the difference between surface and air temperature ( T = Ts − Ta) as the canopy surface is cooler than the surrounding desert surface in this case, but the air temperatures above desert and forest canopy are similar. To expound this apparent contradiction of larger sensible heat flux for smaller T , it is important to recall that, when adopting the simplified big-leaf representation of the forest as a single surface, H
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