The impact of changes in extreme weather during the period of the North American monsoon in late summer are evaluated as they may impact Department of Defense (DoD) facilities in the Southwest United States, with respect to the concerns of exceedance of infrastructural limits and operational capability. The overarching research objective was to evaluate how warm season extreme weather events in the Southwest will change with respect to occurrence and intensity. The project addressed the following: 1) consideration of existing operational protocols for weather and climate related decision making, 2) creation of climate change projection information at an appropriate spatial scale, 3) consideration of extreme weather and climate events, and 4) use of data to inform adaptation strategies.
A new methodological technique to severe weather event projection was developed using convective-permitting modeling with the Weather Research and Forecasting model. The guiding principle was to use a weather forecast based approach to climate projection. Severe weather event days in the Southwest during the monsoon period were objectively identified within an atmospheric reanalysis and several global climate models from the Coupled Model Intercomparison Project (CMIP), based on the thermodynamic conditions necessary for intense monsoon thunderstorms. The severe weather event days were also investigated to verify that known dynamic forcing factors were present, including a favorable positioning of the subtropical ridge over the western United States and the presence of an upper-level disturbance, or transient inverted trough. Convective-permitting model simulations were then performed for the identified severe weather event days, like an operational numerical weather prediction model, with the advantage of explicitly representing convective structures and propagation. Model simulated differences between past and future periods in the various convective-permitting modeling paradigms were used to determine changes in mean and extreme precipitation and downdraft winds from thunderstorm outflows, and these results were verified against long-term changes in available observational data. Changes in precipitation intensity and duration were finally assessed at the DoD facility scale, in a manner that confirms to DoD operational weather watch and warning criteria.
There have been significant long-term changes in the atmospheric thermodynamic and dynamic conditions during the period of the monsoon in the Southwest that have occurred over the past sixty years. Atmospheric moisture and instability have generally increased, but transient inverted troughs have decreased in frequency because of the expansion of the subtropical ridge. Thus, monsoon thunderstorms in the Southwest are tending to be more thermodynamically dominated, with less tendency to organize and propagate. Though there tend to be a fewer number of strong, organized convective events during the monsoon, when they do occur their associated precipitation is tending to be more intense. In the historical climate simulations, monsoon precipitation on average generally is decreasing in the Southwest, while the intensity of precipitation associated with organized convective events increases. The area of southwestern Arizona, where many important and spatially expansive DoD assets are located, appears to be a local hot spot where organized convective events are becoming more intense, in terms heavier rainfall and more intense downdraft winds. These changes were verified in high resolution observed precipitation data. The projected future changes using dynamically downscaled CMIP models were very consistent with trends from the historical record, including identifying southwestern Arizona as a region of intensifying precipitation extremes.
A physically robust and computationally efficient methodological approach to the projection of extreme event weather in the Southwest was developed within this pilot project, that could be easily adapted for other regions of the United States and the world. A convective-permitting modeling approach adds substantial value to projection of extreme weather, pinpointing the spatial locations within the Southwest where precipitation is becoming more intense with a high degree of accuracy. Model data generated within this project will be made available via a cloud data storage system, to be widely available for DoD and civilian use.