What Cools Forests: Evaporation or Aerodynamic conductance?

Abstract:

The diurnal variation in temperature is usually quantified by the diurnal temperature range (DTR) and is predominantly shaped by solar radiation. Solar radiation warms and evaporation typically cools, but how do their effects vary across vegetation types? Here, we derive an analytic expression from the surface energy balance to estimate the contribution of evaporative conditions and aerodynamic properties in shaping diurnal range of surface temperature (DTsR). We use the concept of warming rate, that is, the increase of temperature with a unit increase in solar radiation, which was found to be a relatively constant characteristic in the morning (Panwar et al., 2019). When multiplied by the maximum of absorbed solar radiation, the warming rate provides an approximation of DTR. First, we analyze the relationship of warming rate to evaporative conditions in 51 FLUXNET sites of short vegetation, savanna and forests. We use evaporative fraction as an indicator of evaporative conditions. The vegetation properties are quantified by the aerodynamic conductance that is observed to exhibit similar diurnal variation (~2.5 times of the mean) across vegetation types. We show that in short vegetation, the warming rate of surface temperature decreases by ~23 x 10-3 K/W m-2 from dry to evaporative conditions, indicating stronger evaporative cooling of surface temperature. Contrarily, warming rates of surface and air temperatures are similar at forest sites and carry literally no imprints of evaporative fraction. These contrasting patterns are reproduced by our analytic expression. Using our expression, we show that the high aerodynamic conductance of forests reduces DTsR substantially more (-48%) than evaporative cooling (-18%). We further show that the diurnal variation of aerodynamic conductance reduces DTsR by ~30 % in short vegetation and savanna, but only by ~7% in forests. We conclude that diurnal temperature variations may be useful to predict evaporation for short vegetation. In forests, however, the diurnal variations in temperatures are mainly governed by their high aerodynamic conductance resulting in negligible imprints of evaporative conditions.

Reference: doi:10.1029/2019GL082248, 2019.