William Blumen

CASES-99: A Comprehensive Investigation of the Stable Nocturnal Boundary Layer

Abstract The Cooperative Atmosphere-Surface Exchange Study—1999 (CASES-99) refers to a field experiment carried out in southeast Kansas during October 1999 and the subsequent program of investigation. Comprehensive data, primarily taken during the nighttime but typically including the evening and morning transition, supports data analyses, theoretical studies, and state-of-the-art numerical modeling in a concerted effort by participants to investigate four areas of scientific interest. The choice of these scientific topics is motivated by both the need to delineate physical processes that characterize the stable boundary layer, which are as yet not clearly understood, and the specific scientific goals of the investigators. Each of the scientific goals should be largely achievable with the measurements taken, as is shown with preliminary analysis within the scope of three of the four scientific goals. Underlying this effort is the fundamental motivation to eliminate deficiencies in surface layer and turbul...

Shear layer instability of an inviscid compressible fluid. Part 2

The stability of parallel shear flow of an inviscid compressible fluid is investigated by a linear analysis. The extension of the Rayleigh stability criterion and Howards semi-circle theorem to compressible flows, obtained by Lees & Lin (1946) and Eckart (1963) respectively, are each rederived by a different approach. It is then shown that a subsonic neutral solution of the stability equation may be found when the basic flow is represented by the hyperbolic-tangent velocity profile. With the aid of this solution, the unstable eigenvalues, eigenfunctions and Reynolds stress are determined by numerical methods. A brief discussion of the results follows.

Intermittent Turbulence Associated with a Density Current Passage in the Stable Boundary Layer

Using the unprecedented observational capabilities deployed duringthe Cooperative Atmosphere-Surface Exchange Study-99 (CASES-99),we found three distinct turbulence events on the night of 18October 1999, each of which was associated with differentphenomena: a density current, solitary waves, and downwardpropagating waves from a low-level jet. In this study, we focus onthe first event, the density current and its associatedintermittent turbulence. As the cold density current propagatedthrough the CASES-99 site, eddy motions in the upper part of thedensity current led to periodic overturning of the stratifiedflow, local thermal instability and a downward diffusion ofturbulent mixing. Propagation of the density current induced asecondary circulation. The descending motion following the head ofthe density current resulted in strong stratification, a sharpreduction in the turbulence, and a sudden increase in the windspeed. As the wind surge propagated toward the surface, shearinstability generated upward diffusion of turbulent mixing. Wedemonstrate in detail that the height and sequence of the localthermal and shear instabilities associated with the dynamics ofthe density current are responsible for the apparent intermittentturbulence.

Uniform Potential Vorticity Flow: Part I. Theory of Wave Interactions and Two-Dimensional Turbulence

Abstract Uniform potential vorticity flows are examined. In the quasi-geostrophic system, conservation of total energy and conservation of available potential energy on plane rigid horizontal boundaries imply a restriction on energy exchanges as a result of scale interactions. It is shown that for the Eady problem instability is always associated with energy transfer both up and down the vertical wavenumber spectrum although energy transfer from small to large three-dimensional wavenumbers may occur over a finite range of the spectrum. An inertial theory of two-dimensional turbulence is also presented. The formal analysis, based on Leiths diffusion approximation, predicts two inertial subranges: −5/3 and −1 power dependences on the horizontal wavenumber for available potential energy on horizontal boundaries. In the former range, available potential energy on horizontal boundaries cascades at a constant rate toward higher wavenumbers; in the latter range, the depth-integrated total energy cascades at a c...

Land–Atmosphere Interaction Research, Early Results, and Opportunities in the Walnut River Watershed in Southeast Kansas: CASES and ABLE

Abstract This paper describes the development of the Cooperative Atmosphere Surface Exchange Study (CASES), its synergism with the development of the Atmosphere Boundary Layer Experiments (ABLE) and related efforts, CASES field programs, some early results, and future plans and opportunities. CASES is a grassroots multidisciplinary effort to study the interaction of the lower atmosphere with the land surface, the subsurface, and vegetation over timescales ranging from nearly instantaneous to years. CASES scientists developed a consensus that observations should be taken in a watershed between 50 and 100 km across; practical considerations led to an approach combining long—term data collection with episodic intensive field campaigns addressing specific objectives that should always include improvement of the design of the long—term instrumentation. In 1997, long—term measurements were initiated in the Walnut River Watershed east of Wichita, Kansas. Argonne National Laboratory started setting up the ABLE ar...

