Orographic Convection during the North American Monsoon in Convection Permitting Simulations under Climate Warming

Summertime convective activity in the southwestern United States is principally driven by the North American Monsoon (NAM). Often, orographically forced precipitation, aided by anomalous low-level pulses of moisture during monsoonal ‘gulf surge’ periods, can organize and propagate over adjacent low-elevation regions as downstream mesoscale convective systems. Climate model projections of the monsoonal precipitation response to a doubling of CO2 show a general weakening of the NAM due to enhanced stability. However, orographic convection is poorly resolved by the use of coarser grid spacings and convective parameterizations found in most global and regional climate models and may lead to an underestimation of extreme precipitation events that can accompany gulf surges. We seek to address this underestimation through the use of convection permitting modeling on seasonal time scales. Precipitation extremes associated with the NAM have been studied at convection permitting resolutions before, but not on seasonal scales and not with regards to climate warming and the dynamics of orographic convection.

We analyze results from a set of continental-scale regional climate simulations using the Weather Research and Forecasting model at 4-km grid spacing with a domain across most of North America. These simulations cover a period extending from October 2000 through December 2013. The analysis focuses on a domain encompassing Arizona and the surrounding region for June through September so as to focus on the monsoonal period. To consider how the NAM and associated MCSs may vary with climate, we analyze an additional pseudo-global warming (PGW) run in which a fixed mean warming perturbation derived from climate models is applied to the boundary conditions.

Model performance over the NAM region is validated against station observations and gridded observational datasets. Days are classified based on a monsoonal gulf surge index to analyze how orographic precipitation and accompanying MCSs interact with low-level moisture transport induced by the NAM and how this may change under climate warming. Environmental attributes like CAPE and precipitable water are analyzed for surge and non-surge days between the two climate states. Mesoscale orographic circulations are investigated for interactions with NAM induced synoptic-scale circulations.