Multi-Scale Interactions in the Planetary Boundary Layer over Complex Terrain

US Army's Dugway Proving Ground (DPG), located in the northwest corner of Utah, USA, is characterized and surrounded by complex land-surface and terrain contrasts. Important features include the Great Salt Lake to the northeast, the West Desert playa to the west, numerous narrow mountain ranges in close proximity, and distant broad and high plateaus. Thermally-driven circulations of different scales associated with these geographic features dramatically impact the evolution of the planetary boundary layer (PBL) at DPG. The relative contribution of these different circulations varies both diurnally and seasonally. Further, synoptic scale features remote from DPG such as thermal lows over Nevada and Great Basin Cyclogenesis have a profound impact on the PBL over DPG. A better understanding of these multi-scale circulations at DPG and their interactions will improve operational forecasts at DPG and offer insight to multi-scale interactions elsewhere. Additionally, as part of the MATERHORN program to improve mesoscale model forecasts in complex terrain, this research will allow better identification of model shortcomings by accurate diagnosis of the features that the models do not skillfully predict.

For this study 38 permanently installed automatic weather stations (AWS), each with periods of record (POR) extending 10-15 years, are used to characterize the surface wind field in and around DPG. AWS installations are well distributed spatially and sample the relevant land-surface features. A wind climatology based on this dataset is used to identify persistent features on hourly and monthly time scales. ERA-Interim reanalysis data and a 15-year climatology of upper-air soundings at Salt Lake City International Airport enable examination of synoptic scale influences.

Combining the above datasets the annual PBL evolution at DPG can be divided in to 4 seasonal regimes. The first, encompassing the months December-February, is dominated by persistent valley cold-air pools that induce the lightest mean surface winds of the year despite the peaking of the 700 hPa wind speeds at this time. The second regime, from March-May, is defined by northerly 850 hPa flow that is superimposed upon DPG by the climatological maximum of cyclogenesis in the lee of the Sierra Nevada and southern Utah that occurs during these months. The third regime, from June-August, includes the strongest mean surface wind speeds of the year despite the minimum in 700 hPa wind speed at this time. These strong surface winds are a result of the strong thermal low that forms over Nevada during the summer months, inducing strong southerly flow at 850 hPa. The fourth regime, from September-November, best displays thermally-driven circulations resulting from the local geography at DPG as it is not influenced by any of the aforementioned synoptic-scale phenomena.

During each of these 4 seasonal regimes, mesoscale and microscale circulations operate on individual slopes and valleys. The strength of these smaller-scale circulations varies seasonally as they are modulated by the amount of solar heating. It is onto these flows that successively larger-scale influences are superimposed to create the full climatological wind evolution.