Stephen D. Burk

A Vertically Nested Regional Numerical Weather Prediction Model with Second-Order Closure Physics

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-resolut...

The Summertime Low-Level Jet and Marine Boundary Layer Structure along the California Coast

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. The pronounced east–west slope of the marine planetary boundary layer (MPBL) is not due primarily to colder SST values along the coast. Dynamically forced low-level coastal divergence, coupled with synoptic-scale divergence, appears to be dominant in determining MPBL i...

Highlights of Coastal Waves 1996

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 ...

Mesoscale Simulation of Supercritical, Subcritical, and Transcritical Flow along Coastal Topography

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 t...

Supercritical flow interaction within the Cape Blanco-Cape Mendocino orographic complex

Abstract Supercritical flow interaction occurring in the marine boundary layer between closely spaced coastal capes is investigated with a mesoscale numerical prediction model. As an extension of previous work, the U.S. Navy’s Coupled Ocean/Atmosphere Mesoscale Prediction System (COAMPS) is used to perform idealized model simulations with marine layers of varying upstream Froude number to elucidate the different flow responses for a single convex bend. The impact upon the supercritical flow of introducing a series of closely spaced coastal bends is then investigated. The expansion fan is significantly reduced in magnitude and size by the formation of a compression wave at a blocking, concave bend approximately 150 km downstream. Building upon the idealized marine layer response, real-data forecasts are then examined for several time periods of supercritical flow interaction between Cape Blanco, Oregon, and Cape Mendocino, California. Observations from the Coastal Waves 1996 (CW96) field program were colle...

Summertime Marine Refractivity Conditions along Coastal California

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 b...

A Nonhydrostatic Mesoscale Simulation of the 10–11 June 1994 Coastally Trapped Wind Reversal

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 i...

Mesoscale Modeling of Summertime Refractive Conditions in the Southern California Bight

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 m...

The Dynamics of Wave Clouds Upwind of Coastal Orography

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...

An Investigation of an Arctic Front with a Vertically Nested Mesoscale Model

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 t...