No abstract

Abstract. The Meso-NH Atmospheric Simulation System is a joint e ort of the Centre National de Recherches Me te orologiques and Laboratoire d'Ae rologie. It comprises several elements; a numerical model able to simulate the atmospheric motions, ranging from the large meso-alpha scale down to the micro-scale, with a comprehensive physical package, a¯exible ®le manager, an ensemble of facilities to prepare initial states, either idealized or interpolated from meteorological analyses or forecasts, a¯exible post-processing and graphical facility to visualize the results, and an ensemble of interactive procedures to control these functions. Some of the distinctive features of this ensemble are the following: the model is currently based on the Lipps and Hemler form of the anelastic system, but may evolve towards a more accurate form of the equations system. In the future, it will allow for simultaneous simulation of several scales of motion, by the so-called``interactive grid-nesting technique''. It allows for the in-line computation and accumulation of various terms of the budget of several quantities. It allows for the transport and di usion of passive scalars, to be coupled with a chemical module. It uses the relatively new Fortran 90 compiler. It is tailored to be easily implemented on any UNIX machine. Meso-NH is designed as a research tool for small and meso-scale atmospheric processes. It is freely accessible to the research community, and we have tried to make it as``user-friendly'' as possible, and as general as possible, although these two goals sometimes appear contradictory. The present paper presents a general description of the adiabatic formulation and some of the basic validation simulations. A list of the currently available physical parametrizations and initialization methods is also given. A more precise description of these aspects will be provided in a further paper.

CRA Colorado Research Associates (United States) CSIM Centro Sperimentale per l'ldrologia e la Meteorologia (Italy) CVB Constant volume balloons DIAL Differential absorption lidar DLR Deutsches Zentrum fur Luft und Raumfahrt (Germany) DOW Doppler-on-Wheels Radar (NCAR

Leading NWP centers have agreed to create a database of their operational ensemble forecasts and open access to researchers to accelerate the development of probabilistic forecasting of high-impact weather.Objectives and cOncept. During the past decade, ensemble forecasting has undergone rapid development in all parts of the world. Ensembles are now generally accepted as a reliable approach to forecast confidence estimation, especially in the case of high-impact weather. Their application to quantitative probabilistic forecasting is also increasing rapidly. In addition, there has been a strong interest in the development of multimodel ensembles, whether based on a set of single (deterministic) forecasts from different systems, or on a set of ensemble forecasts from different systems (the so-called superensemble). The hope is that multimodel ensembles will provide an affordable approach to the classical goal of increasing the hit rate for prediction of high-impact weather without increasing the false-alarm rate. This is being taken further within The Observing System Research and Predictability Experiment (THORPEX), a major component of the World Weather Research Programme (WWRP) under the World Meteorological Organization (WMO). A key goal of THORPEX is to accelerate improvements in

The Meso-NH Atmospheric Simulation System is a joint eort of the Centre National de Recherches Me te orologiques and Laboratoire d'Ae rologie. It comprises several elements; a numerical model able to simulate the atmospheric motions, ranging from the large meso-alpha scale down to the micro-scale, with a comprehensive physical package, a¯exible ®le manager, an ensemble of facilities to prepare initial states, either idealized or interpolated from meteorological analyses or forecasts, a¯exible post-processing and graphical facility to visualize the results, and an ensemble of interactive procedures to control these functions. Some of the distinctive features of this ensemble are the following: the model is currently based on the Lipps and Hemler form of the anelastic system, but may evolve towards a more accurate form of the equations system. In the future, it will allow for simultaneous simulation of several scales of motion, by the so-called``interactive grid-nesting technique''. It allows for the in-line computation and accumulation of various terms of the budget of several quantities. It allows for the transport and diusion of passive scalars, to be coupled with a chemical module. It uses the relatively new Fortran 90 compiler. It is tailored to be easily implemented on any UNIX machine. Meso-NH is designed as a research tool for small and meso-scale atmospheric processes. It is freely accessible to the research community, and we have tried to make it as``user-friendly'' as possible, and as general as possible, although these two goals sometimes appear contradictory. The present paper presents a general description of the adiabatic formulation and some of the basic validation simulations. A list of the currently available physical parametrizations and initialization methods is also given. A more precise description of these aspects will be provided in a further paper.

No abstract

T he ChAllenge. Climate and weather forecasting applications share a common ancestry and build on the same physical principles. Nevertheless, climate research and numerical weather prediction (NWP) are commonly seen as different disciplines. The emerging concept of "seamless prediction" forges weather forecasting and climate change studies into a single framework. At the same

A numerical, hydrostatic model is used to investigate the form and magnitude of the pressure drag created by 3D elliptical mountains of various heights (h) and aspect ratios (R) in flows characterized by uniform upstream velocity (U) and stability (N). Three series of simulations, corresponding to increasing degrees of realism, are performed: (i) without rotation and surface friction; (ii) with rotation, but no surface friction; (iii) with rotation and surface friction. For the simulations with rotation, the Coriolis parameter has a typical midlatitude value and the upstream flow is geostrophically balanced. The surface friction is introduced by the use of a typical roughness length.For low values of the nondimensional height (Nh/U), the pressure drag is reduced by the effect of rotation, in agreement with well-known results of linear theory. This seems to be valid until Nh/U ϳ 1.4, that is, in the high drag regime. On the other hand, for large values of Nh/U, that is, in the blocked flow regime, rotation has the opposite effect and increases the drag. The authors propose a simple interpretation of these results: that geostrophic adjustment acts to first order as a relaxation toward the upstream velocity. For low Nh/U, the acceleration above the mountain is a dominating feature of the flow and here the flow is slowed by the presence of rotation. For high Nh/U, when upstream blocking is dominant, the flow is slowed by the mountain and therefore accelerated by rotation. For values of Nh/U ϳ 1.4, the rotation is sufficient to force a transition from the blocked state to the unblocked state. The influence of rotation may therefore extend the range of usefulness of linear theory.Surface friction dramatically suppresses wave breaking at all values of Nh/U. The induced effect on the drag is negligible for Nh/U Ͼ 3, but there is a strong reduction at smaller values of Nh/U. In fact, the high-drag regime is nearly suppressed.The overall combined effect of rotation and surface friction is to constrain the drag (and to some extent, the flow patterns) to values remarkably close to the linear prediction. This sheds some light on recent, but as yet unexplained, results from the PYREX field experiment. The authors conclude this paper by running a real case drawn from this experiment, which reveals a behavior consistent with the idealized scenarios.