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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.

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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.

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William T. Thompson
,
Tracy Haack
,
James D. Doyle
, and
Stephen D. Burk

Abstract

During the summer months, the California coast is under the influence of persistent northwesterly flow. Several times each summer, this regime is disrupted by coastally trapped wind reversals (CTWR) in which the northwesterly flow is replaced by southerlies in a narrow zone along the coast. Controversy exists as to the physical mechanisms responsible for initiation and maintenance of CTWRs. While it is clear that coastal terrain is important in creating the trapped response, the precise role played by terrain is unclear. In the present study, these issues are investigated using a nonhydrostatic mesoscale model to simulate the 10–11 June 1994 CTWR event. The results show that the model successfully reproduces many of the observed features of this event, including anomalous vertical structure involving the relatively shallow boundary layer with a warm, nearly neutral layer above; the northward propagation of southerly flow in advance of a tongue of coastal stratus/fog; and a substantial reduction in propagation speed due to the sea breeze. Of the several mechanisms that have been proposed in the literature to characterize these events, these results are most consistent with a topographically trapped gravity current. Further investigation, required to verify this hypothesis, is ongoing.

Two sensitivity studies are used to examine the role of terrain in producing and maintaining the CTWR. In the first sensitivity study, the coastline from Pt. Conception to Pt. Reyes is replaced with a straight line and a uniform 840-m-high ridge is placed adjacent to the coast. This simplification permits better isolation of the terrain influence on the mesoscale pressure field and the forcing of the CTWR by the pressure distribution. The results show that adiabatic warming associated with flow over the coastal terrain is required to produce the alongshore pressure gradient, which forces ageostrophic southerly flow, and that, in the absence of bays and gaps in this terrain, southerly flow extends to the location of the minimum pressure. In a second sensitivity study, the height of the ridge along the coast is set to zero. In this simulation there is no mesoscale organization of the southerly flow. Moreover, the structure of the marine boundary layer near the coast is altered by removal of downslope flow and the gravity current characteristics seen in the control and first sensitivity study are absent.

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Robert W. Fett
,
Stephen D. Burk
,
William T. Thompson
, and
Thomas L. Kozo

This paper describes unique environmental phenomena observed during LEADEX (Leads Experiment), a multidisciplinary investigation staged from an ice camp in the Beaufort Sea during March and April 1992. The paper focuses on phenomena observed by NOAA, DMSP, and the European ERS-1 satellites. The opening and closing of a lead is studied using the synthetic aperture radar (SAR) data aboardERS-1. With a mesoscale model, the authors examine the three-dimensional nature of meteorological phenomena and their effect on the opening and closing of leads and show that this model is extremely useful for interpreting structures evident in satellite and in situ observations along northern Alaska. Storms or wind events, which result in leads and fractured ice, also cause ice floes to rotate; the authors document this rotation with automated weather stations anchored to the floes. Finally, the authors describe unique thermal streaks that appeared over a large area of the Beaufort Sea during strong northeasterly winds and explore their nature using multichannel satellite data.

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David P. Rogers
,
Clive E. Dorman
,
Kathleen A. Edwards
,
Ian M. Brooks
,
W. Kendall Melville
,
Stephen D. Burk
,
William T. Thompson
,
Teddy Holt
,
Linda M. Ström
,
Michael Tjernström
,
Branko Grisogono
,
John M. Bane
,
Wendell A. Nuss
,
Bruce M. Morley
, and
Allen J. Schanot

Some of the highlights of an experiment designed to study coastal atmospheric phenomena along the California coast (Coastal Waves 1996 experiment) are described. This study was designed to address several problems, including the cross-shore variability and turbulent structure of the marine boundary layer, the influence of the coast on the development of the marine layer and clouds, the ageostrophy of the flow, the dynamics of trapped events, the parameterization of surface fluxes, and the supercriticality of the marine layer.

Based in Monterey, California, the National Center for Atmospheric Research (NCAR) C-130 Hercules and the University of North Carolina Piper Seneca obtained a comprehensive set of measurements on the structure of the marine layer. The study focused on the effects of prominent topographic features on the wind. Downstream of capes and points, narrow bands of high winds are frequently encountered. The NCAR-designed Scanning Aerosol Backscatter Lidar (SABL) provided a unique opportunity to connect changes in the depth of the boundary layer with specific features in the dynamics of the flow field.

An integral part of the experiment was the use of numerical models as forecast and diagnostic tools. The Naval Research Laboratory's Coupled Ocean Atmosphere Model System (COAMPS) provided high-resolution forecasts of the wind field in the vicinity of capes and points, which aided the deployment of the aircraft. Subsequently, this model and the MIUU (University of Uppsala) numerical model were used to support the analysis of the field data.

These are some of the most comprehensive measurements of the topographically forced marine layer that have been collected. SABL proved to be an exceptionally useful tool to resolve the small-scale structure of the boundary layer and, combined with in situ turbulence measurements, provides new insight into the structure of the marine atmosphere. Measurements were made sufficiently far offshore to distinguish between the coastal and open ocean effects. COAMPS proved to be an excellent forecast tool and both it and the MIUU model are integral parts of the ongoing analysis. The results highlight the large spatial variability that occurs directly in response to topographic effects. Routine measurements are insufficient to resolve this variability. Numerical weather prediction model boundary conditions cannot properly define the forecast system and often underestimate the wind speed and surface wave conditions in the nearshore region.

This study was a collaborative effort between the National Science Foundation, the Office of Naval Research, the Naval Research Laboratory, and the National Oceanographic and Atmospheric Administration.

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