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Serge Desjardins
,
Robert Benoit
, and
Val Swail

Abstract

This paper studies the mesoscale wind field during the blizzard of March 1993 off the east coast of North America and examines the influence of the sea surface temperature distribution on surface winds. Can the Gulf Stream and its meanders, by its strong influence on the marine boundary layer, generate mesoscale features in the wind field? Numerical simulations of the storm are carried out using the MC2, a fully elastic nonhydrostatic model. Simulations are conducted at different resolutions (50, 25, 10, 5, and 2 km) with both detailed and smoothed SST fields, so as to examine the influence of these parameters on the marine boundary layer winds. Results from these numerical simulations are compared with surface observations from buoys. The study reveals some mesoscale features in the wind field caused by the Gulf Stream’s meanders and the warm eddies of the SST field. In a stable boundary layer, the meanders shaped a 50–55-kt (26–28 m s−1) band of winds in a general 40–45-kt (21–23 m s−1) wind field. Behind the cold front, local enhancements of 10-kt (5 m s−1) winds were found over the warm water eddies in the unstable boundary layer.

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Pierre Pellerin
,
Harold Ritchie
,
François J. Saucier
,
François Roy
,
Serge Desjardins
,
Michel Valin
, and
Vivian Lee

Abstract

The purpose of this study is to present the impacts of a fully interactive coupling between an atmospheric and a sea ice model over the Gulf of St. Lawrence, Canada. The impacts are assessed in terms of the atmospheric and sea ice forecasts produced by the coupled numerical system. The ocean-ice model has been developed at the Maurice Lamontagne Institute, where it runs operationally at a horizontal resolution of 5 km and is driven (one-way coupling) by atmospheric model forecasts provided by the Meteorological Service of Canada (MSC). In this paper the importance of two-way coupling is assessed by comparing the one-way coupled version with a two-way coupled version in which the atmospheric model interacts with the sea ice model during the simulation. The impacts are examined for a case in which the sea ice conditions are changing rapidly. Two atmospheric model configurations have been studied. The first one has a horizontal grid spacing of 24 km, which is the operational configuration used at the Canadian Meteorological Centre. The second one is a high-resolution configuration with a 4-km horizontal grid spacing. A 48-h forecast has been validated using satellite images for the ice and the clouds, and also using the air temperature and precipitation observations. It is shown that the two-way coupled system improves the atmospheric forecast and has a direct impact on the sea ice forecast. It is also found that forecasts are improved with a fine resolution that better resolves the physical events, fluxes, and forcing. The coupling technique is also briefly described and discussed.

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Robert Benoit
,
Michel Desgagné
,
Pierre Pellerin
,
Simon Pellerin
,
Yves Chartier
, and
Serge Desjardins

Abstract

This paper attempts to document the developmental research and early mesoscale results of the new fully nonhydrostatic atmospheric model called MC2 (mesoscale compressible community). Its numerical scheme is the semi-implicit semi-Lagrangian approach conceived and demonstrated by Tanguay, Robert, and Laprise. The dominant effort required to become a full-fledged mesoscale model was to connect it properly to a full-scale and evolving physics package; the enlarged scope of a package previously dedicated to hydrostatic pressure coordinate-type models posed some new questions. The one-way nesting is reviewed and particularly the self-nesting or cascade mode; the potential implication of this mode is explored with a stand-alone forecast experiment and related to the other existing approach employing hemispheric or global variable meshes. One of the strong assets of MC2 is its growing community of users and developers. To demonstrate the wideband characteristic of MC2, that is, its applicability to a large range of atmospheric flows, two very different cases are studied: an Atlantic winter East Coast cyclogenesis (meso-α scale, mostly hydrostatic) and a local (meso-γ scale, partly nonhydrostatic) downslope windstorm occuring over unexpectedly modest topography (Cape Breton Highlands of Nova Scotia, Canada).

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