Higher-order Turbulence Closure Research Articles

Optical turbulence simulations over the Tibetan Plateau based on a single-column model with high-order closure

Optical turbulence limits the maximum resolution of ground-based telescopes and leads to image degradation. The use of atmospheric numerical techniques to forecast optical turbulence is crucial for observation scheduling and optimization of adaptive optic systems. Current research methods for forecasting optical turbulence are primarily based on mesoscale models with turbulence closure techniques to parameterize the key terms required for C n 2 calculations under the assumption of atmospheric quasi-steady-state balance, and then the integrated astroclimatic parameters related to C n 2 profile can be obtained. In this study, we propose what we believe to be a novel approach to forecast C n 2 using a boundary layer parameterization based on higher-order turbulence closure in the single-column framework of the CLUBB model. Compared to mesoscale models, the CLUBB model serves as a single-column model, which simplifies modifications and reduces compilation time, and is more conducive to testing the performance of C n 2 parameterization scheme. In the design of C n 2 parameterization scheme, we consider a more complete physical process rather than omitting certain terms to obtain a steady-state solution. The performance of the model is evaluated using measurements obtained during a field campaign conducted at Da Qaidam site above the Tibetan Plateau. The results show that the model is able to capture typical features of the C n 2 profile evolution under convective conditions. Comparison of the model with contemporaneous sounding measurements and quantification of the model’s performance using statistical operators demonstrate the statistical agreement between simulations and measurements. In terms of atmospheric seeing, we can observe a bias of 0.01 and a root-mean-square error (RMSE) of 0.31 without any model calibration, which outperforms the results of previous mesoscale modeling studies. In addition, the new parameterization scheme is also compared with two representative C n 2 algorithms previously used in the mesoscale models, with some improvements observed. The potential demonstrated by this approach is expected to bring greater value and advancement to the research of three-dimensional forecasting of optical turbulence in the future by coupling with the mesoscale model.

Surface Turbulent Fluxes during Persistent Cold-Air Pool Events in the Salt Lake Valley, Utah. Part II: Simulations

AbstractRealistically representing the land–atmosphere interactions during persistent cold-air pools (PCAPs) is critical in simulating the strength of PCAPs, where uncertainties in simulating the PCAP strength will impact the ability to model the poor air quality. To quantify the model performance for land–atmosphere exchange, measurements of surface turbulent and radiative energy fluxes during two PCAPs, one weak and one strong, in Utah were compared with simulations from the Weather Research and Forecasting (WRF) Model. The results show that the WRF Model simulated the surface energy fluxes well in the weak PCAP case and that the performance degraded in the strong PCAP case. The significantly overestimated surface sensible heat flux H and latent heat flux (LE) in the strong PCAP were related, in part, to the overestimated net radiation and soil moisture and unsuitable turbulence parameterizations. The simulation using the Mellor–Yamada–Nakanishi–Niino planetary boundary layer scheme produced the least bias in both net radiation and surface turbulent fluxes for the strong PCAP case, which is expected because of the local higher-order (2.5) turbulence closure scheme. The surface exchange coefficient (CH), a crucial variable used to calculate H, was overall overestimated by the WRF Model. The underestimation of the nondimensional vertical temperature gradient in the Monin–Obukhov stability function was responsible for the overestimated CH, where the stability functions deviate significantly from expected values from observations for the stable atmospheric boundary layer. Our study highlights the need to improve the flux–profile parameterizations under stable conditions over complex terrain by including impacts due to mountainous terrain, such as surface radiative flux divergence and the diurnal mountain wind system.

