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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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Everett C. Nickerson, Evelyne Richard, Robert Rosset, and David R. Smith

Abstract

A three-dimensional meso-β model with parameterized microphysics is presented. The model is capable of simulating orographically forced clouds, rain, and airflow. Tests using a two-dimensional version confirm the ability of the model to replicate the linear and nonlinear mountain wave simulations of previous authors. The model is applied to the Rhine valley and surrounding mountainous areas, the Vosges in France and the Black Forest in Germany. Model-predicted rainfall over the mountainous areas is in good agreement with observations in both magnitude and location; however, an absence of model-predicted cloud cover over the Rhine valley suggests the need for an improved mesoscale initialization procedure.

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Jean-Luc Sortais, Jean-Pierre Cammas, Xiao Ding Yu, Evelyne Richard, and Robert Rosset

Abstract

An example of coupling between an upper-level and a low-level jet–front system is analyzed using the mesoscale hydrostatic model SALSA. The case study chosen is the cold front sampled during the intensive observation period 2 of the Mesoscale Frontal Dynamic Project FRONTS 87 experiment (11 November 1987). Two prominent features of the cold front are a well-developed undulation and a frontal band ahead of the undulation.

Dynamic and diabatic processes of the coupling are investigated through the results of two numerical simulations: full physics and adiabatic. In particular, the roles of ageostrophic circulations are investigated through the decomposition of the ageostrophic wind into its isentropic components: the advective inertial and the diabatic inertial components, and the isallobaric component.

In both simulations it is shown that the low-level branch of the indirect and transverse ageostrophic circulation, associated with the exit region of the upper-level jet streak, is the origin of the cold-front undulation beneath the upper-level jet axis. Other effects of the coupling processes are diagnosed in the full-physics simulation: the intensification of the low-level jet, the formation of the frontal band ahead of the cold front on the left side of the low-level jet, the intensification of the convective system close to the ascending branch of the transverse indirect circulation, and the kinetic energy generation in the exit region of the upper-level jet streak.

An examination of the isentropic components of the ageostrophic wind shows that the upper-level branch of the indirect circulation is the inertial advective component of the ageostrophic wind, whereas the low-level branch is mainly isallobaric. Being caught up in its eastward motion by the faster propagating low-level jet, the low-level branch of the indirect circulation reinforces, by its 7 m s−1 ageostrophic component, the advection of heat and moisture toward the convective system close to the ascending branch of the indirect circulation. In that region, the diabatic inertial component of the ageostrophic wind grows at a rate smaller than that needed for mass adjustment between the thermal and wind fields, thereby leading to kinetic energy generation through the inertial rotation mechanism downstream and on the cyclonic side of the upper-level jet streak.

Dynamical and diabatic processes are shown to contribute to the blocking-in phase and to the development of synergistic interactions between the upper- and the low-level jet-front systems. The results of this study are summarized in a conceptual scheme of the coupling mechanism, including an explanation for the formation of the frontal band.

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