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Evelyne Richard and Nadine Chaumerliac

Abstract

A detailed comparison is made between the results obtained from two microphysical parameterizations capable of simulating cloud and precipitation processes in a mesoscale model. The behavior of each microphysical scheme is first investigated in the context of a mountain wave simulation. Major differences are found in raindrop sill distributions as well as in the rates associated with various microphysical processes. An assessment of the accuracy of each scheme is then obtained by comparing model predictions with observational data from well-documented orographically enhanced precipitation episodes in South Wales. The parameterization of Berry and Reinhardt does a better job of reproducing the observed dependency of the precipitation enhancement on the low-level windspeed than does Kessler's.

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Marc Georgelin and Evelyne Richard

Abstract

The intensive observing period (IOP) 6 of the Pyrenees Experiment (PYREX) has been simulated with a hydrostatic three-dimensional model. The PYREX IOP 6 was devoted to the observation of a regional wind, the tramontana, which blows in the vicinity of the Pyrenees Mountains at the French–Spanish border. Under northerly synoptic wind conditions, the low-level flow frequently splits around the Pyrenees barrier (400 km long and 3000 m high), with the eastern branch of the flow, favored by Coriolis effect, forming the tramontana.

Model results are consistent with the tramontana climatology and in good agreement with the PYREX observations at all the different stages of the tramontana development: in the blocking zone where the model reproduces the observed reversed flow as well as its disappearance later in the course of the simulation, in the acceleration zone where the model gives accurate wind intensity and direction, at the land–sea transition where the development of an internal boundary layer is well predicted, and above the Mediterranean Sea where the spatial structure of the tramontana is well reproduced.

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Marc Georgelin, Evelyne Richard, and Monique Petididier

Abstract

The intensive observing period (IOP) 4 of the Pyrenees Experiment (PYREX) has been simulated with a hydrostatic three-dimensional model. The PYREX IOP 4 was devoted to the observation of the orographically generated flow above and around the Pyrenees Mountains located at the French–Spanish border. Two simulations, with or without surface thermal forcing, have been performed. Their results are described and compared with the PYREX observations. The influence of thermal forcing is assessed on the main characteristics of the flow, including the upwind flow reversal, the vertically propagating mountain wave, the regional winds created by the flow deviation around the mountain, and the lee vortices. Prominent alterations of the flow are found downwind of the mountain where the diurnal cooling induces the disappearance of the lee vortices and the heating significantly reduces the intensity of the low-level deviated flow.

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Ahmed Elkhalfi, Marc Georgelin, and Evelyne Richard

Abstract

Two-dimensional numerical simulations of mountain waves observed during the Pyrenees Experiment have been performed. Two intensive observing periods (IOP) have been simulated, IOP 3, which lasted less than one day, and IOP 9, which lasted two and one-half days. The time evolution of the large-scale flow was incorporated in the model through time-dependent boundary conditions that were updated using the closet upwind sounding. The numerically simulated mountain waves agree well with the available aircraft observations. Good agreement is also obtained between the simulated and observed vertical momentum flux profiles. In addition, the model-generated cross-mountain pressure drag accurately follows the time evolution of the observed drag. To get such a good agreement between observations and computations, it has been necessary to take into account in the model surface layer the effects of subgrid-scale orographic elements.

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Evelyne Richard, Patrick Mascart, and Everett C. Nickerson

Abstract

Numerical simulations of the 11 January 1972 windstorm in Boulder, Colorado, were carded out using a hydrostatic model with a turbulent kinetic energy parameterization to investigate the role of fictional effects in the development of nonlinear mountain waves. Sensitivity tests to the roughness length specification and to the turbulent mixing and dissipation length formulations show that surface friction delays the onset of the strong surface winds and also prevents the downstream propagation of the zone of maximum windspeed. Shear production within convectively stable regions is the dominant mechanism for the production of the turbulent kinetic energy. Moreover, these results are consistent with the hypothesis that a hydrostatic amplification mechanism is capable of accounting for the development of strong downslope winds.

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Jean-François Mahfouf, Evelyne Richard, and Patrick Mascart

Abstract

A two-dimensional mesoscale model including a detailed representation of the planetary boundary layer, the soil and the vegetation is developed. A sea breeze over flat terrain is simulated, thereby confirming the ability of the model to reproduce the known properties of this mesoscale phenomenon. The atmospheric response to soil and vegetation inhomogeneities is then examined with no synoptic flow over flat terrain. The results reveal the large influence of soil texture on surface moisture availability. The transition zone between bare soil and vegetation appears to be a preferred location for the initiation of moist convection. A vegetation canopy over very dry or very wet surfaces reduces the spread between sensible and latent heat fluxes.