Improved Low-Cloud Simulation from the Community Atmosphere Model with an Advanced Third-Order Turbulence Closure

Abstract In this study, a simplified intermediately prognostic higher-order turbulence closure (IPHOC) is implemented in the Community Atmosphere Model, version 5 (CAM5), to provide a consistent treatment of subgrid-scale cloud processes, except for deep convection. The planetary boundary layer (PBL) height is prognosticated to better resolve the discontinuity of temperature and moisture above the PBL top. Single-column model tests show that fluxes of liquid water potential temperature and total water, cloud fraction, and liquid water content are improved with this approach. The simplified IPHOC package replaces the boundary layer dry and moist turbulence parameterizations, the shallow convection parameterization, and the liquid-phase part of the cloud macrophysics parameterization in CAM5. CAM5-IPHOC improves the simulation of the low-level clouds off the west coasts of continents and the storm track region in the Southern Hemisphere (SH). The transition from stratocumulus to cumulus clouds is more gradual. There are also improvements on the cloud radiative forcing, especially shortwave, in the subsidence regime. The improvements in the relationships among low cloud amount, surface relative humidity, lower tropospheric stability, and PBL depth are seen in some stratocumulus regions. CAM5-IPHOC, however, produces weaker precipitation at the South Pacific convergence zone than CAM5 because of less energy flux into the SH atmosphere. The more downward surface shortwave radiative cooling and the less top-of-the-atmosphere longwave cloud radiative heating in the SH relative to the Northern Hemisphere explains the anomalous cooling and the lesser energy flux into the SH, which is related to the underestimate of extratropical middle/high clouds in the SH.

Variational Estimation of Wave-Affected Parameters in a Two-Equation Turbulence Model

AbstractA variational method is used to estimate wave-affected parameters in a two-equation turbulence model with assimilation of temperature data into an ocean boundary layer model. Enhancement of turbulent kinetic energy dissipation due to breaking waves is considered. The Mellor–Yamada level 2.5 turbulence closure scheme (MY2.5) with the two uncertain wave-affected parameters (wave energy factor α and Charnock coefficient β) is selected as the two-equation turbulence model for this study. Two types of experiments are conducted. First, within an identical synthetic experiment framework, the upper-layer temperature “observations” in summer generated by a “truth” model are assimilated into a biased simulation model to investigate if (α, β) can be successfully estimated using the variational method. Second, real temperature profiles from Ocean Weather Station Papa are assimilated into the biased simulation model to obtain the optimal wave-affected parameters. With the optimally estimated parameters, the upper-layer temperature can be well predicted. Furthermore, the horizontal distribution of the wave-affected parameters employed in a high-order turbulence closure scheme can be estimated optimally by using the four-dimensional variational method that assimilates the upper-layer available temperature data into an ocean general circulation model.

Coupling the high-complexity land surface model ACASA to the mesoscale model WRF

Abstract. In this study, the Weather Research and Forecasting (WRF) model is coupled with the Advanced Canopy–Atmosphere–Soil Algorithm (ACASA), a high-complexity land surface model. Although WRF is a state-of-the-art regional atmospheric model with high spatial and temporal resolutions, the land surface schemes available in WRF, such as the popular NOAH model, are simple and lack the capability of representing the canopy structure. In contrast, ACASA is a complex multilayer land surface model with interactive canopy physiology and high-order turbulence closure that allows for an accurate representation of heat, momentum, water, and carbon dioxide fluxes between the land surface and the atmosphere. It allows for microenvironmental variables such as surface air temperature, wind speed, humidity, and carbon dioxide concentration to vary vertically within and above the canopy. Surface meteorological conditions, including air temperature, dew point temperature, and relative humidity, simulated by WRF-ACASA and WRF-NOAH are compared and evaluated with observations from over 700 meteorological stations in California. Results show that the increase in complexity in the WRF-ACASA model not only maintains model accuracy but also properly accounts for the dominant biological and physical processes describing ecosystem–atmosphere interactions that are scientifically valuable. The different complexities of physical and physiological processes in the WRF-ACASA and WRF-NOAH models also highlight the impact of different land surface models on atmospheric and surface conditions.

