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Mark J. Mitchell, Raymond W. Arritt, and Ken Labas

Abstract

Hourly observations from the Wind Profiler Demonstration Network during the warm season months of 1991 and 1992 were used to develop a climatology of the low-level jet (LLJ) over the Great Plains of the central United States. The maximum overall frequency of LLJ occurrence was in the southern part of the Great Plains, while the maximum frequency of the stronger LLJs extended farther north and east (Kansas through Nebraska). The frequency of occurrence for the weakest LLJs exhibited little diurnal variation. In contrast, the strongest jets were about six times more likely to occur within a few hours of local midnight than during the day. Southerly wind events that did not satisfy the criteria for low-level jets (i.e., those that did not include the low-level local maximum of the wind speed profile) showed comparatively little diurnal variability regardless of speed.

Analysis of the synoptic patterns associated with LLJ occurrence showed that the LLJ was promoted by the warm sector of an extratropical cyclone and suppressed by a subtropical ridge or polar high. These relationships were most pronounced for the strongest jets. Similar relations to the synoptic pattern also were found for the non-LLJ southerly wind events. A maximum of LLJ frequency occurred in September. This seasonal maximum apparently is due in part to a greater frequency of synoptic patterns that are conducive to the LLJ, possibly in combination with enhanced boundary-layer forcing attributable to a regional decrease in soil moisture. The mean LLJ duration was about 4 h for the weak jets and 2 h for the strongest jets. This implies that twice-daily radiosonde observations will fail to detect many LLJ events and that the strongest jets are especially likely to be missed. Such concerns are especially important in light of our finding that southerly LLJs represented about one-half of all occurrences of strong, southerly low-level winds over the region. The inability of the conventional network to detect strong LLJs points to the utility of the profilers in forecasting severe weather and implies that hydrologic budgets computed from conventional radiosonde data are likely to underestimate the atmospheric flux of water vapor over the central United States.

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