Kinematics of the Low-Level Jet
W. D. Bonner, S. Esbensen, R. Greenberg
1968
Journal of Applied Meteorology
D e p a r t m e n t of M e t e o r o l o g y , U n i v e r s i t y of C a l i f o r n i a a t Los ABSTRACT Geographical and diurnal variations in the frequency of occurrence of strong low level wind maxima arc detcrmined using 2 yr. of wind data from 47 rawinsonde stations in the United States. Maximum frequency of occurrence is found in the Great Plains at approximately 37'N. and 98"W. The vast majority of jets in this region occur with southerly flow. Southerly wind maxima appear on both
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... ng and afternoon soundings but occur with much greater frequency, over a larger area, on the morning observations. Twenty-eight morning jet cases are used to determine average synoptic-scale wind and temperature patterns in the vicinity of the jet. Diurnal wind oscillations are examined by comparisons of jet frequencies, spceds, and altitudes on four-times-daily observations. The oscillation is similar t o that described by Blackadar; however, there is no apparent tendency for the latitudinal variation in period of the oscillation which Blackadar's model implies. ' time variations in the wind at a particular point in space,' or, for a limited period of time, along a single line of stations.2 This study examines the synoptic-scale structure of the low level jet through the standard network of radiosonde and rawinsonde stations. 833 ., , . " 834 Vol. 96, No. 12 MONTHLY WEATHER REVIEW 1). a circulation between the continent and the ocean; c . a circulation between the mountains and the plain. According to Lettau [24], lorn level wind maxima forming at night in the central United States are not accompanied by increasing geostrophic winds and Blackadar [3] has stated that these wind maxima are not produced by diurnal oscillations in the pressure fields. He has attempted to show that the jet profiles arise from an inertial oscillation of the ageostrophic wind vector as the air near the top of the friction layer is decoupled from the air below by the formation of a nocturnal inversion. This diurnal oscillation of the wind, taking place over a broad area, is thought to give rise to large-scale southerly jets of the type originally described by Means [25, 261 and Newton [28]. Wexler [40] has denied that the southerly jet in this area can be explained by the small-scale radiative and frictional effects alone. According to Wexler, the jet forms as a result of northward deflection by the Rocky Mountains of a shallo~v layer of air flowing westward across the Gulf of Mexico. Qualitatively, the increase in the Coriolis parameter as the air moves northward is offset by the development of strong anticyclonic shear. Along the western boundary of the current, where the terrain slopes upward, the air is retarded by friction. The two effects act together to produce a narrow zone of strong southerly wind at low levels along the eastern slopes of the Rocky Mountains. Blackadar's argument is supported by calculations of wind oscillations arising from hypothetical time and alt.itude variations of the coefficient of eddy viscosity (Buajitti and Blackadar [7]). Wexler has drawn a physical analogy between the southerly jet and the Gulf Stream (an analogy suggested by Newton [29]). He has transposed numerical solutions for the Gulf Stream flow (Charney [SI and Morgan [27]) to the case of the southerly jet, demonstrating an appreciable concentration of momentum along the eastern slopes of the Rocky Mountains. The processes proposed by Blackadar and Wexler are not mutually exclusive and, according to Wexler, diurnal oscillations in the speed of the jet are probably correctly explained by Blackadar. Recent studies indicate, however, that the boundary hyer oscillation is more complicated than Blackadar supposed. Holton [19], reviving an original suggestion by Bleeker and Andre [4], has shown that alternate heating and cooling of the slopes of the Rocky Mountains can produce diurnal oscillations of the wind in a boundary layer with constant eddy viscosi.ty. Hoecker [18] and Sangster [34] have shown that geostrophic winds in this region are not constant in time as Blackadar assumes. They vary strongly from day to night in an oscillation nearly 180' out of phase with that of the actual wind.
doi:10.1175/1520-0450(1968)007<0339:kotllj>2.0.co;2
fatcat:b3gtmhyy6jhnphnypxyus7wrii