Higher-Order Turbulence Closure and Its Impact on Climate Simulations in the Community Atmosphere Model

This paper describes climate simulations of the Community Atmosphere Model, version 5 (CAM5), coupled with a higher-order turbulence closure known as Cloud Layers Unified by Binormals (CLUBB). CLUBB is a unified parameterization of the planetary boundary layer (PBL) and shallow convection that is centered around a trivariate probability density function (PDF) and replaces the conventional PBL, shallow convection, and cloud macrophysics schemes in CAM5. CAM–CLUBB improves many aspects of the base state climate compared to CAM5. Chief among them is the transition of stratocumulus to trade wind cumulus regions in the subtropical oceans. In these regions, CAM–CLUBB provides a much more gradual transition that is in better agreement with observational analysis compared to CAM5, which is too abrupt. The improvement seen in CAM–CLUBB can be largely attributed to the gradual evolution of the simulated turbulence, which is in part a result of the unified nature of the parameterization, and to the general improved representation of shallow cumulus clouds compared to CAM5. In addition, there are large differences in the representation and structure of marine boundary layer clouds between CAM–CLUBB and CAM5. CAM–CLUBB is also shown to be more robust, in terms of boundary layer clouds, to changes in vertical resolution for global simulations in a preliminary test.

Unified parameterization of the planetary boundary layer and shallow convection with a higher-order turbulence closure in the Community Atmosphere Model: single-column experiments

Abstract. This paper describes the coupling of the Community Atmosphere Model (CAM) version 5 with a unified multi-variate probability density function (PDF) parameterization, Cloud Layers Unified by Binormals (CLUBB). CLUBB replaces the planetary boundary layer (PBL), shallow convection, and cloud macrophysics schemes in CAM5 with a higher-order turbulence closure based on an assumed PDF. Comparisons of single-column versions of CAM5 and CAM-CLUBB are provided in this paper for several boundary layer regimes. As compared to large eddy simulations (LESs), CAM-CLUBB and CAM5 simulate marine stratocumulus regimes with similar accuracy. For shallow convective regimes, CAM-CLUBB improves the representation of cloud cover and liquid water path (LWP). In addition, for shallow convection CAM-CLUBB offers better fidelity for subgrid-scale vertical velocity, which is an important input for aerosol activation. Finally, CAM-CLUBB results are more robust to changes in vertical and temporal resolution when compared to CAM5.

Numerical study on the three-dimensional characteristics of the tidal current around harbor entrance

A three-dimensional (3-D) numerical model is established to investigate the tidal current motion characteristics around the harbor entrance. Computational approaches consist of the finite difference method, time-splitting technique, C-grid, and a high-order turbulence closure model. The Ocean General Circulation Model (OGCM) inundation scheme is incorporated in the model. An engineering application, Lianyungang Harbor, China, is used as a real-life case. The model is validated by showing that the simulated results agree well with the field data. In the approach channel, the tidal current shows a deflection effect especially in the bottom layer. As for Lianyungang Harbor, a jet flow forms at the tip of the northern breakwater, which drives a circulation flow at the inner side of the harbor entrance. In the vertical, the flow is significantly stratified at the entrance section within the channel, where the flow direction varies between layers and the speed distribution is quite uniform. These 3-D features become less evident to the offshore. Additionally, the sensitivity analysis shows that the flow deflection is related to the channel depth. Simulation results also imply that the siltation rate at the entrance section of the approach channel tends to be more uniform by the 3-D flow motion feature, and could be more significant with the increase of the channel depth.

A PDF-Based Microphysics Parameterization for Simulation of Drizzling Boundary Layer Clouds

Abstract Formulating the contribution of subgrid-scale (SGS) variability to microphysical processes in boundary layer and deep convective cloud parameterizations is a challenging task because of the complexity of microphysical processes and the lack of subgrid-scale information. In this study, a warm-rain microphysics parameterization that is based on a joint double-Gaussian distribution of vertical velocity, liquid water potential temperature, total water mixing ratio, and perturbation of rainwater mixing ratio is developed to simulate drizzling boundary layer clouds with a single column model (SCM). The probability distribution function (PDF) is assumed, but its parameters evolve according to equations that invoke higher-order turbulence closure. These parameters are determined from the first-, second-, and third-order moments and are then used to derive analytical expressions for autoconversion, collection, and evaporation rates. The analytical expressions show that correlation between rainwater and liquid water mixing ratios of the Gaussians enhances the collection rate whereas that between saturation deficit and rainwater mixing ratios of the Gaussians enhances the evaporation rate. Cases of drizzling shallow cumulus and stratocumulus are simulated with large-eddy simulation (LES) and SCM runs (SCM-CNTL and SCM-M): LES explicitly resolves SGS variability, SCM-CNTL parameterizes SGS variability with the PDF-based scheme, but SCM-M uses the grid-mean profiles to calculate the conversion rates of microphysical processes. SCM-CNTL can well reproduce the autoconversion, collection, and evaporation rates from LES. Comparisons between the two SCM experiments showed improvements in mean profiles of potential temperature, total water mixing ratio, liquid water, and cloud amount in the simulations considering SGS variability. A 3-week integration using the PDF-based microphysics scheme indicates that the scheme is stable for long-term simulations.

