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Stephen D. Burk

Abstract

Here we illustrate a method which readily permits determination of the relative contributions of the individual temperature-humidity structure terms to total Cn 2 within the uppermost region of the clear, convective boundary layer. The relative contributions of terms involving CT 2 , CTq and CT 2 to acoustic, optical and microwave Cn 2 are shown to be functions primarily of the ratio, Δ q /Δθ v , of humidity to virtual potential temperature jump across the inversion. A graphical procedure is illustrated for quickly determining the expected degree of error if CT 2 or Cq 2 are directly inferred from Cn 2 .

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Stephen D. Burk

Abstract

A second-moment turbulence closure model is used to investigate the make-up of the refractive structure parameter Cn 2 for acoustic, optical and microwave radiation. For these different forms of radiation, Wesely (1976) develops functions describing the dependencies of Cn 2 on temperature and moisture structure parameters. Expressions are developed here which relate the temperature and moisture structure parameters to model ensemble-averaged turbulence variables. This permits model evaluation of the Wesely functions throughout the planetary boundary layer.

Three numerical experiments are conducted. Two deal with the marine planetary boundary layer (MPBL) and the final one involves an overland simulation. In the MPBL cases, moisture fluctuations play a dominant role in determining microwave Cn 2 and significantly affect acoustic and optical Cn 2 values. In fact, in one MPBL experiment, the near-surface acoustic and optical Cn 2 values are primarily determined by turbulent moisture perturbations. The overland simulation shows large diurnal variations in structure parameters. Moisture fluctuations are dominant aloft in determining microwave Cn 2 , but during the afternoon near the surface, temperature perturbations make a comparable contribution to microwave Cn 2 . Acoustic and optical Cn 2 are determined primarily by temperature fluctuations except near the inversion, near sunrise and near sunset.

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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.

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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.

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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.

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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.

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Stephen D. Burk
and
William T. Thompson

Abstract

This paper examines the strong, summertime northerly low-level jet (LLJ) that frequently exists along the California coast. The persistent synoptic-scale pressure distribution (North Pacific high to the west, thermal low to the east) and baroclinity created by the juxtaposition of the heated continent and the cool marine layer produce the mean structure of this LLJ. Strong diurnal thermal forcing, coupled with topographic influences on the flow, modulate the jet structure, position, and intensity. A mesoscale model is used to examine many of the complex facets of the LLJ flow dynamics. Several sensitivity studies, in addition to a control experiment, aid in this investigation. Principal findings of this study include the following.

  1. The pronounced east–west slope of the marine planetary boundary layer (MPBL) is not due primarily to colder SST values along the coast.

  2. Dynamically forced low-level coastal divergence, coupled with synoptic-scale divergence, appears to be dominant in determining MPBL inversion slope and profoundly impacts the coastal stratus distribution.

  3. Maximum baroclinity occurs in midafternoon, whereas the LLJ maximum occurs in the evening. An analytical treatment of the dynamics shows that diurnal variation of the jet-level baroclinity, coupled with inertial and friction effects, explain this jet timing.

  4. In a no-terrain simulation, the jet is broader, somewhat weaker, and tilts more to the west than in our control case.

  5. A deeper boundary layer occurs over the location of the Central Valley of California in the no-terrain simulation than in the more realistic control run. Consequently, a delay in time of maximum baroclinity aloft occurs in the no-terrain case, and the LLJ maximum occurs later as compared to the control.

  6. The core of the jet, which resides in the inversion capping the MPBL, lowers and moves toward the coast during the day and lifts and moves farther away from the coast at night.

  7. Meso-β-scale structure of the LLJ along the coast is forced by the topography of points and capes. Thee mesoscale model simulation has supercritical flow, showing expansion fan characteristics, in the MPBL around Cape Mendocino.

  8. Model results are consistent with mountain wave theory in that a near-surface wind speed maximum and pressure minimum are modeled on the lee side of Cape Mendocino.

