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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

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

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

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

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
and
Tracy Haack

Abstract

The Naval Research Laboratory’s Coupled Ocean–Atmosphere Mesoscale Prediction System (COAMPS) is used in conjunction with satellite observations and data from the Coastal Waves 1996 experiment to investigate the dynamics of unusual wave clouds that occur upwind and offshore of orographic features along the California coast. Results indicate that supercritical flow within the marine boundary layer, interacting with blocking coastal orography, is forced to decelerate and an atmospheric bow shock forms. The location and orientation of the COAMPS forecast shock matches well with the leading edge of the wave clouds in satellite imagery, and the modeled jump in boundary layer depth across the shock is in good agreement with the aircraft observations. In the parameter space of Froude number and jump strength that develops within the flow (observed and modeled), the shock manifests itself as an undular bore.

On the innermost grid (Δx = ⅓ km), long, lineal variations in the wind, temperature, and moisture fields are forecast to develop on the subcritical side of the shock front and the modeled wavelength of these perturbations is close to the observed ∼4 km wavelength of the cloud lines. Their cellular structure and the quadrature between the vertical velocity and potential temperature fields strongly suggest that these are trapped internal gravity modes. Further, solutions to the Taylor–Goldstein equation for stationary waves, using a model-computed Scorer parameter profile, provide a comparable estimate of ∼3 km for a trapped, resonant wavelength.

The subkilometer forecasts presented are the highest-resolution real data forecasts with COAMPS to date. Time-dependent outer boundary conditions are supplied to COAMPS by the Naval Operational Global Atmospheric Prediction System. The nonhydrostatic nature of the COAMPS model is essential to forecasting these nonhydrostatic, trapped waves.

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Tracy Haack
and
Stephen D. Burk

Abstract

Large vertical gradients of temperature and moisture, which are not uncommon at the top of the marine atmospheric boundary layer (MABL), yield strong gradients in microwave refractivity that can result in anomalous electromagnetic (EM) propagation, including ducting wherein energy is strongly channeled horizontally. Of particular importance to surface radars and other microwave transmitters are surface-based ducts in which energy is ducted throughout the entire depth of the MABL. The Naval Research Laboratory’s Coupled Ocean–Atmosphere Mesoscale Prediction System (COAMPS) is used to define boundary layer structure during two coastal field experiments, and this model’s ability to forecast refractivity, including surface-based ducting, is assessed. At three marine sites, COAMPS shows considerable skill in MABL forecasts during the Variability of Coastal Atmospheric Refractivity experiment, although it contains biases for the MABL to be somewhat shallow and for the forecast duct strength, measured by the refractivity jump at MABL top, to be too weak. Nevertheless, for a total of 95 forecasts at these sites, COAMPS correctly forecasts surface-based ducting/no surface-based ducting events 82% of the time, with a false alarm rate of only 0.15.

Abrupt variations in coastal MABL depth and wind speed have been observed and modeled when supercritical flow (Froude number > 1) interacts with coastal terrain that is uniformly higher than the MABL depth. COAMPS and data from the Coastal Waves 1996 (CW96) experiment conducted along the northern California coast are used to address the implications of MABL variability to the coastal EM propagation environment. Comparison of CW96 research aircraft cross sections and soundings with COAMPS forecast fields indicate that the mesoscale model captures much of the observed MABL vertical structure and horizontal variability, including the temperature inversion and moisture lapse capping the MABL, the along- and cross-shore MABL slopes, and the presence and intensity of a coastal low-level jet. Supercritical expansion fans form in the lee of Cape Blanco, Oregon, and Cape Mendocino, California, on 1 July 1996 during CW96, and COAMPS indicates the presence of a compression jump where the flow is blocked on the upwind (north) side of Cape Mendocino. In conjunction with these MABL features, the EM propagation environment also exhibits substantial alongshore variation. Stronger, near-surface-based ducting occurs in the expansion fans as the marine layer thins and accelerates; weaker, elevated ducting occurs in the blocked flow.

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William T. Thompson
and
Stephen D. Burk

Abstract

Cold-frontal passages over the Gulf of Mexico in late winter or early spring are frequently followed by return-flow episodes in which modified polar air and warm, moist tropical air move toward the Gulf coast. While both advection and airmass modification due to boundary-layer physics are important in this sequence of events, the relative roles of these processes are unclear. In the present study, the authors utilize data from the Gulf of Mexico Experiment and two distinctive numerical models in addressing this issue. In forecasts of a return-flow event, trajectory computations are performed using a mesoscale numerical weather prediction model to determine the source regions of air arriving on the coat at several different levels. A one-dimensional airmass transformation model is also used in order to delineate boundary-layer physical processes. Simulations were conducted at two sites along the Gulf coast to investigate geographic variability in this return-flow episode, including the effect on boundary-layer structure of sea surface temperature variations in shelf waters.

By careful examination of temporal variations in surface flux and advective forcing and by examining changes due both to surface heat flux and differential advection in the forecast vertical profiles of potential temperature and specific humidity, the authors demonstrate that surface fluxes are important in heating and moistening the boundary layer as the air moves south across the Gulf. In the return flow, the complex vertical structure of differential advective heating and drying from multiple source regions plays an important role as well.

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Stephen D. Burk
and
William T. Thompson

Abstract

Large vertical gradients of temperature and moisture, often present at the top of the atmospheric boundary layer, can result in anomalous electromagnetic propagation. Layers in which the modified refractive index M decreases with height can act to trap microwave energy depending on the frequency and angle of incidence of the signal. Here the authors examine the ability of a mesoscale model to forecast the topography of such a trapping layer and to predict temporal trends in trapping-layer structure and depth.

Data from the Variability of Coastal Atmospheric Refractivity (VOCAR) experiment are used to examine the fidelity of model forecasts. The intensive observing period of VOCAR occurred from 23 August to 3 September 1993 in the Southern California bight. The mesoscale numerical weather prediction model used has a sophisticated physics package that includes a second-order closure turbulence scheme, detailed radiative flux computations, and explicit cloud physics.

The impact of several specific mesoscale and synoptic events (e.g., sea–land breezes, a migrating low) upon the refractivity field is examined along with the model’s capacity to forecast these features. The model exhibits significant promise in its ability to forecast trends in the height of the microwave trapping layer. Furthermore, these trends in trapping-layer depth are found to correlate rather well with the temporal behavior of the measured propagation factor.

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