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William T. Thompson
and
Tracy Haack

Abstract

The Wallops-2000 experiment took place in April and May 2000 in the vicinity of Wallops Island, Virginia, to collect high-resolution measurements of microwave propagation and coincident meteorological parameters in a complex coastal environment. These data are used in conjunction with a mesoscale numerical weather prediction model to examine the impact of sea surface temperature (SST) on microwave ducting. Analysis of time series of meteorological fields at the location of an instrumented buoy indicates reliable forecast skill. Statistics from vertical profiles and of derived ducting characteristics (duct frequency, duct strength, duct-base height, and duct thickness) show that the model reproduced observed duct characteristics with modest accuracy, allowing for a 3–6-h error in synoptic airmass transitions. In addition to the control run, two experiments are conducted to examine the impact of SST on ducting. In one experiment a climatological SST field is used, and in the other a diurnal variation in SST is imposed. The higher SST in the diurnally varying simulations promotes stronger turbulent mixing, deep boundary layers, and small vertical gradients in mixing ratio in comparison with the control, which lead to reduced duct frequency and strength in many cases. The study further reveals that, while advection of large-scale air masses (vertical and horizontal) plays a crucial role in determining whether an environment is favorable for microwave ducting, diurnal variations in SST can be influential in determining the onset of ducting and the frequency of surface-based ducting in coastal regions.

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William T. Thompson
and
R. T. Williams

Abstract

A hydrostatic primitive equation model initialized in a highly baroclinically unstable state was used to simulate maritime cyclogenesis and frontogenesis. In order to identify boundary layer physical processes important in maritime frontogenesis, several different simulations were performed. In an effort to isolate impacts due solely to the boundary layer, moist processes were not included. An adiabatic and inviscid simulation provided the control for these experiments. Two different boundary layer parameterizations were used: a K-theory parameterization featuring Richardson-number-dependent eddy diffusivity and a second-order closure scheme with prognostic equations for the turbulence quantities.

Results indicated that strong warm and cold fronts formed in the adiabatic and inviscid case but that the vertical motion fields were weak. In the K-theory simulation, the results were somewhat more realistic with stronger vertical motion. In both the K-theory and second-order closure simulations, the boundary layer in the cold air was highly unstable and deep mixed layers formed in this region with a large generation of turbulence. The largest cross-front temperature gradients existed in the frontal zone above the mixed layer. These structures were in qualitative agreement with observations of maritime cold fronts over the northwest Pacific Ocean. The second-order closure simulations produced a shallower mixed layer in the cold air with a stronger, more narrow front and large vertical motion. These simulations were more consistent with observations. Results from the second-order closure simulations demonstrated that turbulent mixing of momentum was critical in reproducing the frontogenetic (and frontolytic) effects of the transverse secondary circulation.

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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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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.

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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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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

On 28 August 2002, a visually striking sequence of events appeared in satellite imagery showing a coastally trapped disturbance (CTD) propagating northward along the coast of California against a northerly background flow. As a narrow tongue of coastal stratus indicative of the CTD propagated northward, a long, linear set of wave clouds developed ahead of the advancing CTD and angled away from the coast. The CTD and cloud lines moved northward over the next ∼6 h and, as they approached Cape Mendocino (CM), the leading edge of the CTD clouds rolled up into a cyclonic mesoscale eddy—with the wave clouds being wrapped into the eddy. The CTD abruptly stalled and failed to round CM. Further, a second cyclonic mesoscale eddy formed southwest of Point Arena (PA).

Although there has been extensive study of the propagation phase of CTDs, relatively little attention has been paid to the cessation of their propagation wherein mesoscale eddy development is not uncommon. Using the U.S. Navy's Coupled Ocean–Atmosphere Mesoscale Prediction System (COAMPS), run in an operational manner, numerous observed features of this case are forecast, including: (i) the cold, shallow, cloud-filled, northward-propagating CTD; (ii) the development, linear structure, orientation, and movement of an oblique hydraulic jump–like (“shock”) feature; (iii) a southerly wind shift associated with the CTD that precedes the advancing cloud tongue by several hours in both the observations and the model; (iv) the modeled CTD that rounds PA, but fails to round CM; and (v) the formation of modeled cyclonic mesoscale eddies near both CM and PA. North of PA, however, a phase error develops in which the modeled CTD propagation is too slow.

The model forecast cloud tongue behaves as a gravity current, and similar to earlier observational and modeling studies of CTDs, the model forecasts a bore propagating in the stratified atmosphere immediately above the marine boundary layer. Supercritical flow is forecast in the accelerating northerly flow rounding CM, and when the advancing bore interacts with this high Froude number region a pronounced oblique shock develops and the CTD stalls. Vorticity is enhanced along this shock due to vertical stretching and potential vorticity is generated within the shock. Additionally, juxtaposition of the CTD's southerly flow with the background northerly flow creates a vortex sheet–like shear zone along the offshore flank of the CTD, with the horizontal gradient of absolute vorticity changing signs, which is a necessary condition for classic barotropic instability.

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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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