Abstract

An incompressible, mesoscale model is used to estimate wave drag (WD) profiles over inhomogeneous 2D terrain. The goal is twofold: to evaluate the WD based on the model's fields and to analyze the atmospheric boundary layer (ABL) response to wave breaking. The model employs a simplified higher-order closure scheme for turbulent fluxes. A sponge layer mimics a radiative upper boundary condition (BC). Due to the no-slip lower BC for dissipative flows, the Eliassen-Palm theorem is not fulfilled and WD is generally not constant with height. Within the lower troposphere, the model's mean WD values compare to those from theory and other simulations. Above the ABL, where no physical processes dissipate waves, WD attenuates with height to roughly one-third of its theoretical value. This is mainly due to numerical dissipation of the used first-order advection scheme. However, the acceleration identified with the wave pattern reduction is small compared to governing accelerations.

Two simulation sets are presented: one is a linear and another is a nonlinear airflow. The latter exhibits wave overturning and alters the ABL from above. The ABL becomes horizontally inhomogeneous over a distance equal to several times the characteristic width of the ridge.

Abstract

A simple parameterization of turbulence in an analytical study of wave drag (WD) is used upon governing equations for linearized, parallel, adiabatic, dry Boussinesq flow. Besides the known dependencies of WD on vertical profiles of the mean wind and temperature and the shape of orographic disturbance, WD also depends on eddy diffusivity, which parameterizes turbulence. Since eddy diffusivity is present, WD is not constant with height but is continuously dissipated in the atmospheric boundary layer (ABL). This dissipation is sometimes not negligible because characteristic wavelengths associated with an ABL over small ridges are short enough (λ _x ∼ 1 km) to experience overall turbulence. Under certain conditions and when no critical level is encountered (although critical level might be approached asymptotically here), WD can be reduced for roughly 15%–20% of its surface value. Hence, it is revealed that dissipation of WD can support turbulence in the stable ABL.

Abstract

A simple form of the Prandtl model addressing pure katabatic flows is solved. The new analytic solution is valid for almost any assigned eddy diffusivity K(z) and constant Prandtl number. This model assumes a one-dimensional steady state for momentum and heat balance. Its approximate solution, obtained using the WKB method, appears as a generalization and improvement of the classic analytic solution for the constant-K case. It is compared favorably against a numerical solution. A comparison with observations from PASTEX, Austria 1994, shows that the new solution is much closer to the data than the constant-K solution. The dynamics revealed with this new solution is discussed (relatively sharper near-surface profiles, their gradients, and the low-level jet), and a suggestion toward improving boundary layer parameterizations is offered.

Abstract

Fully 3D nonlinear model simulations for supercritical flow along locations at the California coast, at Cape Mendocino, and Point Sur, are presented. The model results are objectively and subjectively verified against measurements from the Coastal Waves 1996 experiment with good results. They are then analyzed in terms of the flow structure, the impact of the local terrain, the atmospheric forcing on the ocean surface, and the momentum budgets. It is verified that the flow is supercritical (Fr > 1) within a Rossby radius of deformation from the coast and that it can be treated as a reduced-gravity, shallow water flow bounded by a sidewall—the coastal mountain barrier. As the supercritical flow impinges on irregularities in the coastline orientation, expansion fans and hydraulic jumps appear. The modeled Froude number summarizes well the current understanding of the dynamics of these events. In contrast to inviscid, irrotational hydraulic flow, the expansion fans appear as curved lines of equal PBL depth and “lens-shaped” maxima in wind speed residing at the PBL slope. This is a consequence of the realistic treatment of turbulent friction. Modeled mean PBL vertical winds in the hydraulic features range ±∼1–2 cm s⁻¹, while larger vertical winds (±∼5–10 cm s⁻¹) are due to the flow impinging directly on the mountain barrier. Local terrain features at points or capes perturb the local flow significantly from the idealized case by emitting buoyancy waves. The momentum budget along straight portions of the coast reveals a semigeostrophic balance modified by surface friction. While being geostrophic in the across-coast direction, the along-coast momentum shows a balance between the pressure gradient force and the turbulent friction. In the expansion fans, the flow is ageostrophic, and the imbalance is distributed between turbulent friction and ageostrophic acceleration according to the magnitude of the former. There is also a good correspondence between the magnitude of the local curl of the surface stress vector and the measured depression in sea surface temperature (SST) in areas where the latter is large and the along-coast flow is relatively weak, implying that a substantial portion of the upwelling is driven locally. Supplying the measured SST in the numerical simulations, with a considerable depression along the coast, had only marginal feedback effects on the character of the flow.

Abstract

A fully nonlinear, primitive equation hydrostatic numerical model is utilized to study coastal flow along central California, combining a realistic atmospheric model, with a higher-order turbulence closure, with highly simplified background flow. Local terrain and surface forcing of the model are treated realistically, while the synoptic-scale forcing is constant in time and space. Several different simulations with different background wind directions were performed. The motivation is to isolate the main properties of the local flow dependent on the coastal mesoscale influence only and to facilitate a study of the structure of the coastal atmospheric boundary layer, the mean momentum budget, and the atmospheric forcing on the coastal ocean for simplified quasi-stationary but still typical conditions. The model results feature the expected summertime flow phenomena, even with this simplified forcing. A coastal jet occurs in all simulations, and its diurnal variability is realistically simulated. The coastal topography serves as a barrier, and the low-level coastal flow is essentially coast parallel.

