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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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Brice Boudevillain, Hervé Andrieu, and Nadine Chaumerliac

Abstract

A very short-term rainfall forecast model is tested on actual radar data. This model, called RadVil, takes advantages of voluminal radar data through vertically integrated liquid (VIL) water content measurements. The model is tested on a dataset collected during the intensive observation period of the Mesoscale Alpine Program (MAP). Five rain events have been studied during this experiment. The results confirm the interest of VIL for quantitative precipitation forecasting at very short lead time. The evaluation is carried out in qualitative and quantitative ways according to Nash and correlation criteria on forecasting times ranging from 10 to 90 min and spatial scales from 4 to 169 km2. It attempts to be consistent with the hydrological requirements concerning the rainfall forecasting, for instance, by taking account of the relation between the catchments' size, their response time, and the required forecasting time. Several versions of RadVil corresponding to several VIL measurement strategies have been tested. Improvements offered by RadVil depend on meteorological situations. They are related to the spatial and temporal evolution of the VIL field structure and the validity of the models assumptions. Finally, a relationship between the temporal structure of VIL fields and forecast quality is established.

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Nicole Audiffren, Sylvie Cautenet, and Nadine Chaumerliac

Abstract

Evidence of the efficient removal of chemicals by ice particles has been deduced from past field experiments and laboratory studies. However, the ice phase has been poorly represented in prior cloud chemistry modeling. This paper uses a two-dimensional Eulerian cloud model to address the impact of ice-phase processes on the chemistry of precipitation in the context of a simulated cumulonimbus cloud. Riming of graupel and the freezing of supercooled rain are the main processes for the transfer of species toward graupel. Even when freezing is the main mode for this transfer, riming still plays an important role by providing a feedback effect that limits the diluting influence of rain. When riming is the only process, sulfate production is more efficient in rainwater, whereas when freezing dominates a decrease in sulfate production is observed.

During the decaying stage, the precipitation (glaciated and/or liquid) has higher concentrations of the hydrogen peroxide and sulfates that originated from the gas phase. However, sulfates chemically produced in the liquid phases are less concentrated than if ice had played no role.

This study demonstrates the potential impact of ice-phase processes in organized cloud systems where strong updrafts exist, as ahead of a cold front.

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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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Evelyne Richard, Nadine Chaumerliac, Jean Francois Mahfouf, and Everett C. Nickerson

Abstract

Orographic precipitation enhancement associated with the feeder mechanism proposed by Bergeron has been simulated using a two dimensions model based upon primitive equations including detailed parameter microphysics. A case-by-case comparison is made between model results and each of 14 well-documented precipitation episodes in southern Wales. The model reproduces the observed strong dependence of the precipitation enhancement on the low-level wind speed, as well as the weak dependence on the upwind precipitation rate. Model results also demonstrate that a satisfactory treatment of orographically enhanced precipitation requires the linking of the dynamical, thermodynamical and microphysical processes.

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