Energetics of African wave disturbances derived from the FSU Global Spectral Model

Summary The FSU Global Spectral Model including comprehensive physical parameterization and high resolution (T 106) is used to make predictions to 48 hours for two cases of African easterly wave disturbances which occurred during FGGE. Model output is compared with FGGE III-b analyses to qualitatively assess the model's performance. The results of energetics calculations for the two case studies are included in this study. These calculations are based on model output to 24 hours. The energetics results confirm that combined barotropic/baroclinic instability provides the primary energy source for the African waves. Such energetics results, based on individual case studies, also provide an interesting and valuable comparison with compositing studies on African waves. An aspect of this study deals with the definition of a double jet stream structure at 700 mb for the two cases under consideration. The energetics patierns in zones to the north and south of each of these jets are separately portrayed.With 15 Figures     

气旋形成与扰动发展机理的数值试验

应用六层全隐式原始方程模式,对地面气旋形成与扰动发展的机制进行了数值试验. 首先,构造了接近实际的理想带状温压场和相应的扰动场.试验表明在带状环流上如果在低空加上一个类似锋区的扰动,便会有锋面气旋很快地发展起来,并逐渐趋于锢囚阶段,和实际大气的锋面气旋生命史很相似.但若扰动在对流层中层,则地面气旋形成过程较慢,强度也弱.如果初始扰动位于对流层高层,则地面形成的气旋在结构上和切断低压类似,不像锋面气旋.     

A numerical experiment of cyclogenesis and the development of disturbances

A numerical experiment has been carried out to study the mechanism of cyclogenesis and the develop-ment of disturbances. First, an idealized field of temperature and pressure is designed, which is very similar to the actual synoptic situation, consisting of the jet-like zonal circulation with some nonzonal waves superim-posed on it. Prediction is made by using a six-level splitting primitive model with the idealized field as an initial one. The results show that if a disturbance like a frontal zone in the lower troposphere is superim-posed on the zonal circulation, a frontal cyclone will quickly develop and then gradually become an oc-cluded one. Its life cycle is similar to that of the actual frontal cyclone on the synoptic map. However, if there is a disturbance superimposed on the zonal circulation in the middle troposphere, the cyclone with weaker intensity will be slowly formed near the surface. Finally, if the initial disturbance is located at the high-level, a situation like a cut-off low rather than a frontal cyclone will develop.     

A study of planetary wave errors in a spectral numerical weather prediction model

Abstract

A series of fifteen 96‐h forecasts made with a spectral numerical weather prediction model is studied with reference to errors in the planetary wavelengths. The major contributor to the short (less than 48‐h) range forecast error is identified as an external mode. The medium range forecast error (96 h) is internal in character and reflects a deficiency in the simulation of the quasi‐stationary components.     

A theoretical explanation of mountain and valley winds by a numerical method

Summary A theoretical model of the local circulation in a valley is obtained. The region under study consists of a V-shaped trough of finite length leading into a flat plain, and a given rate of heat flow from ground to air is assumed. The equations of motion are solved numerically, and give a circulation featuring valley and slope winds, their respective anti-winds (compensating flow at higher levels) and vertical currents.

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Zusammenfassung Es wird ein theoretisches Modell der lokalen Zirkulation in einem Tale entwickelt. Das untersuchte Gebiet ist ein V-förmiges Tal endlicher Länge, das in eine Ebene mündet. Es wird die Annahme zugrunde gelegt, daß eine gegebene Wärmemenge von der Erdoberfläche an die Luft abgegeben wird. Die numerische Lösung der Bewegungsgleichungen gibt die Berg- und Hangwinde sowie die entsprechenden Ausgleichströmungen oder Antiwinde und Vertikalströmungen.

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Résumé Un modèle théorique de l'écoulement local d'une vallée est développé. La région étudiée consiste en une vallée de longueur finie qui débouche sur une plaine horizontale, et on postule, à l'interface air-sol, un flux thermique donné. On résout les équations du mouvement numériquement, et on obtient un écoulement où les vents de vallée (longitudinaux) et de pente (transversaux), leurs contre-vents respectifs, et des courants verticaux, sont les caractères principaux.

