Search Results

You are looking at 1 - 6 of 6 items for :

  • Author or Editor: Stephen D. Burk x
  • Journal of the Atmospheric Sciences x
  • Refine by Access: All Content x
Clear All Modify Search
Stephen D. Burk

Abstract

An atmospheric boundary layer (ABL) model is used to address problems involving the generation, turbulent transport, and deposition of giant sized (1–25 μm) sea-salt aerosol. The surface aerosol generation rate is taken from the production flux expressions developed by Monahan. A simplified second-moment closure formulation for turbulent transport is used, while dry deposition fluxes are computed as functions of Stokes' settling speed and the rate of inertial impaction of particles across the viscous sublayer.

Initially we investigate, starting from first principles, whether the model can develop reasonable sea-salt volume distributions at several different Beaufort wind forces Using the empirical expressions for generation and deposition fluxes, we permit an initially aerosol-free ABL to fill by diffusion until the volume distributions approach equilibrium., we then compare these distributions with the classic Woodcock observations. Further experiments are conducted in which we explore the dynamic behavior of the aerosol spectra when winds are varying, and also we study vertical sea-salt profiles in a humid, trade wind ABL.

Full access
Stephen D. Burk

Abstract

The convective boundary-layer scaling expressions presented by Wyngaard and LeMone (1980) are compared with predictions from a turbulence closure model. We first examine a model experiment involving a clear-air, convectively driven boundary layer overland. The model results agree well with scaling expressions and observations in the lower boundary layer and near the inversion. In the mid-boundary layer region, however, the closure model underestimates the temperature structure parameter C T 2 and overestimates the humidity structure parameter C q 2.

A cloud-topped marine boundary layer is examined in a second experiment which uses AMTEX data. Order-of-magnitude differences are found here between interfacial-layer scaling expressions and closure model predictions. Potential sources of this disagreement are discussed.

Full access
Stephen D. Burk

Abstract

Large, diurnally varying surface temperature gradients occur at the polar cap periphery during Martian spring and summer. A primitive equation numerical model having grid points lying in the meridional plane is developed to calculate the wind field in this intensely baroclinic region. The atmosphere is assumed at rest initially, with the developing circulation being driven solely by the oscillating surface temperature gradient.

Maximum winds of approximately 20 m s−1 develop when the atmosphere is initially isothermal. Model sensitivity to surface boundary layer depth is examined, while in other experiments the initial lapse rate is varied. Heating rates due to planetary radiation, though large, are found to have a negligible influence upon the flow. Convective heat transfer is the dominant diabatic process.

Bagnold's (1941) theory of sand-grain movement, adapted to Martian surface conditions, is utilized to investigate the dust-lifting potential of the polar winds. As modelled, the surface wind stresses appear insufficient to raise dust, but this conclusion could be altered with inclusion of additional physical processes in the model.

Full access
Stephen D. Burk

Abstract

A one-dimensional higher order turbulence closure model is used to investigate moisture structure within the diurnally varying planetary boundary layer. The diurnal character of the moist boundary layer as a whole and a variety of micrometeorological features are examined in a series of experiments having differing lower boundary conditions on the moisture field. In one case, midafternoon surface evaporation and turbulent moisture transfer to higher levels act as competing processes in determining low-level moisture content. A double wave in low-level daily specific humidity results (specific humidity minima in early morning and midafternoon). In another experiment, a moisture inversion develops when there is a strong nocturnal moisture flux to the surface such as occurs with dew formation.

A simple, analytic method of calculating the moist layer's growth rate is compared with the numerical results. The analytic method provides good flux estimates when the shoulder in the specific humidity profiles (where the moisture lapse first sharply deviates from its mixed-layer value) is treated as being the top of the moist boundary layer.

The specified initial moisture distribution has a considerable lapse above 0.5 km. However, during the afternoon a well-mixed moist layer develops despite dry air entrainment above and surface moisture influx from below. This suggests that rapid growth into a dry environment cannot explain the coincidence of strong moisture lapses with thermally well-mixed regions.

Full access
Stephen D. Burk
and
Dean O. Staley

Abstract

Abstract not available.

Full access
Stephen D. Burk
,
Tracy Haack
, and
R. M. Samelson

Abstract

A mesoscale atmospheric model is used to address the characteristics of stratified flow bounded by a side wall along a varying coastline. Initial Froude number values are varied through alteration of marine inversion strength, permitting examination of supercritical, subcritical, and transcritical flow regimes encountering several coastal configurations. Consistent with shallow water models, sharp drops in boundary layer depth and flow acceleration occur in flow rounding convex bends; however, significant flow response occurs in the stratified layer aloft, which is unexplained by conventional shallow water theory. The strongest flow acceleration occurs in the transcritical case while, regardless of inversion strength, the deformation of the isentropes aloft shows general structural similarity.

Advection of horizontal momentum is an important component of the horizontal force balance. A simulation having several coastline bends exhibits a detached, oblique hydraulic jump upwind of a concave bend that strongly blocks the flow. For the single-bend case, a shallow water similarity theory for stratified flow provides qualitative, and partial quantitative, agreement with the mesoscale model, in the boundary layer and aloft. Horizontal structure functions for these similarity solutions satisfy a set of equivalent shallow water equations. This comparison provides a new perspective on previous shallow water models of supercritical flow around coastal bends and suggests that the existence of the supercritical flow response may depend more on the presence of a low-level jet than on a sharp boundary layer inversion.

Full access