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Jean-Pierre Pinty, Robert Benoit, Evelyne Richard, and René Laprise

Abstract

The fully compressible 3D nonhydrostatic semi-implicit semi-Lagrangian MC2 (mesoscale compressible community) model described by Tanguay et al. has been modified in order to incorporate orography through the Gal-Chen and Somerville transformation of the vertical coordinate by Denis. In this study, a 2D version of the model is tested against classical solutions covering various mountain-wave regimes for continuously stratified flows. A close inspection of the propagation of the vertical momentum flux is performed to asses the accuracy and stability of the numerical method. The study emphasizes also the fact that a correct representation of forced hydrostatic gravity waves is reliable for Courant numbers less than 0.5. This limitation may be less severe as the flow becomes more nonhydrostatic. Furthermore, the sensitivity of the isothermal reference state for flows with realistic static stability and over steep slope mountain is explored.

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Ernest N’Dri Koffi, Marc Georgelin, Bruno Benech, and Evelyne Richard

Abstract

The main objective of the present paper is the use of a constant volume balloon (CVB) as a tool to (i) study trapped lee waves and (ii) assess the forecasting capability of a nonhydrostatic numerical model. Then, CVB data obtained during the Pyrénées Experiment (PYREX) are compared with nonhydrostatic two-dimensional trapped lee waves simulated by the Meso-NH model. This model is a community research model based on the Lipps–Hemler form of the anelastic system, which has been recently developed by the CNRM of Météo-France and the Laboratoire d’Aérologie of Université Paul Sabatier in France.

To analyze how the CVB responds to lee waves, a simple CVB model is first applied to academic atmospheric stationary wave flows, analogous to those encountered during PYREX. This model takes into account the vertical velocity of the surrounding air, geometrical parameters of the balloon, and the atmospheric heating processes. Results show that the CVB reacts well to the atmospheric wave period, with a phase delay of only a few minutes.

Three CVB trajectories obtained during the third Intensive Observation Period of PYREX are then computed within Meso-NH 2D simulations from the balloon’s starting point, using the CVB model. The simulated quantities are compared to the experimental CVB data, focusing especially on the lee-wave vertical movements. The simulated lee-wave vertical velocities and amplitude are found to be in good agreement with the observations, as shown by the statistical analysis. The computed CVB heights deviate by less than 13% from the altitude of the measured trajectories. This comparison of the model output to the CVB experimental data demonstrates the good performance of the Meso-NH model in the prediction of the vertical excursions of the lee waves.

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Nadine Chaumerliac, Evelyne Richard, Robert Rosset, and Everett C. Nickerson

Abstract

Two widely used microphysical schemes are compared to evaluate their possible impact on wet deposition mechanisms. They are based upon different spectral distributions for raindrops (Marshall-Palmer and lognormal distributions) and use different formulations for the autoconversion and evaporation process, as well as for the fall velocity of raindrops. A comparative study of these two schemes is carried out for a two-dimensional mountain wave simulation in a mesoscale meteorological model. Differences in the spatial and temporal evolution of microphysical fields are investigated. The two schemes are compared for simple chemical scenarios: gas dissolution in cloud and rain, gas scavenging by raindrops, and wet deposition. Results contrast the differing behavior of the two schemes in describing processes such as the direct scavenging of gases by raindrops and the release of chemical species back into the atmosphere because of below-cloud evaporation of rain.

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Jean-Pierre Pinty, Patrick Mascart, Evelyne Richard, and Robert Rosset

Abstract

Many recent studies have suggested that heterogeneities in soil properties or vegetation characteristics many trigger mesoscale circulations in planetary boundary layer (PBL). Unfortunately, these flows appear to be very sensitive to the choice of the model characteristics and therefore require a careful calibration of the parameterization representing the vegetation/atmosphere interface.

In this paper, the micrometeorological data from the HAPEX-MOBILHY field experiment are used to calibrate an evapotranspiration parameterization scheme over three types of dense vegetation typical of western Europe. This parameterization is then used a 2D mesoscale model to investigate the atmospheric response to a discontinuity in vegetation type (cereal crop to conifer forest). The results show a significant circulation when the soil is moist, associated with substantial PBL modification, whereas only a negligible atmospheric response is obtained when the soil is dry in the conifer forest). The results show a significant circulation when the soil is moist, associated with substantial PBL modification, whereas only a negligible atmospheric response is obtained when the soil is dry in the cereal crop area. A precise knowledge of the soil moisture therefore appears to be required, even with dense vegetation cover, to use advanced evapotranspiration schemes in mesoscale models.

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