Nonhydrostatic Mesoscale Model Research Articles

Local simulations of land-sea breeze cycles in Athens based on large-scale operational analyses

Two different episodes of land-sea breeze evolution in the Athens area are numerically investigated using a non-hydrostatic mesoscale model, with E- ϵ turbulence closure, in the framework of the APSIS project. The initialization and the time evolving boundary conditions are derived from the ECMWF operational analyses. The simulated surface winds have been compared with available surface measurements. On these two situations, the use of operational analyses imposed on the boundary of a local model provides more realistic simulations than those based only on local soundings. The results obtained are closer to more sophisticated nested simulations except in a relaxation zone near the boundaries. The urbulence predicted by the E- ϵ turbulence model is very weak in early morning and increases to values representative of moderately convective conditions during the day in correspondence with the observed poor dispersion of pollutants.

Influence of topography and biogenic volatile organic compounds emission in the state of Baden‐Württemberg on ozone concentrations during episodes of high air temperatures

A nonhydrostatic mesoscale model which is coupled with a transport and diffusion model and the gas phase mechanism RADM2 is used to study the influence of biogenic volatile organic compounds (VOC) emission on the ozone concentration during episodes of high air temperatures in the state of Baden‐Württemberg, Germany. All model parts, including the determination of the biogenic VOC emissions, are run in a coupled mode. The results of the model simulations are compared with observations for a summer smog episode which occurred in August 1990. Simulations which are carried out without biogenic VOC emissions show a maximum difference in the ozone concentration of 18 ppb, while the maximum ozone values are of the order of 100 ppb. This shows that the biogenic VOC emissions play an important role when high temperatures are present in Baden‐Württemberg. Simulations which are carried out with homogeneous terrain height show differences in the biogenic emissions caused by changes in the temperature distribution. Large differences are also found for the concentration distributions. The differences in the results which are obtained with and without biogenic VOC emissions are decreasing; the maximum difference is about 10 ppb. This demonstrates the necessity of a sufficient treatment of topographic effects in the regional scale.

The EUMAC Zooming Model, a tool for local-to-regional air quality studies

The EUMAC Zooming Model (EZM) is a comprehensive model system for simulations of wind flow and air pollutant transport and transformation at the local-to-regional scale. Core models of the EZM are the nonhydrostatic mesoscale model MEMO and the photochemical dispersion model MARS. Manifold air quality studies, primarily for large urban agglomerations, have been already performed with the EZM. In most of these studies the results of the EZM agree fairly well with available observations. An example for a successful study with the EZM is its application to simulate the formation of photochemical smog in the case of stagnant meteorological conditions in Athens. By applying a suitable nesting technique, MEMO reproduces satisfactorily the observed diurnal wind pattern in Athens. The results of MARS elucidate the characteristics of a severe photosmog episode in Athens and are in general very similar to available observations.

A nonhydrostatic model for simulation of airflow over mesoscale bell-shaped ridges

This paper describes a nonhydrostatic and incompressible mesoscale model formulation using a terrain-following coordinate system. A tensor transformation procedure is used to derive a diagnostic equation for the nonhydrostatic pressure field. The model features a simplified second-order turbulence closure scheme. The two-dimensional version of the nonhydrostatic model, as well as the corresponding hydrostatic model, are applied to simulate stably stratified airflow over mesoscale bell-shaped mountain ridges. The results show that the nonhydrostatic model is capable of simulating nonhydrostatic dynamics of mesoscale lee wave systems such as the trapped wave phenomenon.

Simulations of the wind field in Athens with the nonhydrostatic mesoscale model MEMO

MEMO is a fully vectorized nonhydrostatic mesoscale model using terrain-following coordinates. The numerical solution is based on second-order discretization applied on a staggered grid which is allowed to be non-equidistant in all directions. Special care is taken that conservative propeprties are preserved within the discrete model equations. The discrete pressure equation is solved with a direct elliptic solver in conjuction with a generalized conjugate gradient method. Advective terms are treated with an explicit, monotonicity-preserving discretization scheme with only small implicit diffusion. Turbulent diffusion is described using an one-equation turbulence model, while at roughness height similarity theory is applied. An efficient scheme is applied to calculate radiative transfer. The algebraic surface heat budget equation and an one dimensional heat conduction equation are solved to obtain the surface temperature over land and the soil temperature. In the frame of the APSIS exercise A the model MEMO was used to simulate the mesoscale flow in the Athens basin on May 25, 1990. Being in satisfactory agreement with observations, the results indicate that weak pressure gradients accompanied by warm advection aloft may lead to stagnant conditions and thus to severe air pollution episodes in Athens.

Numerical Modeling of Orographically Forced Postfrontal Rain

Abstract A nonhydrostatic mesoscale model is used to simulate the dynamics and microphysics of postfrontal flow in the mountainous region of southeastern Australia. The aim of the paper is to determine if it is possible to use 2D models to simulate the characteristics of the liquid water field upstream from Baw Baw Plateau under postfrontal conditions. Results from both 2D and 3D simulations are compared with aircraft and surface observations taken during the Australian Winter Storms Experiment I, conducted during July and August 1988. The observations and both the 2D and 3D simulations show that under postfrontal conditions, the main feature of the flow is a series of standing lee waves downstream from Baw Baw Plateau. The microphysical fields are characterized by a cap cloud over Baw Baw Plateau and a region of high liquid water content extending at least 50 km upstream from the plateau. Convective elements form upstream from the plateau and are subsequently advected to the northeast. As the convective ...

