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Meso- and Microscale Features of a Colorado Cold Front

George S. YoungDepartment of Atmospheric Science, Colorado State University, Fort Collins, CO 80523

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Richard H. JohnsonDepartment of Atmospheric Science, Colorado State University, Fort Collins, CO 80523

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Abstract

Data from the NOAA BAO (Boulder Atmospheric Observatory) tower and the PROFS (Program for Regional Observing and Forecasting Services) surface mesonetwork have been used to detect the meso- and microscale flow patterns associated with the passage of a shallow cold front over complex terrain. This front moved across the PROFS surface mesonetwork and the BAO tower site on the morning of 3 December 1981. Partial blocking of the cold airflow by the higher terrain to the north led to a westward movement of the cold front in the upper reaches of the South Platte River drainage basin. A meso-β-scale anticyclonic eddy subsequently formed in the lee of this terrain obstruction.

The micro-α-scale vertical eddy structure in the cold frontal zone is defined using data from the BAO tower. Updrafts of >6 m s−1 at 200 m AGL occurred at the wind-shift and temperature-drop line. Immediately behind this feature, micro-α-scale eddies entrained air through the frontal surface, diluting the low-level flow which was overtaking the front from behind. These features have been observed by others in atmospheric and laboratory gravity currents.

Abstract

Data from the NOAA BAO (Boulder Atmospheric Observatory) tower and the PROFS (Program for Regional Observing and Forecasting Services) surface mesonetwork have been used to detect the meso- and microscale flow patterns associated with the passage of a shallow cold front over complex terrain. This front moved across the PROFS surface mesonetwork and the BAO tower site on the morning of 3 December 1981. Partial blocking of the cold airflow by the higher terrain to the north led to a westward movement of the cold front in the upper reaches of the South Platte River drainage basin. A meso-β-scale anticyclonic eddy subsequently formed in the lee of this terrain obstruction.

The micro-α-scale vertical eddy structure in the cold frontal zone is defined using data from the BAO tower. Updrafts of >6 m s−1 at 200 m AGL occurred at the wind-shift and temperature-drop line. Immediately behind this feature, micro-α-scale eddies entrained air through the frontal surface, diluting the low-level flow which was overtaking the front from behind. These features have been observed by others in atmospheric and laboratory gravity currents.

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