Quantifying the biophysical effects of forests on local air temperature using a novel three-layered land surface energy balance model

Abstract

The well-documented energy balance dynamics within forest ecosystems are poorly implemented in studies of the biophysical effects of forests. This results in limitations to the accurate quantification of forest cooling/warming on local air temperature. Taking into consideration the forest air space, this study proposes a three-layered (canopy, forest air space and soil [CAS]) land surface energy balance model to simulate air temperature within forest spaces (T_af) and subsequently to evaluate its biophysical effects on forest cooling/warming, i.e., the air temperature gradient (∆T_a) between the T_af and air temperature of open spaces (T_ao) (∆T_a = T_af - T_ao). We test the model using field data for 23 sites across 10 cities worldwide; the model shows satisfactory performance with the test data. High-latitude forests show greater seasonal dynamics of ∆T_a, generating considerable cooling of local air temperatures in warm seasons but minimal cooling or even warming effects during cool seasons, while low-latitude tropical forests always exert cooling effects with less interannual variability. The interannual dynamics of ∆T_a are significantly related to the seasonality of solar geometry and canopy leaf phenology. The differences between forest canopy temperature (T_c) and T_ao, which are the two most important terms attributed by the CAS model in impacting T_af, explain a large part of forest cooling and warming (May-July: R² = 0.35; November-January: R² = 0.51). The novel CAS model provides a feasible way to represent the energy balance within forest ecosystems and to assess its impacts on local air temperatures globally.

Keywords: Climate warming mitigation; Forest biophysical effects; Forest cooling/warming; Land-atmosphere model; Local environmental health; Radiative transfer equation.