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With 12 Figures

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Contribution No. 98 of the Department of Atmospheric Sciences, University of Washington, Seattle, USA.     

A numerical study of geostrophic adjustment and frontogenesis

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A numerical modelling study of coastal flooding

Summary A coastal ocean model capable of modelling tides, storm surge and the overland flow of floodwaters has been further developed to include the flux of water from tributaries and the forcing from wave breaking that leads to wave setup in the nearshore zone. The model is set up over the Gold Coast Broadwater on the east coast of Australia. This complex region features a coastal lagoon into which five tributaries flow and is subject to flooding from extreme oceanic conditions such as storm surge and wave setup as well as terrestrial runoff. Weather conditions responsible for storm surge, waves and flooding include cyclones of both tropical and mid-latitude origin. Two events are modelled. The first is an east coast low event that occurred in April 1989. This event verified well against available observations and analysis of the model simulations revealed that wave setup produced a greater contribution to the elevated water levels than the storm surge. The second case to be modelled was tropical cyclone Wanda, responsible for the 1974 floods. Modelled water levels in the Broadwater were reasonably well captured. Sensitivity experiments showed that storm surge and wave setup were only minor contributors to the elevated sea levels and their contribution was confined to the earlier stage of the event before the runoff reached its peak. The contribution due solely to runoff exhibited a tidal-like oscillation that was 180° out-of-phase with the tide and this was attributed to the greater hydraulic resistance that occurs at high tide. A simulation of this event with present day bathymetry at the Seaway produced sea levels that were 0.3–0.4 m lower than the simulation with 1974 bathymetry highlighting the effectiveness of deepened Seaway channel to reduce the impact of severe runoff events in the Broadwater. Received October 16, 2001 Revised December 28, 2001     

A numerical study of the convective boundary layer

Computations of the buoyantly unstable Ekman layer are performed at low Reynolds number. The turbulent fields are obtained directly by solving the three-dimensional time-dependent Navier-Stokes equations (using the Boussinesq approximation to account for buoyancy effects), and no turbulence model is needed. Two levels of heating are considered, one quite vigorous, the other more moderate. Statistics for the vigorously heated case are found to agree reasonably well with laboratory, field, and large-eddy simulation results, when Deardorff's mixed-layer scaling is used. No indication of large-scale longitudinal roll cells is found in this convection-dominated flow, for which the inversion height to Obukhov length scale ratio –z _i /L _*=26. However, when heating is more moderate (so that –z _i /L _*=2), evidence of coherent rolls is present. About 10% of the total turbulent kinetic energy and turbulent heat flux, and 20% of the Reynolds shear stress, are estimated to be a direct consequence of the observed cells.     

A numerical study of the Chicago lake breeze

A two-dimensional numerical model is presented for the study of the Chicago area lake breeze. The hydrostatic approximation used by previous workers in this field is not employed. Instead, the complete z-momentum equation is solved and the equation of continuity is retained in its original form. The numerical model employs a staggered mesh, and Alternating Direction Implicit methods are used for the integration of the equations. Computational stability is achieved without external filters, upstream space differencing, or artificial horizontal diffusion terms.The results obtained for realistic initial conditions but simplified boundary conditions compare reasonably well with observations obtained by Lyons in his 1967 observational study of the Chicago lake breeze. Limiting forms of the lower boundary condition bracket the observed wind speeds and other kinematic features of the breeze. Among the various eddy diffusivity models, that with constant eddy diffusivity with height produces the most realistic results. The models with a decrease of eddy diffusivity with height show unrealistic flow patterns in the upper layers. The stability of the atmosphere at the onset of the breeze is shown to have a pronounced effect on the intensity of the circulation, but not on the thickness of the inflow layer, which appears quite insensitive to large changes in the parameters involved.     

