LOW-LEVEL JET CLIMATOLOGY FROM ENHANCED RAWINSONDE OBSERVATIONS AT A SITE IN THE SOUTHERN GREAT-PLAINS

A climatology of the Great Plains low-level jet (LLJ) is developed fro m 2 yr of research rawinsonde data obtained up to eight times per day at a site in north-central Oklahoma. These data have better height and time resolution than earlier studies, and show that jets are stronger than previously reported and that the heights of maximum wind speed a re closer to the ground. LLJs are present in 47% of the warm season so undings and 45% of the cold season soundings. More than 50% of the LLJ s have wind maxima below 500 m above ground level (AGL). Because the 4 04-MHz radar profiler network in the central United States has its fir st data points at 500 m AGL, it is likely to miss some LLJ events and will have inadequate Vertical resolution of LLJ wind structure. Previo us studies have identified LLJs on the basis of a wind speed profile c riterion. This criterion fails to separate the classical southerly LLJ s from the less frequent northerly jets, which differ in both structur e and evolution. Classical southerly jets are more frequent; they occu r year round, with the highest frequency in the summer and at night. S outherly LLJ wind speed maxima are most frequently found at 300-600 m AGL, and peak speeds, typically between 15 and 21 m s(-1), are attaine d at 0200 CST. The height of the wind speed maximum varies little duri ng nightime--a period when surface-based inversions grow in depth but generally remain below the jet. Winds at the nose of the southerly jet s exhibit a distinct diurnal clockwise turning in wind direction and a n oscillation in speed. Northerly jets occur year round. They are gene rally associated with cold air outbreaks and are found in the cold air behind southward-moving cold fronts. In winter, their frequency of oc currence rivals that of the southerly jets. Their occurrence, however, is less dependent on time of day, with a weak daytime maximum. They a re more variable in the heights of their wind speed maxima, are associ ated more frequently with elevated frontal inversions, and do not exhi bit a clockwise turning with time. The heights of the jet speed maxima are found to increase with distance behind the surface cold front.