  9. The LLJ maxima in the lee of points and capes produce local maxima in surface stress. The position of these wind stress maxima correlate well with the location of cold pools observed in the SST, implying that locally enhanced, wind-forced upwelling plays a major role in the creation of such cold SST patches.

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Stephen D. Burk
and
William T. Thompson

Abstract

The model we describe involves a unique strategy in which a high vertical resolution grid is nested within the coarse vertical resolution grid of a regional numerical weather prediction (NWP) model. Physics computations performed on the high vertical resolution grid involve time-dependent solution of second-order turbulence equations, the transfer equations for long- and shortwave radiation, and moist thermodynamic calculations which include liquid water content and fractional cloudiness. The dynamical computations involving advection, pressure gradient, and Coriolis terms are performed on the regional model grid. The two grids interact fully each model time step.

This approach represents an extension into NWP of the general practice of supplying coarse large-scale dynamical forcing to high-resolution boundary layer models. Aside from the computational savings of performing dynamical calculations only at the coarser resolution, we also avoid difficulties which can arise with high vertical-resolution dynamical computations in regions of significant topography. This model can, however, be easily made to take on the appearance of a standard, nonnested model by specifying everywhere one fine grid paint per coarse grid layer.

Several preliminary model forecasts are presented. The first is a 36-hour forecast over the Mediterranean and adjacent regions during midsummer. This provides a good test of the model's ability to develop a realistic cool marine mixed layer over the Mediterranean, while properly treating the extreme diurnal variations in the boundary layer over North Africa. Our second numerical forecast takes place in a much more active synoptic regime involving a wintertime frontal passage at a weather station ship in the North Atlantic.

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Stephen D. Burk
and
William T. Thompson

Abstract

A one-dimensional turbulence model has been coupled with the large-scale fields of a hemispheric model so as to produce a high-resolution marine boundary layer forecast system. Model initialization is performed either by use of individual ship soundings or from standard fields of the hemispheric model. Detailed boundary layer forecasts in specified oceanic regions are desirable for many purposes, but large-scale model forecasts with such high resolution are computationally impractical. This paper presents results from approximately 90 different 24 h forecasts at the location of four different ocean station vessels.

We statistically compare model forecast profiles of temperature and moisture with verifying soundings, and also evaluate persistence as a forecast. Results consistently show a significant improvement of the model forecasts relative to persistence. The one-way influence driving force provided by large-scale time derivative terms derived from the hemispheric model is found to be very important to this coupled forecast system.

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William T. Thompson
and
Stephen D. Burk

Abstract

A vertically mesoscale regional numerical weather prediction model is used to simulate an arctic front. The front was observed during the Arctic Cyclone Expedition of 1984. The regional model employs a unique vertical nesting scheme in which the dynamics computations are performed on a low vertical-resolution (coarse) grid and the physics computations are performed on a high vertical resolution (fine) grid nested within the coarse grid. Turbulent fluxes are parameterized using a second-order closure approach. The model forecast compares favorably with the observations. Moreover, the model develops detailed mesoscale and boundary layer structure that verifies against the observations when initialized using only sparse, synoptic-scale data.

A control experiment is run in which identical, high vertical resolution is used on both the dynamics and the physics grids. Several additional simulations are performed in order to demonstrate the utility and accuracy of the vertical nesting methodology. With the typical nested configuration (14 coarse grid levels, 24 fine levels), the evolution of the front is nearly identical to the control. When the resolution is degraded to 14 points on both grids, significant structural differences in the boundary layer arise.

The terms of the frontogenetic forcing function are evaluated in each of the experiments. In all of the simulators, the horizontal deformation is the dominant frontogenetic effect while the tilting term is the dominant frontolytic term for this arctic front, just as it is for midlatitude cold fronts. The diabatic term is predominantly frontolytic with the strongest heating occurring in the cold air as the front moves off the ice edge and out over the Barents Sea. In an experiment in which surface sensible and latent heat fluxes are deleted, a slightly stronger front having more pronounced ageostropic circulation develops.

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