Characterizing the North American monsoon in the Community Atmosphere Model: Sensitivity to resolution and topography

This work examines the effect of horizontal resolution and topography on the North American monsoon (NAM) in experiments with an atmospheric general circulation model. Observations are used to evaluate the fidelity of the representation of the monsoon in simulations from the Community Atmosphere Model version 5 (CAM5) with a standard 1.0 degrees grid spacing and a high-resolution 0.25 degrees grid spacing. The simulated monsoon has some realistic features, but both configurations also show precipitation biases. The default 1.0 degrees grid spacing configuration simulates a monsoon with an annual cycle and intensity of precipitation within the observational range, but the monsoon begins and ends too gradually and does not reach far enough north. This study shows that the improved representation of topography in the high-resolution (0.25 degrees grid spacing) configuration improves the regional circulation and therefore some aspects of the simulated monsoon compared to the 1.0 degrees counterpart. At higher resolution, CAM5 simulates a stronger low pressure center over the American Southwest, with more realistic low-level wind flow than in the 1.0 degrees configuration. As a result, the monsoon precipitation increases as does the amplitude of the annual cycle of precipitation. A moisture analysis sheds light on the monsoon dynamics, indicating that changes in the advection of enthalpy and moist static energy drive the differences between monsoon precipitation in CAM5 1.0 degrees compared to the 0.25 degrees configuration. Additional simulations confirm that these improvements are mainly due to the topographic influence on the low-level flow through the Gulf of California, and not only the increase in horizontal resolution.