Tracy Haack

Bora event variability and the role of air‐sea feedback

1) was conducted to improve forecast momentum and heat flux fields, and to evaluate surface flux field differences for two consecutive bora events during February 2003. (COAMPS 1 is a registered trademark of the Naval Research Laboratory.) The strength, mean positions and extensions of the bora jets, and the atmospheric conditions driving them varied considerably between the two events. Bora 1 had 62% stronger heat flux and 51% larger momentum flux than bora 2. The latter displayed much greater diurnal variability characterized by inertial oscillations and the early morning strengthening of a west Adriatic barrier jet, beneath which a stronger west Adriatic ocean current developed. Elsewhere, surface ocean current differences between the two events were directly related to differences in wind stress curl generated by the position and strength of the individual bora jets. The mean heat flux bias was reduced by 72%, and heat flux RMSE reduced by 30% on average at four instrumented over-water sites in the two-way coupled simulation relative to the uncoupled control. Largest reductions in wind stress were found in the bora jets, while the biggest reductions in heat flux were found along the north and west coasts of the Adriatic. In bora 2, SST gradients impacted the wind stress curl along the north and west coasts, and in bora 1 wind stress curl was sensitive to the Istrian front position and strength. The two-way coupled simulation produced diminished surface current speeds of 12% over the northern Adriatic during both bora compared with a one-way coupled simulation.

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...

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...

Mesoscale Modeling of Boundary Layer Refractivity and Atmospheric Ducting

Abstract In this study four mesoscale forecasting systems were used to investigate the four-dimensional structure of atmospheric refractivity and ducting layers that occur within evolving synoptic conditions over the eastern seaboard of the United States. The aim of this study was to identify the most important components of forecasting systems that contribute to refractive structures simulated in a littoral environment. Over a 7-day period in April–May of 2000 near Wallops Island, Virginia, meteorological parameters at the ocean surface and within the marine atmospheric boundary layer (MABL) were measured to characterize the spatiotemporal variability contributing to ducting. By using traditional statistical metrics to gauge performance, the models were found to generally overpredict MABL moisture, resulting in fewer and weaker ducts than were diagnosed from vertical profile observations. Mesoscale features in ducting were linked to highly resolved sea surface temperature forcing and associated changes i...

Summertime Influence of SST on Surface Wind Stress off the U.S. West Coast from the U.S. Navy COAMPS Model

Abstract High-resolution mesoscale model sea surface temperature (SST) analyses and surface wind stress forecasts off the U.S. West Coast are analyzed on monthly time scales for robust signatures of air–sea interaction as the surface winds encounter ocean surface features such as SST fronts, filaments, and eddies. This interaction is manifest by the linear relationship, or coupling coefficient, between the downwind SST gradient and wind stress divergence and between the crosswind SST gradient and wind stress curl evident from analysis of fields averaged over 29 days. This study examines fields from the Coupled Ocean–Atmosphere Mesoscale Prediction System (COAMPS) model, spanning the summer months, June–September, for four consecutive years, 2002–05. Relative to several models evaluated previously, coupling coefficients are much closer to those calculated from Quick Scatterometer (QuikSCAT) satellite measurements. In addition, the summertime correlation between the wind stress derivative field and its corr...

U.S. West Coast Surface Heat Fluxes, Wind Stress, and Wind Stress Curl from a Mesoscale Model

Abstract Monthly averages of numerical model fields are beneficial for depicting patterns in surface forcing such as sensible and latent heat fluxes, wind stress, and wind stress curl over data-sparse ocean regions. Grid resolutions less than 10 km provide the necessary mesoscale detail to characterize the impact of a complex coastline and coastal topography. In the present study a high-resolution mesoscale model is employed to reveal patterns in low-level winds, temperature, relative humidity, sea surface temperature as well as surface fluxes, over the eastern Pacific and along the U.S. west coast. Hourly output from successive 12-h forecasts are averaged to obtain monthly mean patterns from each season of 1999. The averages yield information on interactions between the ocean and the overlying atmosphere and on the influence of coastal terrain forcing in addition to their month-to-month variability. The spring to summer transition is characterized by a dramatic shift in near-surface winds, temperature, a...

Toward the Assimilation of the Atmospheric Surface Layer Using Numerical Weather Prediction and Radar Clutter Observations

Radio wave propagation on low-altitude paths over the ocean above 2GHz is significantly affected by negative refractivity gradients in the atmospheric surface layer, which form what is often referred to as an evaporation duct (ED). Refractivity from clutter (RFC) is an inversion approach for the estimation of the refractivity profile from radar clutter, and RFC-ED refers to its implementation for the case of evaporation ducts. An approach for fusing RFC-ED output with evaporation duct characterization that is based on ensemble forecasts from a numerical weather prediction (NWP) model is examined here. Three conditions of air‐sea temperature difference (ASTD) are examined. Synthetic radar clutter observations are generated using the Advanced Propagation Model. The impacts of ASTD on the evaporation duct refractivity profile, atmospheric parameter inversion, and propagation factor distributions are studied. Relative humidity at a reference height and ASTD are identified as state variables. Probability densities from NWP ensembles, RFC-ED, and joint inversions are compared. It is demonstrated that characterization of the near-surface atmosphere by combining RFC-ED and NWP reduces the estimation uncertainty of ASTD and relative humidity in an evaporation duct, with respect to using either method alone.