Exploring atmospheric boundary layer characteristics in a severe SO<sub>2</sub> episode in the north-eastern Adriatic

Abstract. Stable atmospheric conditions are often connected with the occurrence of high pollution episodes especially in urban or industrial areas. In this work we investigate a severe SO2 episode observed on 3–5 February 2002 in a coastal industrial town of Rijeka, Croatia, where very high daily mean concentrations (up to 353.5 μg m−3) were measured. The episode occurred under high air pressure conditions, which were accompanied with a fog and low wind speeds. Three air quality models (50-km EMEP model, 10-km EMEP4HR model and 1-km CAMx model) were used to simulate SO2 concentrations fields and to evaluate the relative contribution of distant and local pollution sources to observed concentrations. Results suggest that the episode was caused predominately by local sources. Furthermore, using three-dimensional, higher-order turbulence closure mesoscale meteorological model (WRF), the wind regimes and thermo-dynamical structure of the lower troposphere above the greater Rijeka area (GRA) were examined in detail. Modelled atmospheric fields suggest several factors whose simultaneous acting was responsible for elevated SO2 concentrations. Established small scale wind directions supported the transport of air from nearby industrial areas with major pollution sources towards Rijeka. This transport was associated with strong, ground-based temperature inversion and correspondingly, very low mixing layer (at most up to about 140 m). Additionally, the surface winds in Rijeka were light or almost calm thus, preventing ventilation of polluted air. Finally, a vertical circulation cell formed between the mainland and a nearby island, supported the air subsidence and the increase of static stability.

Elucidating Model Inadequacies in a Cloud Parameterization by Use of an Ensemble-Based Calibration Framework

AbstractEvery cloud parameterization contains structural model errors. The source of these errors is difficult to pinpoint because cloud parameterizations contain nonlinearities and feedbacks. To elucidate these model inadequacies, this paper uses a general-purpose ensemble parameter estimation technique. In principle, the technique is applicable to any parameterization that contains a number of adjustable coefficients. It optimizes or calibrates parameter values by attempting to match predicted fields to reference datasets. Rather than striving to find the single best set of parameter values, the output is instead an ensemble of parameter sets. This ensemble provides a wealth of information. In particular, it can help uncover model deficiencies and structural errors that might not otherwise be easily revealed. The calibration technique is applied to an existing single-column model (SCM) that parameterizes boundary layer clouds. The SCM is a higher-order turbulence closure model. It is closed using a multivariate probability density function (PDF) that represents subgrid-scale variability. Reference datasets are provided by large-eddy simulations (LES) of a variety of cloudy boundary layers. The calibration technique locates some model errors in the SCM. As a result, empirical modifications are suggested. These modifications are evaluated with independent datasets and found to lead to an overall improvement in the SCM’s performance.

Combining Non-local Scalings with a TKE Closure for Mixing in Boundary-layer Clouds

A new approach to the parametrization of the cumulus-capped boundary layer is described. It combines a traditional higher-order turbulence closure, appropriate for boundary layers where the skewness of thermodynamic variable probability distributions is low (typically stratocumulus-capped), with non-local scaled similarity functions. These are introduced in order to represent explicitly that part of the distribution arising from skewed cumulus elements and the scalings are found to work very well against equilibrium shallow cumulus large-eddy simulations. Results from a wide range of single column model simulations, from stratocumulus to shallow cumulus to cumulus rising into stratocumulus, are presented that demonstrate the validity of the approach as a means of parametrizing the cloudy boundary layer. Sensitivity tests show that enhancement of the turbulence length scales and the buoyancy production of TKE are especially important.

