Imprints of evaporation and vegetation type in diurnal temperature variations

Diurnal temperature variations are strongly shaped by the absorption of solar radiation, but evaporation, or the latent heat flux, also plays an important role. Generally, evaporation cools, but its relation to diurnal temperature variations is unclear. This study investigates the diurnal response of surface and air temperatures to solar radiation and how evaporation and vegetation modify their response. We used the warming rate of temperature to absorbed solar radiation in the morning under clear-sky conditions and evaluated how the warming rates change for different evaporative fractions. Results for 51 FLUXNET sites show that air temperature carries very weak imprints of evaporation across all vegetation types. However, surface temperature warming rates of short vegetation decrease significantly by ~23 x 10^-3 K/W m^-2 from dry to wet conditions. Contrarily, warming rates of surface and air temperatures are similar at forest sites and carry literally no imprints of evaporation. We explain these contrasting patterns with a surface energy balance model. The model reveals a strong sensitivity of the warming rates to evaporative fraction and aerodynamic conductance. However, for the large aerodynamic conductance, the sensitivity to the evaporative fraction is strongly reduced. We then show that in addition to the higher aerodynamic conductance of forests, imprints of evaporation in the warming rate of surface temperature are reduced by 50% through an enhanced aerodynamic conductance under dry conditions. This contribution is comparatively weak (28%) for short vegetation. These findings have implications for the interpretation of land-atmosphere interactions and the roles of moisture limitation and vegetation on diurnal maximum temperatures, which is of key importance for ecological functioning. We conclude that surface temperature warming rate is a promising predictor of evaporation for short vegetation. These findings are in agreement with our previous study (Panwar et al., 2019) where the weaker response of air temperature to the evaporative fraction is explained by the larger growth of the boundary layer on drier days. In forests, however, the diurnal variation in temperatures is governed by their aerodynamic properties resulting in no imprint of evaporation in diurnal temperature variations.

Reference: Panwar, A., Kleidon, A. and Renner, M.: Do Surface and Air Temperatures Contain Similar Imprints of Evaporative Conditions?, Geophysical Research Letters, 46(7), 3802–3809, doi:10.1029/2019GL082248, 2019.