The unstable surface layer above forest: Regional evaporation and heat flux

Abstract

Radiosonde measurements above the heterogeneous forest of the Landes region in southwestern France provided vertical profiles of potential temperature and specific humidity in the atmospheric boundary layer. For all of the 62 profiles analyzed under unstable atmospheric conditions a surface sublayer could be identified within which the Monin‐Obukhov similarity was consistent with the regional surface fluxes of sensible heat and latent heat (evaporation). For the potential temperature the vertical extent of this sublayer was found to be 41 (±30) ≤ ( z − d 0 )/ Z 0 ≤ 130 (±49), where Z 0 = 1.2 m is the roughness height and d 0 = 6.0 m is the displacement height; for the specific humidity it was 48 (±36) ≤ ( z − d 0 )/ z 0 ≤ 153 (±63). These results show that irregular forest surfaces are not anomalous, as regards the lower limit of the surface layer in comparison with surfaces with smaller roughness. The surface fluxes derived from the profile measurements were compared with flux measurements obtained by means of an eddy correlation system atop a 29 m mast, some 9 m above a mature section of forest some 4.5 km away from the launching site of the radiosondes; these independent measurements were made by a team from the Institute of Hydrology (Wallingford, England). On average, these two types of estimates were in good agreement. For the sensible heat flux the correlation coefficient was r = 0.75. For the evaporation rate it was r = 0.66. For the evaporation obtained by means of the energy budget from the sensible heat flux it improved to r = 0.82. The profile‐derived fluxes did not compare as favorably with corresponding flux values measured above agricultural crops in clearings. This confirms that the forest was the dominant surface at the regional scale.