Feasibility of the Direct Method to Solve the Anelastic Pressure Equation in Nonhydrostatic Two-Dimensional Mesoscale Models

Abstract For anelastic nonhydrostatic mesoscale models, the pressure has to be solved from the Poisson partial differential equation. This can be done in various ways. Here the usually neglected direct method is compared to widely used (iterative) relaxation methods. The feasibility of the direct method is limited to the two-dimensional (2D) case. For this case, the direct method appears to compare favorably, provided the number of grid levels is sufficiently small. Furthermore, it is shown that nonhydrostatic calculations can proceed faster than hydrostatic ones in certain cases, in spite of the fact that the amount of computation time per time stop is longer for the former. The reason is that the time step can often be chosen much larger for the nonhydrostatic than for the hydrostatic calculations without violating stability requirements.

Observations and Numerical Model Simulation of a Partialy Trapped Lee Wave over the Welsh Mountains

Abstract A large-amplitude lee-wave event detected in radiosonde ascents during a field experiment in the Welsh mountains is described and wave characteristics are deduced. The synoptic-flow pattern accompanying this wave field is used to initialize the U.K. Meteorological Office's nonhydrostatic mesoscale model (with a 3-km horizontal grid length) in an attempt to simulate the gravity-wave response over the Welsh mountains. After 4 h of integration time the resulting flow held is quasi-study and exhibits a fairly regular wave field with a dominant wavelength at about 22 km and nearly vertical phase lines. Verification is achieved by comparing rate-of-ascent variations with model vertical velocity variations along the trajectory of a radiosonde: the agreement is very good. The vertical structure equation for linear gravity waves is solved using the smoothed wind and temperature fields obtained from the sondes. A “leaky” resonance is identified at the dominant wavelength of the model simulation. Limited co...

A Nonhydrostatic Mesoscale Model Designed to Simulate Scale Interaction

Abstract A three-dimensional nonhydrostatic mesoscale model is presented that is designed to optimally represent the scale-interaction process among inertially balanced and unbalanced modes occurring within convective weather systems. Because scale-interaction simulations are long-term integrations that emphasize the evolution of the three-dimensional kinetic energy spectrum, the model is built to conserve enstrophy, as well as kinetic energy against numerical sources and sinks in three dimensions. A non-Boussinesq and quasi-compressible framework is employed to maintain applicability on meso-α and larger scales as well as for situations of relatively large local density variation. Sample integrations in two and three dimensions are presented that show enstrophy conservation to be effective in improving the prediction of nonlinear evolution as truncation errors act to force anomalous bifurcations from the physical solution.

Multicell stage of the Munich storm of 12 July 1984: a numerical study

The multicell stage of the Munich hailstorm is investigated using a three-dimensional, non-hydrostatic mesoscale model. During this stage, the storm developed in a wind profile that had little directional shear yet an extensive series of two-dimensional numerical experiments could not produce a self-sustaining and realistic convective system. The modelled storm is a multicellular, three-dimensional system featuring right-flank development of new cells. It is constituted of two principal flow branches, namely, an intense updraught that travels at the speed of a mid-level steering current and a weak downdraught. A mid-level relative inflow overtakes the storm from the rear but undergoes unsaturated descent of only about 100 mb, fails to establish the classical downdraught associated with mid-latitude storms, and exits ahead of the storm. Weak, shallow downdraughts originate beneath an inversion and from ahead of the storm. Viewed in a cross section parallel to the direction of storm motion, trajectory analyses show that the cold pool is fed by two types of shallow downdraught, namely a gravity or solitary wave undergoes minimal vertical displacement and an overturning jump that causes substantial evaporative cooling despite its small vertical displacement. A density current circulation fails to form in this plane but one does develop in the perpendicalur section and its interaction with the convergence field and vertical shear causes secondary cell initiation at right angles to that found in multicell squall lines. DOI: 10.1034/j.1600-0870.1992.t01-3-00006.x

A spectral fully compressible nonhydrostatic mesoscale model in hydrostatic sigma coordinates: Formulation and preliminary results

The set of fully compressible nonhydrostatic equations governing a broad spectrum of atmospheric motion was transformed fromz coordinates to sigma coordinates under a hydrostatic base state. The hydrostatic base state may be either time-independent, such as a hydrostatic balance with-out motion or with motion such as a thermal wind balance, or time-dependent such as might be obtained from the result of integrating a hydrostatic model. The transformed set of equations can be used to predict and study all scales of at mospheric phenomena.

