Simulations of cold-air pool formation inside forest-covered valleys: A sensitivity study

A common feature of nocturnal boundary layers in valleys and basins are cold-air pools, defined as topographically confined, stagnant layers of air that are colder than the overlying air. The limited numbers of research studies that have examined the impact of forest cover on cold-air pool development inside valleys have generally identified forest cover as promoting cooling. However, a recent numerical modeling study has shown that forest cover on sidewalls may retard cooling inside valleys due to weakened downslope flows and associated adiabatic cooling, compared to valleys with bare sidewalls. In this study, we examine systematically the effect of forest cover on cold-air pools using a canopy flow modeling system, ARPS-CANOPY, which is based on the Advanced Regional Prediction System (ARPS) model. Two-dimensional simulations are performed with idealized terrain and canopy configurations intended to represent topographic structures ranging from small, shallow valleys to broad, deep valleys and canopy structures spanning from bare ground to dense canopies. Background wind speed is also controlled to evaluate the possible role of the forest canopy as a shelter from background turbulence. The findings of this study may be of interest to air quality, climate change, agriculture, and wind energy researchers and practitioners.