Search Results

You are looking at 1 - 7 of 7 items for :

  • Author or Editor: Stephen D. Burk x
  • Journal of Applied Meteorology and Climatology x
  • Refine by Access: All Content x
Clear All Modify Search
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 .

Full access
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.

Full access
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.

Full access
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.

Full access
Stephen D. Burk
and
William T. Thompson

Abstract

Several numerical models are used to examine strong air-sea fluxes and resultant airmass modification following a cold-frontal passage over the Gulf of Mexico. Data from the Gulf of Mexico Experiment (GUFMEX), which was conducted in February-March 1988, are used for model validation. To provide a benchmark by which to evaluate the role of diabatic processes in airmass modification, the mesoscale model was initially run with surface fluxes deleted. Subsequent full physics runs show profound alterations to the boundary layer due to the diabatic processes. A one-dimensional airmass transformation (AMT) boundary-layer model is also tested and compared with the mesoscale model and GUFMEX data. The Lagrangian character of the AMT model is a useful compliment to the mesoscale model output. Further, at least in one forecast, the AMT model yields a better forecast of boundary-layer depth.

Strong sensible and latent heat fluxes in the vicinity of the cold front act frontolytically, while a subsidence-induced local maximum in latent heat flux appears in the return flow that is established in the western Gulf. The precipitable-water field shows a tongue of moist air returning to the Louisiana coast and indicates that substantial mesoscale horizontal gradients in the moisture field are to be expected in the return flow.

Full access
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.

Full access
Qin Xu
,
Binbin Zhou
,
Stephen D. Burk
, and
Edward H. Barker

Abstract

An air–soil layer coupled scheme is developed to compute surface fluxes of sensible heat and latent heat from data collected at the Oklahoma Atmospheric Radiation Measurement–Cloud and Radiation Testbed (ARM–CART) stations. This new scheme extends the previous variational method of Xu and Qiu in two aspects: 1) it uses observed standard deviations of wind and temperature together with their similarity laws to estimate the effective roughness length, so the computed fluxes are nonlocal; that is, they contain the contributions of large-eddy motions over a nonlocal area of O(100 km2); and 2) it couples the atmospheric layer with the soil–vegetation layer and uses soil data together with the atmospheric measurements (even at a single level), so the computed fluxes are much less sensitive to measurement errors than those computed by the previous variational method. Surface skin temperature and effective roughness length are also retrieved as by-products by the new method.

Full access