4 Complex Flow Interactions over a Very High-Resolution Surface Weather Station Network

Tuesday, 6 August 2013
Holladay-Halsey (DoubleTree by Hilton Portland)
Matthew E. Jeglum, 125 S State St Rm 1311, Salt Lake City, UT; and S. W. Hoch, C. D. Whiteman, and F. W. Gallagher III

Dugway Proving Ground, host to the observational portion of the Mountain Terrain Atmospheric Modeling and Observations (MATERHORN) program, operates a permanently installed array of 51 automated weather stations (miniSAMS) aligned on a roughly rectangular grid with a grid spacing of one mile. The dataset includes 1-minute averages of temperature, pressure, wind speed, wind direction and solar radiation at 2 m height and wind speed and direction at 2 m and 10 m heights. The array sits in a broad, flat-floored basin surrounded by mountainous terrain with relief on the order of 500 m in all directions except northwest. To the northwest lies nearly flat terrain, including unvegetated playa (dry alkali flats which fill with water seasonally to form shallow lakes).

The relatively high spatial and temporal resolution of the array allows investigation of atmospheric boundary layer structures that are either poorly observed or unobserved using typical surface networks. Data from the array shows signatures of synoptic cold fronts, diurnal slope and valley flows, and turbulent wakes, often interacting with one-another in complex ways.

The array data was analyzed for the MATERHORN Fall campaign period (25 Sept. through 21 Oct. 2012), but one illustrative day will be investigated as a case study. On 23 October 2012 a synoptic cold front associated with a 700-mb shortwave trough crossed the miniSAMS grid in the late afternoon, leaving moderate northwesterly winds in its wake. Shortly thereafter a second, weaker boundary propagated across the grid from the northeast, with northeasterly winds behind it. This boundary displayed traits common to downslope windstorms. The winds switched from northwest to northeast as this boundary passed, shifting counterclockwise through 270 degrees of rotation. Normally one would expect a 90-degree shift in wind direction in this situation, rather than a 270-degree rotation. This boundary was followed by the return of the pre-frontal southwesterlies, with a coincident rise in temperature despite the nighttime hours. Finally, northwesterlies again propagated across the grid, accompanied by a drop in temperature. Through the remainder of the night a confluence zone was maintained over the array, with northwesterly winds on the west side and northeasterly winds on the east side. As the confluence zone wavered back and forth, individual stations sifted from NW to NE and back. When this happened, however, the wind direction rotated through the large angle (~330 deg).

The high spatial and temporal resolution of the miniSAMS observations offers a unique opportunity to characterize and study the contribution of differently forced flows and their interactions. The knowledge gained can be applied to meso- and micro-scale processes that occur elsewhere in the world.

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