Land Surface Cooling by Extended Growing Season in Northern Forests through Reduced Aerodynamic Resistance

Abstract

Growing season changes of northern forests are important for land surface temperature (LST) as well as seasonal climate. However, biophysical sensitivity of LST to earlier leaf onset and later leaf senescence is uncertain. Here, we investigated the LST sensitivity to advanced start of growing season (SOS) and delayed end of growing season (EOS) during corresponding two periods, namely, green-up and senescence periods, over northern deciduous forests. Main biophysical cause of LST sensitivity was investigated by applying physically based two resistance mechanism (TRM) method to an experimental set using Community Land Model version 5 (CLM5). Results show that biophysical sensitivities of LST to advanced SOS and delayed EOS were ？2.1×10？2 K day？1 and ？2.8×10？2 K day？1 during green-up and senescence periods, respectively, indicating surface cooling due to extended growing season. Larger LST sensitivity to EOS than that to SOS is explained by three times larger aerodynamic resistance change to EOS than that to SOS. This surface cooling effect during senescence period is caused by enhanced turbulence heat transfer according to reduced aerodynamic resistance. Our results suggest that the reduced aerodynamic resistance according to growing season change can be an important consideration for projecting seasonal climate change.