An Analysis of Dryline Structure and Propagation Influenced by the Black Hills

A dryline is a boundary intersecting the earth's surface that separates moist, maritime tropical air from dry, continental tropical air. These boundaries are common in the Great Plains of the United States during the spring and summer months and are typically oriented meridionally, orthogonal to the gradually sloping terrain to the west. The dryline is a favorable location for isolated convective initiation during the warm season, which makes understanding them an important factor in forecasting thunderstorms and their associated hazards. In addition, forecasting the propagation of a dryline is crucial during wildfires. The rapid change in low-level moisture and wind shifts associated with drylines can abruptly change the path and strength of an active fire. The objective of this project is to determine how the dryline structure and evolution is impacted when a dryline interacts with the Black Hills of western South Dakota. The Black Hills form a unique topographic feature in the Northern Plains creating an abrupt, isolated elevation change roughly 100 miles east of the Rocky Mountains. The Hills also represent a distinct change in landuse from the surrounding grassland prairie as they are primarily composed of a ponderosa pine forest. Drylines are often observed to form west of the Hills, over eastern Wyoming, and subsequently move eastward, crossing the hills. To date there has been little study of how a dryline may evolve when crossing an isolated terrain feature such as the Black Hills; however, past studies have shown that terrain slope and soil moisture can have significant effects on the dryline structure and propagation. In light of this, it is expected that the combined sudden change in topography and landuse/soil moisture afforded by the Black Hills may impact the structure and propagation of the dryline in the Northern Plains. As an initial step toward addressing this objective, archived data from the Rapid Update Cycle (RUC) forecast model were analyzed to document drylines in the vicinity of the Black Hills. Gradients in surface-specific humidity were used to determine the strength and location of the drylines, and background synoptic conditions were assessed as well. In multiple cases, a local maximum of specific humidity was maintained over the Hills while the dryline advanced eastward further to the south, effectively stalling the boundary's progression across the Hills. This concentration of moisture would often be maintained, even once the dryline established itself further east across the plains, producing a “bullseye” of high specific humidity within the dry airmass west of the dryline. In other cases, however, the dryline appeared to accelerate over the Hills creating a bulge in the dryline directly to the east. These contrary behaviors appear to occur within different background synoptic patterns, suggesting that the Black Hills' influence on dryline activity varies with the synoptic scale environmental conditions. To further evaluate the role of the Black Hills in producing the dryline characteristics observed with the RUC data, a series of Weather Research and Forecasting (WRF) simulations are being run for the 2010 dryline season. These simulations consist of one control run designed to simulate observed dryline behavior, and three sensitivity tests designed to isolate the impacts of vegetation and terrain on dryline propagation over the Black Hills. The first of these adjusts the landuse and soil moisture of the Black Hills to match the vegetation of the surrounding prairies. This run removes the effects of moisture contained by the ponderosa pine trees to assess how the landuse of the Hills may impact the evolution of a dryline. The next test modifies the elevation of the Hills to remove the abrupt increase in terrain height, while retaining the appropriate soil moisture and landuse characteristics. Flattening the Black Hills in this simulation eliminates the effects of topographically forced flows near the Hills, and illustrates how the obstructed air flow from the Hills influences the dryline. The final simulation considers both vegetation and terrain alterations. This run removes the forest vegetation and elevated terrain to represent the evolution of a dryline without the influence of the isolated terrain or landuse features. The output of these simulations will be compared to the control run to evaluate whether the landuse or elevation change of the Black Hills have a greater effect on the morphology of dryline propagation in the Northern Plains.