Robert Benoit

The Canadian MC2: A Semi-Lagrangian, Semi-Implicit Wideband Atmospheric Model Suited for Finescale Process Studies and Simulation

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 l...

Inclusion of a TKE boundary layer parameterization in the Canadian regional finite-element model

Abstract The formulation of the regional model recently implemented by the Atmospheric Environment Service of Canada for its operational 48 h NWP forecasts is presented. The emphasis is put on the parameterization of the physical processes, especially those affecting the atmospheric boundary layer. The originality of this model, in addition to the use of 3-D finite elements, of variable meshes in both the horizontal and vertical, and of being non-nested (as previously described by Staniforth and Daley), consists in the treatment of the time-dependent turbulent Kinetic energy (TKE) and the inclusion of the full diurnal cycle. The overall organization of the model calculations it also presented in order to convey a more accurate description of this integrated system. Sample results from the well-known case of the Presidents Day Cyclone of 1979 and general performance are covered in the last section.

The Real-Time Ultrafinescale Forecast Support during the Special Observing Period of the MAP

Recent developments in numerical modeling and computer technology will soon allow for limited-area production-type numerical weather prediction at a resolution of 1–2 km. This advance opens exciting prospects for the prediction of airflow and precipitation phenomena in and around mountainous regions, by improving the representation of the underlying topography, and by explicitly simulating (rather than parameterizing) moist convection. During the Special Observation Period (SOP; 7 Sept–15 Nov 1999) of the Mesoscale Alpine Programme (MAP) the Canadian Mesoscale Compressible Community Model (MC2) has been run in real time at a horizontal resolution of 3 km on a computational domain of 350 × 300 × 50 grid points, covering the whole of the Alpine region. An overview of the model configuration and performance will be presented along with simulation and validation results from selected MAP cases. Some critical aspects that require particular attention in future research will also be addressed.

On the Use of Coupled Atmospheric and Hydrologic Models at Regional Scale

Abstract. The purpose of this study is to present the possibilities offered by coupled atmospheric and hydrologic models as a new tool to validate and interpret results produced by atmospheric models. The advantages offered by streamflow observations are different from those offered by conventional precipitation observations. The dependence between basins and sub-basins can be very useful, and the integrating effect of the large basins facilitates the evaluation of stateof-the-art atmospheric models by filtering out some of the spatial and temporal variability that complicate the point-by-point verifications that are more commonly used. The streamflow predicted by the coupled atmospheric-hydrologic model versus the measured streamflow is sufficiently sensitive to clearly assess atmospheric model improvements resulting from increasing horizontal resolution and altering the treatment of precipitation processes in the model. A case study for several southern Ontario river basins is presented with the Watflood hydrologic model developed at the University of Waterloo. It is passively coupled to a nonhydrostatic mesoscale atmospheric model (mc2) that is integrated 318 HIGH PERFORMANCE COMPUTING SYSTEMS AND APPLICATIONS at horizontal resolutions of 35, 10 and 3 km. The Watflood model is also driven by radar derived precipitation amounts from King City Radar observations. It is demonstrated that the hydrological model is sufficiently sensitive and accurate to diagnose model and radar errors. This tool brings an additional degree of verification that will be very important in the improvement of technologies associated with atmospheric models, radar observations and the water resources management.

Moisture Transport Diagnosis of a Wintertime Precipitation Event in the Mackenzie River Basin

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 ...

Simple Tests of a Semi-Implicit Semi-Lagrangian Model on 2D Mountain Wave Problems

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.

A Regional Model Intercomparison Using a Case of Explosive Oceanic Cyclogenesis

Abstract The authors evaluate the performance of current regional models in an intercomparison project for a case of explosive secondary marine cyclogenesis occurring during the Canadian Atlantic Storms Project and the Genesis of Atlantic Lows Experiment of 1986. Several systematic errors are found that have been identified in the refereed literature in prior years. There is a high (low) sea level pressure bias and a cold (warm) tropospheric temperature error in the oceanic (continental) regions. Though individual model participants produce central pressures of the secondary cyclone close to the observed during the final stages of its life cycle, systematically weak systems are simulated during the critical early stages of the cyclogenesis. Additionally, the simulations produce an excessively weak (strong) continental anticyclone (cyclone); implications of these errors are discussed in terms of the secondary cyclogenesis. Little relationship between strong performance in predicting the mass field and skil...

Diabatic initialization of the Canadian Regional Finite-Element (RFE) Model Using Satellite Data. Part I: Methodology and Application to a Winter Storm

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 o...

Inclusion and verification of a predictive Cloud-Water Scheme in a Regional Numerical Weather Prediction Model

Abstract The implementation of a predictive cloud-water scheme in a regional finite-element weather prediction model is presented. The model employed in this study includes efficient and accurate numerical techniques and is equipped with a relatively extensive parameterization of the planetary boundary layer, surface process, and radiation. The modifications made to the meteorological model in this study include the addition of the advection equation for cloud water to the set of primitive meteorological equations. In our implementation of the predictive cloud-water scheme, the cloud-water equation represents the grid-resolved cloud-water field, whereas the effects of subgrid convective clouds are parameterized with the convective scheme of Kuo. The numerical solution of the advection equation for cloud water is analogous to the solution of the moisture equation using the semi-Lagrangian advection algorithm applied previously in regional weather forecast. The advection of cloud water is performed using th...

Finescale Topography and the MC2 Dynamics Kernel

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.