Comparative Analysis of Low-Level Cold Fronts: Wavelet, Fourier, and Empirical Orthogonal Function Decompositions

Abstract Atmospheric cold fronts observed in the boundary layer represent relatively sharp transition zones between air masses of disparate physical characteristics. Further, wavelike features and/or eddy structures are often observed in conjunction with the passage of a frontal zone. The relative merits of using both global and local (with respect to the span of a basis element) transforms to depict cold-frontal features are explored. The data represent both tower and aircraft observations of cold fronts. An antisymmetric wavelet basis set is shown to resolve the characteristics of the transition zone, and associated wave and/or eddy activity, with a relatively small number of members of the basis set. In contrast, the Fourier transformation assigns a significant amplitude to a large number of members of the basis set to resolve a frontal-type feature. In principle, empirical orthogonal functions provide an optimal decomposition of the variance. The observed transition zone, however, has to be phase alig...

An Analysis of Ekman Boundary Layer Dynamics Incorporating the Geostrophic Momentum Approximation

Abstract A system of equations that describe motions in the boundary layer are derived. This system doors from the Ekman boundary layer equations by the inclusion of inertial terms through the geostrophic momentum approximation. The equations are solved, subject to the condition that the, horizontal motions approach the ageostrophic interior flow at the top of the boundary layer. The vertical velocity field at the top of the boundary layer is also determined. Interpretations of results are provided. A model of a circular vortex is to display characteristics of the velocity field in the boundary layer. In comparison with the Ekman boundary layer solution: 1) the magnitude of the horizontal and vertical velocities are relatively higher in an anticyclonic vortex and relatively lower in a cyclonic vortex, and 2) the depth of the boundary layer, which is a function of the vortex radius and the Rossby number (Ro ≤0.3), is higher in an anticyclonic vortex and lower in a cyclonic vortex than the constant Ekman la...

A Comparison between the Hoskins-Bretherton Model of Frontogenesis and the Analysis of an Intense Surface Frontal Zone

Abstract The Hoskins-Bretherton model of frontogenesis employed here represents the counterpart of the two-dimensional Eady problem expressed in geostrophic coordinate space. The fundamental characteristics of the model solution are shown to be derivable from the properties of the nonlinear one-dimensional advection equation and the linearized Eady problem. Detailed comparisons are made between the predictions of this model and the analysis of an intense frontal zone presented by Sanders. Qualitative agreement is found in details of the horizontal wind field and potential temperature distributions. The major discrepancy occurs in the vertical velocity field: the most intense vertical velocities occur at midlevel in the model and are significantly smaller in magnitude than the rising narrow jet above the analyzed zone of maximum cyclonic relative vorticity. The presence of this jet is responsible for the most significant frontogenetical properties of the front associated with vertical tilting of potential ...

An observational study of instability and turbulence in nighttime drainage winds

A detailed analysis of two nighttime drainage wind events that commenced on the evenings of 7 and 8 October, 1980 is presented. Data on wind velocity and temperature (10-s values), obtained from each of the eight instrumented levels of the Boulder Atmospheric Observatory, are used to construct 10-min means and root-mean-square values of all the variables. Additional information is provided by acoustic sounder data for 8 October.The analyses reveal that the passage of the drainage front occurs abruptly, between two 10-s observations, on both days. Relatively intense root-mean-square variability in the velocity and temperature fields accompanies the passage of the drainage front. In addition, the undercutting cold drainage air initiates significant variability in the vertical velocity field that extends above the 300 m level of the tower. The most significant variability in the other meteorological fields is primarily restricted to the lowest 150 m with the passage of the drainage front.A principal feature of the analysis is the delineation of Kelvin-Helmholtz instability and billow development, breakdown into turbulence and ultimate decay to a less turbulent state that occurs intermittently behind the drainage front. These features are interpreted in light of Thorpes (1973a, b) experimental work on stability and turbulence in stratified shear flow and the predictions of linear instability theory. The interpretations are carried out by considering the distributions of the Richardson number, the peak shears of the mean flow and the vertical fluxes of horizontal momentum associated with the unstable growth of the disturbances. Additional comparisons are made between the turbulent structures in Thorpes laboratory experiments and the turbulence exhibited in traces of the 10-s vertical velocity data measured at various levels both above and below the interface.The relevance of the present results to the design of future field programs, and to the observational data requirements that should be met to incorporate turbulent entrainment processes in models of pollutant dispersal is discussed.

On Short-Wave Baroclinic Instability

Abstract The Physical process responsible for short-wave baroclinic instability (zonal wavenumbers>10) is examined by means of a linearized two-layer Eady model. The static stability is uniform but different in each layer and the wind shear is uniform throughout both layers. Analysis of the unstable growth rates reveals that the instability is associated with the delta function distribution of potential vorticity at one boundary and at the interface between the two layers. This interpretation complements the interpretation of the unstable modes of a multi-layer model by Staley and Gall (1977). However, the present analysis also demonstrates how the short- and long-wave baroclinic instabilities depend on the relative layer depths as well as on the jump in static stability between the two layers. The effect of a jump in zonal wind shear is shown to be analogous to a jump in static stability in the present model. Finally, some implications of modeling atmospheric flows by multi-layered models, exhibiting dis...