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Robert Benoit

Abstract

New forms of the exact primitives (Nickerson and Smiley, 1975) for the Businger et al. (1971) surface layer gradients are described for the unstable case. In addition to all the theoretical advantages of the Nickerson and Smiley solution, the new formulas do not suffer from a numerical divergence problem which so far has hindered the free use of the exact primitives for the slightly unstable case.

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Gilles Verner
and
Robert Benoit

Abstract

The technique of nonlinear normal mode initialization is applied to a limited area model by interpolating initialized fields obtained from a hemispheric spectral model.

The vertical structure of the two models is identical and topography and physical parameterization are common to both models.

Results show that the procedure successfully transfers the properties of nonlinear normal mode initialization from the spectral model to the regional model. There are also indications that if we impose differences in vertical structure between the forecast and the initialization models, these have only minor effects on the time evolution.

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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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Claude Girard
,
Robert Benoit
, and
Michel Desgagné

Abstract

The Canadian Mesoscale Compressible Community (MC2) model provided daily forecasts across the Alps at 3-km resolution during the Mesoscale Alpine Programme (MAP) field phase of 1999. Among the results of this endeavor, some have had an immediate impact on MC2 itself as it increasingly became evident that the model was spuriously too sensitive to finescale orographic forcing. The model solves the Euler equations of motion using a semi-implicit semi-Lagrangian scheme in an oblique terrain-following coordinate. To improve model behavior, typical approaches were tried at first. These included a generalization of the coordinate transformation to make the terrain influence decay much more quickly with height as well as the introduction of nonisothermal basic states to diminish the amplitude of numerical truncation errors. The concept of piecewise-constant finite elements was invoked to reduce coding arbitrariness. But it was later pointed out that the problem was very specific and due to a numerical inconsistency. The true height of model grid points is fixed and known in height-based coordinates. Nevertheless, it was discovered that for this semi-Lagrangian scheme to be consistent, the departure height is an unknown that must be obtained in the same manner as the other unknowns.

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Nicolas Gasset
,
Robert Benoit
, and
Christian Masson

Abstract

The large size of modern wind turbines and wind farms triggers processes above the surface layer, which extend to the junction between microscales and mesoscales, and pushes the limits of existing approaches to predict the wind. The main objectives of this study are thus to introduce and evaluate an approach that will better account for physical processes within the atmospheric boundary layer (ABL), and allow for both microscale and mesoscale modeling. The proposed method, in which mathematical model and main numerical aspects are presented, combines a mesoscale approach with a large-eddy simulation (LES) model based on the Compressible Community Mesoscale Model (MC2). It is evaluated relying on a shear-driven ABL case allowing the authors to assess the model behavior at very high resolution as well as more specific numerical aspects such as the vertical discretization and time and space splitting of turbulence-related terms. The proposed LES-capable mesoscale model is shown to perform on par with other similar reference LES models, while being slightly more dissipative. A new vertical discretization of the turbulent processes eliminates a spurious numerical mode in the solution. Finally, the splitting of horizontal and vertical turbulence-related terms is shown to have no impact on the results of the test cases. It is thus demonstrated that the revised MC2 is suitable at both microscales and mesoscales, thus setting a strong foundation for future work.

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Benoit Cushman-Roisin
,
James J. O'Brien
, and
Robert L. Smith

Abstract

This note studies the response of a simple linear baroclinic coastal-upwelling model to fluctuating longshore winds. Correlations between wind stress and velocities are computed explicitly. It is shown that these correlations depend primarily upon the wind-stress spectrum and that, for a realistic spectrum, the wind stress leads the longshore velocity by approximately one day. The computed correlations agree remarkably well with observations and dismiss the belief that time lags ought to be directly related to the local inertial period, i.e., some fraction of the local pendulum day.

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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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Olli M. Turpeinen
,
Louis Garand
,
Robert Benoit
, and
Michel Roch

Abstract

The usefulness of numerical weather prediction models in very short-range forecasting is limited by the spinup problem, resulting in an underestimation of both the divergent wind component and the precipitation.

To alleviate the spinup problem, latent-heating profiles were directly assimilated into the Canadian regional finite-element (RFE) model. The estimates of latent heating were based on the precipitation rates inferred from GOES infrared and visible imagery. The latent heating was distributed in the vertical according to the stratiform condensation scheme of the model, but the heating rates were normalized to correspond to the satellite-inferred rain rates. The initial relative humidity field was enhanced to 95% between sigma-level 0.875 and the cloud top wherever the probability of precipitation, derived from satellite imagery, was larger than 40%.

The results of a case study from the Canadian Atlantic Storms Program (CASP) indicated that the spinup time of the vertical motion, initially of the order of 9 hours, could be practically eliminated. The forecast precipitation rates in the frontal zone agreed closely with Nimbus-7 SMMR microwave observations as early as 1–2 hours after the initialization.

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Gary M. Lackmann
,
John R. Gyakum
, and
Robert Benoit

Abstract

Wintertime precipitation events in the Mackenzie River basin (MRB) play an important role in the hydrology of the region because they contribute substantially to water storage prior to the spring runoff maximum. The Mesoscale Compressible Community (MC2) Model is used to simulate a representative wintertime MRB precipitation event. The MC2 simulation, gridded analyses, and raw observations are used to (i) document meteorological conditions associated with the precipitation event, (ii) assess the ability of the model to reproduce the precipitation event and antecedent large-scale moisture transport, and (iii) identify which planetary- and synoptic-scale features are responsible for the observed moisture transport using piecewise quasigeostrophic potential vorticity (QGPV) inversion.

Precipitation in the MRB develops north of an intense frontal boundary as a southwesterly flow of moisture originating over the Pacific Ocean is lifted over cold, dense arctic air near the surface. A lee cyclone forms along the frontal boundary as an upper-tropospheric disturbance approaches from the west. The MC2 model adequately represents the lee cyclone formation, the observed precipitation event, and large-scale moisture transport, as determined through comparison of the model output with analyses and raw observations. A plume of moisture advances northeastward from the subtropical Pacific Ocean toward the MRB during the 24–36-h period prior to the precipitation event. Piecewise QGPV inversion demonstrates that the background climatological flow and a cyclonic QGPV anomaly located over the eastern Pacific Ocean are associated with the initial moisture transport into the Gulf of Alaska. Later, a second cyclonic QGPV anomaly centered over the Gulf of Alaska is associated with moisture transport from over the Gulf of Alaska into the MRB. The moisture flux is generally largest in the lower troposphere owing to the larger concentration of water vapor there. The Rocky Mountains, located west of the MRB, block much of the eastward moisture transport below the 800-hPa level. Moisture transport in the layer between 700 and 800 hPa is therefore crucial for MRB precipitation in situations where the moisture originates over the Pacific. QGPV inversions based on a vertically partitioned QGPV field indicate that QGPV anomalies located below the dynamic tropopause are associated with larger moisture transport at the 700-hPa level than their tropopause-based counterparts.

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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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