Three-dimensional modelling of estuarine turbidity maxima in a tidal estuary

A new numerical model for simulating estuarine dynamics is introduced here. This model, called General Estuarine Transport Model (GETM), has been specifically designed for reproducing baroclinic, bathymetry-guided flows where the tidal range may exceed the mean water depth in large parts of the domain such that drying and flooding processes are relevant. Several physical and numerical features of the model support exact and stable results for such domains. For the physics, high-order turbulence closure schemes guarantee proper reproduction of vertical exchange processes. Among the specific numerical features, generalised vertical coordinates, orthogonal curvilinear horizontal coordinates, high-order TVD advection schemes and stable drying and flooding algorithms have been implemented into GETM. The model is applied here to simulate the dynamics of estuarine turbidity maxima (ETMs), a complex feature present in most tidal estuaries. First, idealised simulations for a two-dimensional domain in the xz space will be shown to reproduce the basic generation mechanisms for ETMs. Then, a realistic three-dimensional simulation of the Elbe estuary in Northern Germany will be carried out. It is demonstrated that for a given forcing situation the model reproduces a stable ETM at the correct location.

Continuous four-dimensional source attribution for the Berlin area during two days in July 1994. Part I: the new Euler–Lagrange-model system LaMM5

The multiple nested three-dimensional mesoscale Eulerian grid point model MM5 was directly coupled with a Lagrangian particle trajectory model in order to perform a four-dimensional source attribution for the area of Berlin based on the import probability density (IPD) distribution of the according receptor box. Within the resulting model system LaMM5 introduced here, the IPD distributions are not based on backward trajectories, which lack the recognition of the turbulent environment, but on the forward integration of a huge amount (order of Million per day) of particle releases according to an emission scenario which is approximately continuous in space and time. Hence the receptor import record yields an accordingly continuous IPD distribution. Much attention has been paid on spatial and temporal resolution at the interface between both model parts (online-coupling) and the interface itself has been extended by the turbulent quantities resulting from the higher-order turbulence closure of the Eulerian model part. LaMM5 is applied on an episode with high photochemical activity across Berlin at two consecutive days (25th and 26th of July in 1994) with varying meteorological conditions leading to an accordingly different source attribution. The main results are: • The decay of the Berlin IPD with increasing source-receptor distance and time appears in an exponential manner if only sources out of a constant level ( z=25 m) are regarded. • Heterogeneous wind fields in time and space enhance the contributions (emissions) of nearby sources to the total import of the receptor in contrast to stationary wind fields which increase the scope of the IPD distribution in upstream direction. There are further results from several additional sensitivity studies presented in a companion paper B (Part II).

An Investigation of the Boundary Layer Dynamics of Sardinia Island under Sea-Breeze Conditions

A numerical mesoscale model is used to study the wind field and the boundary layer structure of the island of Sardinia during typical summer conditions. The numerical model is three-dimensional and employs a higherorder turbulence closure scheme. The model simulations were performed for summer conditions characterized by weak synoptic forcing from the northwest and clear skies. These conditions favor the development of thermal circulations, the most significant of which are the sea-breeze systems. The nighttime wind patterns generally are dominated by topography, which leads to the development of strong drainage flow. On the other hand, as revealed in the simulated wind field, at midday the wind has an onshore component at virtually every coastline. The well-organized sea-breeze systems interact to produce convergence zones. Another interesting feature is the development of a cyclonic eddy pattern during late-afternoon hours. The model results are compared with observations taken at a network of near-surface wind stations and rawinsonde profiles from the Cagliari airport. Available observations agreed relatively well with model predictions.

An Improved Method for Evaluating the non-Linear Terms in the Nlmsfd Model

The non-linear terms in the non-linear mixed spectral finite difference (NLMSFD) model are evaluated by a simple yet more efficient method. The explicit expressions for the non-linear terms, which can number O(100), are no longer required. The improved algorithm results in source code of smaller size, faster execution time, and provides a more reliable way to evaluate the non-linear terms. Results from numerical experiments have shown that this approach is particularly useful in higher-order turbulence closure schemes.

