THREE SOUTHERLY LOW-LEVEL JET SYSTEMS DELINEATED BY THE WEATHER BUREAU SPECIAL PIBAL NETWORK OF 1961

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  • 1 U.S. Weather Bureau, Washington, D.C.
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Abstract

For the first time in meteorological history the broad aspects of the southerly low-level jet over the Western Plains have been studied in fine detail both in time and space. This was accomplished 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 are described. High pressure cells east of the Great Plains, whether of polar or tropical origin, produced low-level southerly jet systems. Jet speed maxima occurred between 300 and 800 m. above the ground (generally well below the 850-mb. level) and were often found in several horizontally-arranged cells. By day the jet was incoherent (with one exception) and 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 mesosystems. On those nights with surface inversions the speed maxima were lower in elevation 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 jet is easily applicable for the practicing forecaster, but Wexler's inertial boundary layer interpretation predicts the jet by day as well as by night on a basic southerly current. Both theories seemed to apply at times during the existence of the jet systems described here.

Abstract

For the first time in meteorological history the broad aspects of the southerly low-level jet over the Western Plains have been studied in fine detail both in time and space. This was accomplished 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 are described. High pressure cells east of the Great Plains, whether of polar or tropical origin, produced low-level southerly jet systems. Jet speed maxima occurred between 300 and 800 m. above the ground (generally well below the 850-mb. level) and were often found in several horizontally-arranged cells. By day the jet was incoherent (with one exception) and 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 mesosystems. On those nights with surface inversions the speed maxima were lower in elevation 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 jet is easily applicable for the practicing forecaster, but Wexler's inertial boundary layer interpretation predicts the jet by day as well as by night on a basic southerly current. Both theories seemed to apply at times during the existence of the jet systems described here.

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