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 morning 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 su...

Analysis of 1 week's data in August 1960 shows significant diurnal variations in surface geostrophic wind over the south-central United States. The oscillation in the southerly component (V,) is driven by the response of the thermal wind t o the diurnal temperature cycle over sloping terrain. A smaller oscillation in U, derives from spatial variations in the amplitude of the diurnal pressure wave. The amplitude of the oscillation in V g is about 3 to 5 m see-1 a t the surface, decaying exponentially with height to near 0 a t ' 2 km. Examination of 11 yr of summertime rawinsonde data at Fort Worth, Tex., shows a very regular diurnal variation in boundary layer wind with maximum amplitude of about 3 m sec-* a t 600 m abovc the ground. This oscillation is forced by periodic variations in both eddy viscosity and geostrophic wind. Using a simplified model of the boundary layer, we obtain solutions for the diurnally periodic wind resulting from "reasonable" variations in eddy viscosity and "observed" variations in geostrophic wind.

Using simulated atmospheric data from the National Center for Atmospheric Research (NCAR) community climate model (CCM), we test the hypothesis that the global positioning system (GPS) can be used to detect global and regional climate change. We examine how the fundamental GPS variables (wet and total delays and vertical profiles of refractivity) as well as precipitable water as estimated by ground‐based GPS receivers would change in a climate with 2 times the present concentration of carbon dioxide (CO2). Because of the higher water vapor content in the doubled CO2 simulation the wet delay and the precipitable water show a significant increase in the tropics and middle latitudes. Refractivity also shows an increase in the lower troposphere. We also simulate the changes in the GPS signal delay in a doubled CO2 climate as would be measured by a radio occultation technique using low Earth‐orbiting (LEO) satellites equipped with GPS receivers. Increases in temperature and water vapor in the lower troposphere of the model atmosphere produce opposite effects on the occultation delay. Increased temperature tends to decrease the delay, while increased water vapor increases the delay. Amplified by the long “lever arms” of the LEO‐atmosphere‐GPS link, a strong “greenhouse warming” signal is simulated, with increases in occultation delay of nearly 100 m using the globally averaged data. This increase indicates that globally the effect of increased water vapor dominates. However, significant regional differences are present in the occultation delay response. In the tropics, where the temperature increase is smallest and the water vapor increases are largest, increases in delay of about 300 m are simulated. In contrast, in the polar regions where the increased temperatures are greatest and the increases in water vapor are smallest, the temperature effect dominates and a decrease in occultation delay of nearly 70 m is simulated. When compared to expected errors in measuring the occultation delay of about 1 m, these results indicate that monitoring trends in occultation delays would be a practical way to detect global and regional climate change.

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The relationship between the low-level jet and thunderstorm activity in the south-central United States is examined through mesoanalysis of surface data from Weather Bureau and NSSP stations. Separate squall systems moved through Kansas and Oklahoma during the night; the systems in Kansas persisted while those in Oklahoma died out. The most likelyexplanation for this is the synoptic-scalevertical velocities in the vicinity of the low-level jet.