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Clive Temperton and Michel Roch

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.

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Luc Fillion and Michel Roch

Abstract

Recent studies have demonstrated that variational nonlinear normal-mode initialization can be efficiently implemented in the context of shallow-water models, provided one uses a physical space formulation. The implicit nonlinear normal-mode initialization (INMI) technique provides essentially the same balancing benefit as standard ”explicit“ nonlinear NMI but does not require the explicit computation of the linear free modes of the model. This allows variational initialization with arbitrary horizontal variation of the weights that specify the changes to the analyzed fields during initialization. As a consequence, the variational extension of INMI allows more flexibility to control the relative adjustment of mass and wind fields over data-rich and data-poor regions.

The purpose of this paper is to demonstrate the feasibility of variational implicit normal-mode initialization (VINMI) for multilevel models. This new scheme is illustrated on the presently operational Canadian baroclinic regional finite-element (RFE) model. It is shown that the VINMI scheme efficiently controls the relative magnitude of the changes to the analyzed mass and wind fields during the balancing (initialization) processes. A comparison is also made of the impact of the VINMI scheme versus that of the presently operational unconstrained version of the initialization scheme (INMI). Future development and applications of the method are discussed at the end of the paper.

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Jean Côté, Michel Roch, Andrew Staniforth, and Luc Fillion

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 smoothly away from the area of interest.

To demonstrate the potential of this strategy, we have developed a shallow-water prototype using highly efficient numerical techniques, such as a two time-level semi-implicit semi-Lagrangian integration scheme. This model is a generalization of that of Côté and Staniforth (1990) to variable resolution on an arbitrarily rotated latitude-longitude mesh. Sample integrations indicate that it is possible to almost exactly reproduce a 2-day forecast on an 80° × 60° uniform-resolution (0.5°) subdomain (covering North America) of the variable-resolution mesh, for one-seventh the cost (both computational and storage) of running the model with uniform resolution (0.5°) everywhere, and for a cost about two orders of magnitude lower than running a conventional uniform-resolution Eulerian spectral model. For this variable-resolution mesh, fully 70% of the points in each direction are on the uniform-resolution area of interest. Thus, the overhead associated with using a model of global extent for short-range forecasting is indeed small, and is a small price to pay to avoid the lateral boundary condition problems of regional models.

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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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Jean Côté, Sylvie Gravel, André Méthot, Alain Patoine, Michel Roch, and Andrew Staniforth

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.

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Jean Côté, Jean-Guy Desmarais, Sylvie Gravel, André Méthot, Alain Patoine, Michel Roch, and Andrew Staniforth

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.

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Kao-San Yeh, Jean Côté, Sylvie Gravel, André Méthot, Alaine Patoine, Michel Roch, and Andrew Staniforth

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 Laprise's 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 interpolations for the semi-Lagrangian advection are of fourth-order accuracy except for the trajectory estimation. The resulting set of nonlinear equations is solved iteratively using a motionless isothermal reference state that gives the usual semi-implicit problem as a preconditioner. The Helmholtz problem that needs to be solved at each iteration is vertically separable, the impact of nonhydrostatic terms being simply a renormalization of the separation constants. The convergence of this iterative scheme is not greatly modified by the nonhydrostatic perturbation. Three numerical experiments are presented to illustrate the model's performance. The first is a test to show that hydrostatic balance at low resolution is well maintained. The second one is a mild orographic windstorm case, where the flow should remain hydrostatic, to test that hydrostatic balance at high resolution is also well maintained. The third one is a convective case taken from the Verification of the Origins of Rotation in Tornadoes Experiment (VORTEX). Although these results are encouraging, rigorous testing of the model's performance for strongly nonhydrostatic flows still remains to be done.

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Stéphane Bélair, Michel Roch, Anne-Marie Leduc, Paul A. Vaillancourt, Stéphane Laroche, and Jocelyn Mailhot

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 reveal gains of at least 12 h and even up to 72 h for medium-range QPFs (i.e., day 3 to day 5 predictions). The new global system even performs slightly better than MSC’s operational regional 15-km system for short-range QPFs.

In a more absolute manner, results suggest that QPFs from the new global system may still have accuracy and value even at the medium range. This seems to be true at least for the smallest precipitation threshold, related to precipitation occurrence, for which the positive area under curves of relative economic value remains important, even for day 5 QPFs.

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Gregory C. Smith, Jean-Marc Bélanger, François Roy, Pierre Pellerin, Hal Ritchie, Kristjan Onu, Michel Roch, Ayrton Zadra, Dorina Surcel Colan, Barbara Winter, Juan-Sebastian Fontecilla, and Daniel Deacu

Abstract

The importance of coupling between the atmosphere and the ocean for forecasting on time scales of hours to weeks has been demonstrated for a range of physical processes. Here, the authors evaluate the impact of an interactive air–sea coupling between an operational global deterministic medium-range weather forecasting system and an ice–ocean forecasting system. This system was developed in the context of an experimental forecasting system that is now running operationally at the Canadian Centre for Meteorological and Environmental Prediction. The authors show that the most significant impact is found to be associated with a decreased cyclone intensification, with a reduction in the tropical cyclone false alarm ratio. This results in a 15% decrease in standard deviation errors in geopotential height fields for 120-h forecasts in areas of active cyclone development, with commensurate benefits for wind, temperature, and humidity fields. Whereas impacts on surface fields are found locally in the vicinity of cyclone activity, large-scale improvements in the mid-to-upper troposphere are found with positive global implications for forecast skill. Moreover, coupling is found to produce fairly constant reductions in standard deviation error growth for forecast days 1–7 of about 5% over the northern extratropics in July and August and 15% over the tropics in January and February. To the authors’ knowledge, this is the first time a statistically significant positive impact of coupling has been shown in an operational global medium-range deterministic numerical weather prediction framework.

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Natacha B. Bernier, Jose-Henrique G. M. Alves, Hendrik Tolman, Arun Chawla, Syd Peel, Benoit Pouliot, Jean-Marc Bélanger, Pierre Pellerin, Mario Lépine, and Michel Roch

Abstract

A global deterministic wave prediction system (GDWPS) is used to improve regional forecasts of waves off the Canadian coastline and help support the practice of safe marine activities in Canadian waters. The wave model has a grid spacing of ¼° with spectral resolution of 36 frequency bins and 36 directional bins. The wave model is driven with hourly 10-m winds generated by the operational global atmospheric prediction system. Ice conditions are updated every three hours using the ice concentration forecasts generated by the Global Ice–Ocean Prediction System. Wave forecasts are evaluated over two periods from 15 August to 31 October 2014 and from 15 December 2014 to 28 February 2015, as well as over select cases during the fall of 2012. The global system is shown to improve wave forecast skill over regions where forecasts were previously produced using limited-area models only. The usefulness of a global expansion is demonstrated for large swell events affecting the northeast Pacific. The first validation of a Canadian operational wave forecast system in the Arctic is presented. Improvements in the representation of forecast wave fields associated with tropical cyclones are also demonstrated. Finally, the GDWPS is shown to result in gains of at least 12 h of lead time.

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