A CLIMATOLOGY OF THE WARM-SEASON GREAT-PLAINS LOW-LEVEL JET USING WIND PROFILER OBSERVATIONS

Hourly observations from the Wind Profiler Demonstration Network durin g the warm season months of 1991 and 1992 were used to develop a clima tology 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 Neb raska). The frequency of occurrence for the weakest LLJs exhibited lit tle diurnal variation. In contrast, the strongest jets were about six times more likely to occur within a few hours of local midnight than d uring 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 lo cal maximum of the wind speed profile) showed comparatively little diu rnal variability regardless of speed. Analysis of the synoptic pattern s associated with LU occurrence showed that the LW was promoted by the warm sector of an extratropical cyclone and suppressed by a subtropic al ridge or polar high. These relationships were most pronounced for t he strongest jets. Similar relations to the synoptic pattern also were found for the non-LLJ southerly wind events. A maximum of LLJ frequen cy occurred in September. This seasonal maximum apparently is due in p art 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 LU dura tion was about 4 h for the weak jets and 2 h for the strongest jets. T his implies that twice-daily radiosonde observations will fail to dete ct many LLJ events and that the strongest jets are especially likely t o be missed. Such concerns are especially important in light of our fi nding that southerly LLJs represented about one-half of all occurrence s of strong, southerly low-level winds over the region. The inability of the conventional network to detect strong LLJs points to the utilit y of the profilers in forecasting severe weather and implies that hydr ologic budgets computed from conventional radiosonde data are likely t o underestimate the atmospheric flux of water vapor over the central U nited States.