Among the conclusions are the following. (i) The boundary layer for a northerly jet is more shallow and more variable than that for a southerly jet. One reason is an interaction between waves generated by the coastal mountains and the boundary layer. A realistic inclusion of the Sierra Nevada is important, even for the near-surface coastal atmosphere. (ii) The transition from southerly to northerly flow, when changing the background flow direction, is abrupt for a change in the latter from west to northwest and more gradual for a change east to south. (iii) The low-level flow is in general semigeostrophic. The across-coast momentum balance is geostrophic, while the along-coast momentum balance is dominated by vertical stress divergence and the pressure gradient. Local acceleration and spatial variability close to the coast arise as a consequence of the balance among the remaining terms. For southeasterly background flow, the across-coast momentum balance is dominated by the background synoptic-scale and the mesoscale pressure gradients, sometimes canceling the forcing, thus making this case transitional. (iv) Smaller-scale flow transitions arise for some background flow directions, including an early morning jet reversal north of Monterey, California, and a morning-to-noon low-level eddy formation in the Southern Californian Bight. (v) The model turbulence parameterization provides realistic patterns of the atmospheric forcing on the coastal ocean. (vi) Characteristic signals measured in propagating wind reversals related to boundary layer depth and inversion structure here are seen to correspond to different quasi-stationary conditions.

Abstract

Winds through the Vratnik Pass, a mountain gap in the Dinaric Alps, Croatia, are polarized along the gap axis that extends in the northeast–southwest direction. Although stronger northeasterly wind at the Vratnik Pass is frequently related to the Adriatic bora wind, especially at the downstream town of Senj, there are many cases in which the wind at Senj is directionally decoupled from the wind at the Vratnik Pass. A cluster analysis reveals that this decoupling is sometimes related to lower wind speeds or a shallow southeasterly sirocco wind along the Adriatic, but in many cases the bora blows over a wider region, while only Senj has a different wind direction. Several mechanisms can be responsible for the latter phenomenon, including the formation of a lee wave rotor. A numerical model simulation corroborates the decoupling caused by a rotor for a single case. The prevalence of northeasterly winds at the Vratnik Pass during southeasterly sirocco episodes is another result that challenges the current understanding. It is shown that, at least in one of these episodes, this phenomenon is related to a secondary mesoscale low pressure center in the eastern lee of the Apennines that forms as a subsystem of a broader Genoa cyclone. Less frequent southwesterly winds through the gap are predominantly related to the thermal sea breeze and anabatic circulations that are sometimes superimposed on the geostrophic wind.

Abstract

This paper presents a turbulence closure for neutral and stratified atmospheric conditions. The closure is based on the concept of the total turbulent energy. The total turbulent energy is the sum of the turbulent kinetic energy and turbulent potential energy, which is proportional to the potential temperature variance. The closure uses recent observational findings to take into account the mean flow stability. These observations indicate that turbulent transfer of heat and momentum behaves differently under very stable stratification. Whereas the turbulent heat flux tends toward zero beyond a certain stability limit, the turbulent stress stays finite. The suggested scheme avoids the problem of self-correlation. The latter is an improvement over the widely used Monin–Obukhov-based closures. Numerous large-eddy simulations, including a wide range of neutral and stably stratified cases, are used to estimate likely values of two free constants. In a benchmark case the new turbulence closure performs indistinguishably from independent large-eddy simulations.

Abstract

No Abstract available.

Abstract

Strong winds crossing elevated terrain and descending to its lee occur over mountainous areas worldwide. Winds fulfilling these two criteria are called foehn in this paper although different names exist depending on the region, the sign of the temperature change at onset, and the depth of the overflowing layer. These winds affect the local weather and climate and impact society. Classification is difficult because other wind systems might be superimposed on them or share some characteristics. Additionally, no unanimously agreed-upon name, definition, nor indications for such winds exist. The most trusted classifications have been performed by human experts. A classification experiment for different foehn locations in the Alps and different classifier groups addressed hitherto unanswered questions about the uncertainty of these classifications, their reproducibility, and dependence on the level of expertise. One group consisted of mountain meteorology experts, the other two of master’s degree students who had taken mountain meteorology courses, and a further two of objective algorithms. Sixty periods of 48 h were classified for foehn–no foehn conditions at five Alpine foehn locations. The intra-human-classifier detection varies by about 10 percentage points (interquartile range). Experts and students are nearly indistinguishable. The algorithms are in the range of human classifications. One difficult case appeared twice in order to examine the reproducibility of classified foehn duration, which turned out to be 50% or less. The classification dataset can now serve as a test bed for automatic classification algorithms, which—if successful—eliminate the drawbacks of manual classifications: lack of scalability and reproducibility.

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.