Michel Roch

The Operational CMC–MRB Global Environmental Multiscale (GEM) Model. Part I: Design Considerations and Formulation

Abstract An integrated forecasting and data assimilation system has been and is continuing to be developed by the Meteorological Research Branch (MRB) in partnership with the Canadian Meteorological Centre (CMC) of Environment Canada. Part I of this two-part paper motivates the development of the new system, summarizes various considerations taken into its design, and describes its main characteristics.

The Operational CMC–MRB Global Environmental Multiscale (GEM) Model. Part II: Results

Abstract An integrated forecasting and data assimilation system has been and is continuing to be developed by the Meteorological Research Branch (MRB) in partnership with the Canadian Meteorological Centre (CMC) of Environment Canada. Part II of this two-part paper presents the objective and subjective evaluations of the intercomparison process that led to the operational implementation of the new Global Environmental Multiscale model. The results of a “proof of concept” experiment and those of a meso-γ-scale simulation further demonstrate the validity and versatility of this model.

The CMC–MRB Global Environmental Multiscale (GEM) Model. Part III: Nonhydrostatic Formulation

Abstract An integrated forecasting and data assimilation system has been and is continuing to be developed by the Meteorological Research Branch (MRB) in partnership with the Canadian Meteorological Centre (CMC) of Environment Canada. Part III of this series of papers presents the nonhydrostatic formulation and some sample results. The nonhydrostatic formulation uses Laprises hydrostatic pressure as the basis for its vertical coordinate. This allows the departure from the hydrostatic formulation to be incorporated in an efficient switch-controlled perturbative manner. The time discretization of the model dynamics is (almost) fully implicit semi-Lagrangian, where all terms including the nonlinear terms are (quasi-) centered in time. The spatial discretization for the adjustment step employs a staggered Arakawa C grid that is spatially offset by half a mesh length in the meridional direction with respect to that employed in previous model formulations. It is accurate to second order, whereas the interpolat...

Medium-Range Quantitative Precipitation Forecasts from Canada's New 33-km Deterministic Global Operational System

Abstract The Meteorological Service of Canada (MSC) recently implemented a 33-km version of the Global Environmental Multiscale (GEM) model, with improved physics, for medium-range weather forecasts. Quantitative precipitation forecasts (QPFs) from this new system were compared with those from the previous global operational system (100-km grid size) and with those from MSC’s short-range (48 h) regional system (15-km grid size). The evaluation is based on performance measures that evaluate bias, accuracy, and the value of the QPFs. Results presented in this article consistently show, for these three aspects of the evaluation, that the new global forecast system (GLBNEW) agrees more closely with observations, relative to the performance of the previous global system (GLBOLD). The biases are noticeably smaller with GLBNEW compared with GLBOLD, which severely overpredicts (underpredicts) the frequencies and total amounts associated with weak (strong) precipitation intensities. The accuracy and value scores r...

A Variable-Resolution Semi-Lagrangian Finite-Element Global Model of the Shallow-Water Equations

Abstract To meet the needs of short- and medium-range operational forecasting, the authors propose a unified strategy based on the use of a global variable-resolution model, run in two different configurations. These are as follows: (i) a variable-resolution “regional” configuration (with resolution focused over an area of interest) for detailed forecasts to 2 days, and (ii) a uniform-resolution “medium-range” one, for forecasts to 7 days or longer. This otters significant economy in an operational environment, since there is only one model—instead of the usual two—to maintain, develop, and optimize. It also provides an efficient and conceptually simple solution to the nesting problem for regional forecasting: the planetary waves are adequately resolved around a high-resolution subdomain (which resolves mesoscale disturbances), there are no artificial lateral boundaries with their attendant problems, and there is no abrupt change of resolution across an internal boundary since the resolution varies smooth...

Staggered Vertical Discretization of the Canadian Environmental Multiscale (GEM) Model Using a Coordinate of the Log-Hydrostatic-Pressure Type

AbstractThe Global Environmental Multiscale (GEM) model is the Canadian atmospheric model used for meteorological forecasting at all scales. A limited-area version now also exists. It is a gridpoint model with an implicit semi-Lagrangian iterative space–time integration scheme. In the “horizontal,” the equations are written in spherical coordinates with the traditional shallow atmosphere approximations and are discretized on an Arakawa C grid. In the “vertical,” the equations were originally defined using a hydrostatic-pressure coordinate and discretized on a regular (unstaggered) grid, a configuration found to be particularly susceptible to noise. Among the possible alternatives, the Charney–Phillips grid, with its unique characteristics, and, as the vertical coordinate, log-hydrostatic pressure are adopted. In this paper, an attempt is made to justify these two choices on theoretical grounds. The resulting equations and their vertical discretization are described and the solution method of what is formi...

The Stratospheric Extension of the Canadian Global Deterministic Medium-Range Weather Forecasting System and Its Impact on Tropospheric Forecasts

AbstractA new system that resolves the stratosphere was implemented for operational medium-range weather forecasts at the Canadian Meteorological Centre. The model lid was raised from 10 to 0.1 hPa, parameterization schemes for nonorographic gravity wave tendencies and methane oxidation were introduced, and a new radiation scheme was implemented. Because of the higher lid height of 0.1 hPa, new measurements between 10 and 0.1 hPa were also added. This new high-top system resulted not only in dramatically improved forecasts of the stratosphere, but also in large improvements in medium-range tropospheric forecast skill. Pairs of assimilation experiments reveal that most of the stratospheric and tropospheric forecast improvement is obtained without the extra observations in the upper stratosphere. However, these observations further improve forecasts in the winter hemisphere but not in the summer hemisphere. Pairs of forecast experiments were run in which initial conditions were the same for each experiment ...

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...

Implicit Normal Mode Initialization for an Operational Regional Model

Abstract In a previous study based on the shallow-water equations, it was shown that nonlinear normal mode initialization (NMI) can be implemented without knowing the normal modes of a model; this implicit form of nonlinear NMI is particularly useful in models for which computing the horizontal normal modes is impracticable. The present paper extends the technique to the multilevel Canadian Operational Finite-Element Regional Model. This paper shows that the method yields well-balanced initial conditions and consistent vertical velocity fields. Forecasts from these initial conditions using a semi-Lagrangian time-integration scheme with relatively large time steps are free from unrealistic high-frequency oscillations.

Numerical forecasting of winter coastal storms during CASP: Evaluation of the regional finite‐element model

Abstract As part of the Canadian Atlantic Storms Program (CASP), a meso‐α scale version of the regional finite‐element model was set up for the short‐term forecasting of East Coast storms during the CASP field phase. The main features of the mesoscale model compared with the operational continental version include a 100‐km resolution over a reduced domain, a modified physics package, improved surface and geophysical field analyses, and the refinement of initial moisture analyses using satellite imagery. Field evaluations of the mesoscale and continental models by CASP meteorologists indicated that both models generally underpredicted storm speed and deepening and that the CASP model yielded more accurate storm locations. Mesoscale forecasts of precipitation types and boundary‐layer winds were quite realistic when compared with actual station reports. Objective verification scores for 11 storm cases confirm the better meteorological performance of the mesoscale model. The most striking improvements are fou...