Boundary layer structure over an inhomogeneous surface: Simulation with a non-hydrostatic mesoscale model

A three-dimensional, non-hydrostatic mesoscale model is used to study boundary-layer structure over an area characterized by the city of Copenhagen, the Oresund strait, and adjacent coastal farmland. Simulations are compared with data obtained on June 5, 1984 during the Oresund experiment.

Numerical Experiments with the Adjoint of a Nonhydrostatic Mesoscale Model

Abstract For the purpose of assimilation of radar data into a nonhydrostatic mesoscale forecast model, the adjoint method is considered. For the case of dry convection, a set of identical twin experiments shows that it is possible to construct the initial conditions for the temperature only moderately well from observations of one velocity component only. This behavior can be explained with an ill-posedness of the problem in absence of temperature information caused by the discrete representation of gradients. Small pieces of additional thermodynamic information improve the results noticeably. Contamination of the data with random error has little impact on the quality of the retrieved initial state, as long as there are enough data available. Phenomena not resolved by the data are not retrievable by the algorithm. For the case considered, the adjoint method proves to be a robust tool for data assimilation purposes, but it also leaves the question of how to avoid the ill-posedness open.

Prediction of orographic precipitation using cartesian coordinates and a single prognostic equation for the water substance

The three-dimensional compressible non-hydrostatic mesoscale model ADREA, developed at NCSR “Demokritos” for wind field and dispersion predictions in complex terrain, is used to perform two-dimensional simulations of orographically induced precipitation. The model makes use of a simple cartesian grid by allowing blocked areas in the grid volumes and grid faces which are crossed by the irregular topography (the so-called porous-medium concept). Additionally, since we are seeking both simplicity and low computational requirements, only one prognostic equation for the whole of the water substance is implemented. The model equations are based on the theory developed for multicom-ponent/multiphase mixtures and give full account of the effects arising from the relative motion between water particles and moist air. The evaluation of the feasibility of this approach is obtained by comparing model predictions with measurements and analytical results concerning orographic precipitation episodes in the mountain barrier of Sierra Nevada.

A test of a semi-implicit integration technique for a fully compressible non-hydrostatic model

A semi-implicit scheme, in which the time-step is limited only by the advective criterion, is analysed and successfully tested in a fully compressible non-hydrostatic mesoscale model.

Non-hydrostatic model of airflow over irregular topography — Theoretical bases

This article deals with some problems connected with the formulation of a non-hydrostatic mesoscale model of airflow in the atmosphere. Due to an irregular surface a terrain-following coordinate system is used and the equations of the model are transformed into this system. Sound waves are eliminated by the use of the anelastic approximation. The influence of boundaries is minimized by the use of open boundary conditions at the lateral boundaries of the computational domain and of the absorbing layer beneath the upper boundary.

Numerical studies on cold fronts Part I: Gravity flows in a neutral and stratified atmosphere

We have used a two dimensional version of a nonhydrostatic mesoscale model to simulate atmospheric gravity currents for different thermal stratification. The horizontal and vertical grid increments are chosen such, that the major features of a current like head and elevated nose are resolvable.

Channeling and Countercurrent in the Upper Rhine Valley: Numerical Simulations

In the upper Rhine Valley, located in the southwest part of the Federal Republic of Germany, a pronounced channeling of the airflow is observed and occasionally also a countercurrent, although the valley is very flat and very broad (35 km), and its boundary mountains are no more than 500 m above the valley floor. Both channeling and countercurrents are simulated numerically with the nonhydrostatic mesoscale model FITNAH. These simulations are carried out for steady state conditions in a moderately stable stratified atmosphere for five different directions of the geostrophic wind (W, WNW, NW, NNW and N; the orientation of the valley is south to north). The results are shown by maps with streamlines at 30 m above ground by vertical cross sections and by three-dimensional trajectories.

An application of an efficient non-hydrostatic mesoscale model

This paper deals with a non-hydrostatic mesoscale model that achieves full vectorization on computers like the CYBER 205. The model formulation ensures the conservation of all fluxes and takes into account the terrain inhomogeneities by the aid of suitable transformations. The diagnostic equation for the pressure change is solved using a very efficient vectorized elliptic solver. By imposing appropriate boundary conditions no additional precautions at the boundaries are necessary to achieve meaningful results. As an application, the steady-state inviscid flow over a single mountain is simulated.

Numerical Simulation of Organized Convection. Part I: Model Description and Preliminary Comparisons with Squall Line Observations

Abstract A numerical model designed for the simulations of mesoscale flows perturbed by deep convective clouds is discussed. It is based on the time dependent coupling between a three-dimensional nonhydrostatic mesoscale model and a quasi-one-dimensional cloud model. The evolution and motion of individual convective cells are simulated by the cloud model since they cannot be explicitly resolved by the mesoscale model. This implies that in cloudy areas each model simulates the time evolution of the same variable at the same location and at the same time, under the influence of different processes (large-scale processes for the mesoscale model and microphysical processes for the cloud model). The comparison between the cloud and mesoscale rates of change leads to an evaluation of the coupling terms which transmit the cloud influences into the mesoscale model so that mesoscale fields are perturbed accordingly. For example, the nonhydrostatic pressure field reacts to the cloud development by building up an ov...