Guy Schayes

Development of a 3-dimensional Meso-gamma Primitive Equation Model - Katabatic Winds Simulation in the Area of Terra-nova Bay, Antarctica

The spatial evolution of Antarctic katabatic winds in the area of Terra Nova Bay is examined using the three-dimensional version of the Universite Catholique de Louvain-Modele Atmospherique Regional (UCL-MAR) mesoscale primitive equation models. The ability of the model to replicate classical linear mountain wave simulations is verified. Then, three-dimensional experiments are performed for the terrain configuration of Terra Nova (Ross Sea coastal zone) using different horizontal resolutions (5, 10, and 20 km). The model converges for resolutions lower than 10 km. Results are in qualitative agreement with available observations and previous modeling work. Strong katabatic winds are simulated with a jet over Terra Nova Bay. The model seems able to initiate the mesocyclonic activity in the Ross Sea due to the katabatic circulation.

Dynamical aspects of katabatic wind evolution in the Antarctic coastal zone

The spatial evolution of katabatic winds along idealized slopes representative of Antarctic terrain is examined using a hydrostatic, two-dimensional primitive equation model with high resolution. A downslope momentum-forces analysis is made of simulations in which katabatic flow reaches steady state, with emphasis on physical mechanisms in the coastal zone. The importance of the reversal of the pressure gradient force in the coastal zone, causing the sudden decay of katabatic winds, is discussed.

Formulation of an Index for Sea Breezes in Opposing Winds

A scaling analysis is conducted to explore the dependence of sea-breeze speed and inland occurrence in the presence of opposing winds on a set of dynamical parameters. The overall aim of the analysis is to develop an index for sea-breeze occurrence in the face of opposing winds, similar to the Biggs and Graves lake-breeze index. Most studies separate sea-breeze speed and sea-breeze inland occurrence or, at best, link the two in linear analyses. This work analyzes the output of a nonlinear numerical mesoscale model ( in idealized simulations) using scaling methods commonly applied in observational studies. It is found that the scaled sea-breeze speed, in response to increasing magnitude of opposing wind, shows two distinct phases: a phase of increasing speed while the sea breeze progresses inland and a phase of sharply decreasing speed when the sea breeze is no longer detected inland. The analysis also allows the development of an index for sea-breeze inland occurrence. This index is an improvement over existing analyses through the use of nonlinear scaling and the use of surface heat flux as opposed to simpler land-sea temperature contrasts.

Sudden Cessation of Katabatic Winds in Adélie Land, Antarctica

The evolution of summer katabatic wind events over the steep slopes of Adelie Land is examined, with emphasis on the sudden cessation of these events. Different idealized large-scale forcings are considered, including a situation that comes very close to one observed during the IAGO (Interaction Atmosphere Glace Ocean) campaign, held in the region in November-December 1985. The hydrostatic meso-gamma-scale atmospheric model MAR (Modele Atmospherique Regional) is used to assess the sensitivity of the simulated cessation process to a prescribed large-scale forcing.

Atmospheric Turbulence Decay During the Solar Total Eclipse of 11 August 1999

The studies of turbulence decay were based in the past on measurements carried out in neutrally stratified wind tunnels and, more recently, on large-eddy simulation runs. Here the atmospheric turbulence decay process during the solar total eclipse of 11 August 1999 is examined. Thus a rapid transition from convective boundary-layer turbulence to that of a neutral or slightly stable one is considered. A u-v-w propeller anemometer and a fast response temperature sensor located in northern France on top of a 9-m mast recorded the turbulence observations. The measurements, in terms of turbulent kinetic energy decay with time, were found to be in good agreement with those prescribed by a theoretical model of turbulence decay recently proposed. In particular, it was found that the exponent of the power law describing the decay process has the value -2.

Marine air intrusion into the Adelie Land sector of East Antarctica: A study using the regional climate model (MAR)

[1] Marine air intrusion and subsequent cloud formation plays a dominant role in the energy budget and mass balance of the Antarctic. However, the intrusion is very difficult to understand using the ground-based measurements alone. In this paper we present simulations of marine air intrusion into the Adelie Land, East Antarctica, using the Modele Atmospherique Regional (MAR), for July 1994 and January 1995. The model is nested into the European Centre for Medium-Range Weather Forecasts (ECMWF) analyses. The simulations show a strong influence of large-scale disturbances, over the ocean, which helped in the penetration of marine air into the interior and the formation of clouds. Each marine air intrusion episode resulted in cloud formation in July 1994. Blocking anticyclones have also been found to be responsible for much of the moisture transport far into the interior elevated locations. MAR simulations, as well as ECMWF analyses, show influence of cyclones in strengthening and prolonging the surface layer flow. The study also indicated that the influence of depressions on surface winds is pronounced during the period when the depression is approaching the Adelie Land coast.

Impact of Turbulence Closures on Diurnal Temperature evolution for Clear Sky Situations Over Belgium

The aim of this study is to quantify the impact of turbulence closure on the simulation of surface air temperature at screen height (1.5 m) over Belgium. The mesoscale model MAR (Modèle Atmosphérique Régional), developed at the Université catholique de Louvain, is used to examine one-dimensional situations. A new second-order closure (level 2.5) is implemented containing prognostic equations for all three velocity variances, and diagnostic or prognostic formulations for the dissipation. This closure is compared with first and one-and-a-half order closures. Idealized nearly-neutral and convective cases underline the differences between first and second-order closures, and between diagnostic and prognostic equations for the dissipation. The one-and-a-half and second-order closures give satisfying results, but the first-order closure produces generally less appropriate vertical diffusion. Observed clear sky and weak horizontal advection situations have shown the sensitivity of 24 h temperature evolution to the choice of the turbulent closure.

Sensitivity of Air Trajectories in Sea Breeze Environment to Turbulence and Surface Formulations

Using models to represent the behaviour of air and its pollutants necessitates several approximations. Two important ones are the turbulence closure and the representation of the ground characteristics. In most meso-scale models, the effect of turbulence is represented through eddy exchange coefficients determined by mixing lengths. In the experiments presented here, the sensitivity to mixing length formulation is analysed with a meso-scale model using a 1.5 order turbulent closure scheme and two associated mixing lengths: the length lk for the turbulent diffusion coefficient and the length le for the dissipation term. The two turbulence formulations we compare are those of Therry- Lacarrere (1983) and of Bougeault-Lacarrere (1989).

Intercomparison on the Flow Field over the Attic Peninsula with Two Models

The circulation of air masses for the control of air pollution is particularly difficult to estimate in areas of complex terrain, due to the presence of mountains and hills or to the effect of large water basins. In the Athens area, the local circulation is mainly driven by the sea-land breezes, which are strongly influenced by a mountain ridge culminating at 1460m (Parnitha mountain fairly close to the sea-shore). Actual terrain slopes can be locally very large, which renders the simulation particularly challenging for mesoscale models. In this paper two different models participating in the APSIS intercomparison exercice are presented and initialized in order to describe the typical summer sea-breeze in the region of Athens.

The SEVEX Project, an Integrated Dispersion and Contamination Calculation for Complex Terrain and Accidental Releases

Following the EEC Seveso directive of 1982, industries using dangerous materials in large quantities and Public authorities are obliged to establish a distance of possible contamination occuring during hypothetical major accident. Three Belgian universities were charged of the practical study.