Three Southerly Low-level Jet Systems Delineated by the Weather Bureau Special Pibal Network

For the first time in metcorological history the broad aspects of the southerly low-level jet o\er the Western Plains have been studied in fine detail both in time and space. This as acconiplished in the spring of 1961 by means of a line of 13 pibal stations established between Amarillo, Tex., and Little Rock, Ark. Twenty-five consecutive hourly observations were taken at each station on a total of five different 24-hr. periods in April, May, and June. Three of the observational periods were analyzed and the jet characteristics arc described. High prtssure cells east of the Great Plains, whether of polar or tropical origin, produced lo\i-le\el southerly jet systems. Jet speed maxima occurred between 300 and 800 m. above the ground (generally ~ c l l below the 850-nib. level) &’ CE were often found in several horizontally-arranged cells. By day the jet was incoherent (with one exception) aTld speeds were sub-geostrophic, while at night the jet was well organized and coherent and speed maxima were as high as 1.95 times the sea level geostrophic speed. The jet lifted over mountainous regions and over intruding mcsosystems. On those nights with surface inversions the speed maxima were lower in elcvation and required a smaller pressure gradient for a given maximum nighttime speed than when lapse conditions existed, but their elevation apparently had no correlation with the height of the inversion itself. Blackadar’s theory of the inertial oscillation for producing the low-level nocturnal jct is easily applicable for the practicing forecaster, but Wexler’s inertial bouildary layer interpretation predicts the jet by day as well as by night on a basic southerly current. Both theories seemed to apply a t times during the existence of the jet systems described here.