Diurnal Variations in Boundary Layer Winds Over the South-Central United States in Summer

Abstract

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.

Analytical description of a nocturnal low‐level jet

“…However, although the theory yields peak jet wind speeds that exceed the geostrophic values by ∼70% (which is consistent with many observations), the theory cannot explain observed cases where the peak winds are several hundreds of per cent of the geostrophic values. Synoptic-or terrain-associated baroclinicity may well be a factor in the development of these latter jets (Holton, 1967;Bonner and Paegle, 1970;Shapiro and Fedorovich, 2009). We plan to extend our current methodology to include this effect.…”

“…Since a descending wind maximum was a more robust feature of the numerical simulations over flat terrain without an imposed thermal wind than of the observations (which were frequently over gentle, but generally non-zero, terrain slopes, and subject to synoptic influences), we speculate that the tendency of the height of some observed wind maxima to increase with time may be a consequence of synoptic-or terrain-associated baroclinicity. Other aspects of low-level jet structure and climatology have also been attributed to baroclinicity (Holton, 1967;Bonner and Paegle, 1970;Shapiro and Fedorovich, 2009). …”

“…Although a diurnal wind oscillation could be induced by the heating function, Holton's results did not correctly reproduce the observed phase of the diurnal oscillations, and arguably the flow was not as jet-like as observations show. Bonner and Paegle (1970) considered a gradually varying (single-frequency, 24 h period) eddy viscosity and geostrophic wind, with the periodicity of the geostrophic wind ascribed to the diurnal temperature cycle over sloping terrain. Their results were in reasonable agreement with observations, but the amplitude of the oscillation was sensitive to the magnitude of the geostrophic wind, the choice of eddy viscosity, and the phase difference between variations of the eddy viscosity and the geostrophic wind.…”

Assessments of moisture fluxes east of the Andes in South America in a global warming scenario

“…The studies by Bonner and Paegle (1970) as well as Paegle (1998) and Marengo et al (2004) suggest similarities between the Rocky Mountains and the Andes Cordillera and their effects on the occurrence of the jet. These mountains extend from the tropical regions to high latitudes, blocking the zonal circulation in lower topographic blocking by the Andes and then flow parallel to the Andes towards south/southeast South America (southern Brazil and northern Argentina).…”

Development and mechanisms of the nocturnal jet

“…These include an inertial oscillation due to frictional decoupling in the early evening (Blackadar, 1957) and Thorpe & Guymer (1977), stress divergence within the nocturnal inversion generating a 'quasi-steady' jet (implied by the model of Nieuwstadt, 1984), baroclinicity produced in inclined boundary layers over the sloping terrain (Bonner & Paegle, 1970) and large-scale baroclinicity and interactions between upper-level jet streaks and diabatic processes associated with cyclogenesis (Johnson & Uccellini, 1979). However, if the terrain is flat then two main mechanisms, an inertial oscillation and a 'quasi-steady' boundary layer, could potentially explain the jet.…”