Using the SKAGEX dataset for evaluation of ocean model skills

Numerical ocean models are now being applied in numerous oceanographic studies. However, the qualities of the model results are often uncertain and there is a great need for standards and procedures for evaluation of the skills of numerical general circulation models. In this paper measurements from repeated hydrographical sections across Skagerrak taken in 1990, the SKAGEX dataset, are used to evaluate the skills of two σ-coordinate ocean models and to study the sensitivity of these models to model parameters. A methodology for quantification of model skills based on observations from repeated hydrographical sections in general is suggested. Area averages of absolute differences are for Skagerrak completely dominated by the discrepancies in the upper few meters of the ocean and may not be used to assess models' abilities to reproduce the fields in the larger and deeper part of the ocean. Therefore, discrepancies between average values in time from the observed fields and time averaged values from model outputs are related to the natural variability of the fields. The numbers produced with the suggested measure are relative numbers that will be specific for each section and for each series of observation. Ideally we would therefore like to see the measures computed for a number of sections for various models and choices of model parameters in order to assess model skills. The value of the SKAGEX dataset as a tool for model improvements is demonstrated. Evidence to support the importance of applying non-oscillatory, gradient preserving advection schemes in areas with sharp density fronts is given. The method is used to identify that the forcing/initial values/boundary values for the temperature field are inferior to the corresponding values for the salinity field. With the present coarse resolution, 11 layers in the vertical, it is shown that it is far from obvious that the quality of the model results improve when replacing simple Richardson number formulations for vertical mixing processes with higher order turbulence closure in the Skagerrak area.

Idealized Simulations of Atmospheric Coastal Flow along the Central Coast of California

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 synopticscale 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 nearsurface 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.

Flow dynamics in Athens area under moderate large-scale winds

The purpose of the present paper is to study the wind regime in Greater Athens Area (GAA) during a day when the sea breeze was opposing a moderately strong synoptic-scale flow. The approach adopted in the present study is to analyze data from a field campaign and from numerical model simulations. The field data used in the analysis are from the MEDditerranean CAmpaign of PHOtochemical Tracers-TRAnsport and Chemical Evolution (MEDCAPHOT-TRACE) which took place in the GAA during the period 20 August–20 September 1994. The numerical simulations were performed using a three-dimensional, higher-order turbulence closure model. All model equations are transformed to a terrain-following coordinate system, which enables studies of flow over complex terrain. The results from the simulations compare reasonably well with the field data. The analysis of the field data and the numerical simulation results reveals that, under the present conditions, the temporal and spatial variations of the wind field are not as pronounced as in days with weak synoptic background flow. A sea breeze circulation is developing over the sea and reaches the shoreline but is counterbalanced by the opposing synoptic flow and it does not penetrate deeply over the land. The characteristics of the sea breeze systems developing in GAA under weak large-scale ambient winds are presented in a companion paper.

Anatomy of the sea-breeze circulation in Athens area under weak large-scale ambient winds

The city of Athens is located near an irregular coastline and is surrounded by moderately high mountains. The complex structure of the sea breezes developing in Greater Athens Area (GAA) during a day with weak large-scale ambient winds is examined using consolidated field data and results from numerical model simulations. The data used in the analysis are from the MEDditerranean CAmpaign of PHOtochemical Tracers-TRAnsport and Chemical Evolution (MEDCAPHOT-TRACE) which took place in the GAA during the period 20 August–20 September 1994. The numerical simulations were performed using a three-dimensional, higher-order turbulence closure model. All model equations are transformed to a terrain-influenced coordinate system, which enables studies of flow over complex terrain. A full diurnal cycle has been simulated for the typical sea-breeze day of 15 September 1994. Major characteristics of the sea-breeze circulation including surface wind pattern, inland penetration of the sea-breeze front and vertical extension and intensity as well as diurnal variation of the vertical eddy thermal diffusivity are identified and discussed. Model predictions show a good agreement with the comprehensive data set.