Sea breeze in Mallorca. A numerical study

Summary The Spanish island of Mallorca is located almost in the centre of the western Mediterranean. Very often from April to October, and almost every day during the summer, the sea breeze circulation develops. The shape and the topography of the island determine that the main characteristic feature of the sea breeze is the existence of a convergence line in the centre of the island. In this paper we describe the sea breeze circulation. A very simple non-eulerian numerical model allows us to simulate the evolution of the circulation during 24 hours.With 10 Figures     

A numerical study of the marine stratocumulus cloud layer

A one-dimensional grid-level model including longwave radiative transfer and a level-4 second-order turbulent transfer closure which contains prognostic equations for turbulent quantities, is used to study the physics and dynamics of inversion-capped marine stratocumulus clouds.A set of numerical experiments had been performed to examined the role of sea surface temperature, large-scale vertical velocity, wind speed, and vertical wind shear in the formation and the structure of low-level clouds. For a given sea surface and geostrophic wind speed, stratocumulus clouds can grow higher with smaller large-scale subsidence as less dry air entrains into the cloud. Clouds grow higher with higher sea surface temperature for a given geostrophic wind speed and large-scale subsidence as a result of enhanced moist convection. In high wind speeds, the entire cloud deck is lifted up because of larger surface energy flux. In the budget studies of the turbulent kinetic energy (TKE), the buoyancy term is a major source term when the wind speed and the vertical shear are small across the inversion top. When the wind speed and the vertical wind shear across the inversion top become large, the mixed layer is decoupled into a cloud and a subcloud layer. In the TKE budget studies, the shear generation term becomes an important term in the budgets of the TKE and the variance of vertical velocity.     

A high resolution numerical study of Gulf of Mexico fronts and eddies

Summary The Gulf of Mexico (GOM) circulation is simulated using the DieCAST ocean model, with a horizontal resolution of 1/12° and 20 vertical layers. The results compare well with observations of both large and small scale features, including Loop Current frontal occlusions associated with frontal eddies. The simulation is carried out without any data assimilation. The frontal eddies tend to be spaced at about 90° intervals around the Loop Current, leading to a Loop Current head shaped like a square with rounded corners. The pattern rotates as the eddies circle the Loop, and frontal eddies elongate as they squeeze through the Florida Strait. Major warm core eddies separate regularly from the Loop Current and propagate to the western GOM. Old warm core eddies in the western Gulf dissipate through bottom drag effects, which also generate cyclonic parasitic eddies. Newly arrived warm core eddies merge with old ones in the western GOM. Recently separated elongated Loop Current eddies can rotate and reattach temporarily to the Loop Current. The barotropic flow component develops eddies between the main separated warm core eddy and the Loop Current due to eastward dispersion, as the main eddy itself propagates westward into the Gulf.With 10 Figures     

Sensitivity of a numerical wave model on wind re-analysis datasets

Wind is the dominant process for wave generation. Detailed evaluation of metocean conditions strengthens our understanding of issues concerning potential offshore applications. However, the scarcity of buoys and high cost of monitoring systems pose a barrier to properly defining offshore conditions. Through use of numerical wave models, metocean conditions can be hindcasted and forecasted providing reliable characterisations.This study reports the sensitivity of wind inputs on a numerical wave model for the Scottish region. Two re-analysis wind datasets with different spatio-temporal characteristics are used, the ERA-Interim Re-Analysis and the CFSR-NCEP Re-Analysis dataset. Different wind products alter results, affecting the accuracy obtained. The scope of this study is to assess different available wind databases and provide information concerning the most appropriate wind dataset for the specific region, based on temporal, spatial and geographic terms for wave modelling and offshore applications. Both wind input datasets delivered results from the numerical wave model with good correlation. Wave results by the 1-h dataset have higher peaks and lower biases, in expense of a high scatter index. On the other hand, the 6-h dataset has lower scatter but higher biases. The study shows how wind dataset affects the numerical wave modelling performance, and that depending on location and study needs, different wind inputs should be considered.