On 15 March 2005, the Meteorological Service of Canada (MSC) proceeded to the implementation of a four-dimensional variational data assimilation (4DVAR) system, which led to significant improvements in the quality of global forecasts. This paper describes the different elements of MSC's 4DVAR assimilation system, discusses some issues encountered during the development, and reports on the overall results from the 4DVAR implementation tests. The 4DVAR system adopted an incremental approach with two outer iterations. The simplified model used in the minimization has a horizontal resolution of 170 km and its simplified physics includes vertical diffusion, surface drag, orographic blocking, stratiform condensation, and convection. One important element of the design is its modularity, which has permitted continued progress on the three-dimensional variational data assimilation (3DVAR) component (e.g., addition of new observation types) and the model (e.g., computational and numerical changes). This paper discusses some numerical problems that occur in the vicinity of the Poles where the semi-Lagrangian scheme becomes unstable when there is a simultaneous occurrence of converging meridians and strong wind gradients. These could be removed by filtering the winds in the zonal direction before they are used to estimate the upstream position in the semi-Lagrangian scheme. The results show improvements in all aspects of the forecasts over all regions. The impact is particularly significant in the Southern Hemisphere where 4DVAR is able to extract more information from satellite data. In the Northern Hemisphere, 4DVAR accepts more asynoptic data, in particular coming from profilers and aircrafts. The impact noted is also positive and the short-term forecasts are particularly improved over the west coast of North America. Finally, the dynamical consistency of the 4DVAR global analyses leads to a significant impact on regional forecasts. Experimentation has shown that regional forecasts initiated directly from a 4DVAR global analysis are improved with respect to the regional forecasts resulting from the regional 3DVAR analysis.
A major set of changes was made to the Environment Canada global deterministic prediction system during the fall of 2014, including the replacement of four-dimensional variational data assimilation (4DVar) by four-dimensional ensemble–variational data assimilation (4DEnVar). The new system provides improved forecast accuracy relative to the previous system, based on results from two sets of two-month data assimilation and forecast experiments. The improvements are largest at shorter lead times, but significant improvements are maintained in the 120-h forecasts for most regions and vertical levels. The improvements result from the combined impact of numerous changes, in addition to the use of 4DEnVar. These include an improved treatment of radiosonde and aircraft observations, an improved radiance bias correction procedure, the assimilation of ground-based GPS data, a doubling of the number of assimilated channels from hyperspectral infrared sounders, and an improved approach for initializing model forecasts. Because of the replacement of 4DVar with 4DEnVar, the new system is also more computationally efficient and easier to parallelize, facilitating a doubling of the analysis increment horizontal resolution. Replacement of a full-field digital filter with the 4D incremental analysis update approach, and the recycling of several key variables that are not directly analyzed significantly reduced the model spinup during both the data assimilation cycle and in medium-range forecasts.
In this paper, the operational 3D variational data assimilation system (3D-var)
Despite the availability of several atmospheric reanalyses (e.g. ERA-Interim) there exists both considerable uncertainty in surface forcing fields for ice/ocean modelling and sensitivity to the choice of product used. Here we introduce a relatively high-resolution alternative forcing dataset for ice-ocean models derived from the Canadian Meteorological Centre's (CMC) global deterministic prediction system (GDPS). A set of daily 30 h reforecasts is produced using the GDPS 33 km resolution model providing hourly atmospheric forcing fields for the period 2002-2011. The CMC GDPS reforecasts (CGRF) are compared with ERA-Interim and several observational datasets to evaluate their suitability for forcing ocean models. In particular, the surface temperature, humidity and winds of the CGRF show equivalent biases to those found in ERA-interim. Moreover, the higher resolution of the CGRF permit a more detailed representation of atmospheric structures and topographic steering, resulting in finer-scale coastal features and wind-stress curl. Although the CGRF dataset is not a reanalysis and thus is expected to be less well constrained by available observations, its higher resolution and small bias make it an attractive alternative for forcing ice-ocean models.
The impact of the physical parametrization called subgrid-scale orographic blocking, recently introduced in the physics of the Canadian Global Environmental Mutiscale (GEM) model is described. It is based on a formulation by Lott and Miller (1997) and represents the unresolved component of the drag on low-level winds that are blocked at the flanks of mountains. The blocking term plus the gravity-wave drag are now part of a unified parametrization of the subgrid orographic drag that became operational in the global GEM model on 11 December 2001. Results from tests made with various configurations of the model are shown, Lott et Miller (1997), permet de représenter la composante non-résolue de l'effet de résistance sur les vents de surface due aux montagnes. Le terme de blocage ainsi que le paramétrage de la résistance sur l'écoulement due aux ondes de gravité déferlantes forment maintenant un paramétrage unifié des effets orographiques sous-maille devenu opérationnel dans le modèle GEM global le 11 décembre 2001. illustrating how the blocking term impacts the large-scale flow and improves both the short-and the medium-range forecasts, especially in winter. It is shown that at day 5 of the model integrations, the influence of the blocking force applied near the surface is felt by the entire tropospheric and the lower-stratospheric circulation. A mechanism based on perturbations of the Eliassen-Palm flux caused by the low-level forcing is proposed to explain the vertical propagation of the signal generated by the blocking term. RÉSUMÉ L'impact d'un blocage orographique sous-maille sur le modèle canadien GEM (Global Environnemental Multi-échelle) est étudié. Le paramétrage, basé sur une formulation deDes résultats provenant de tests faits avec différentes configurations du modèle sont présentés, démontrant l'impact du terme de blocage sur l'écoulement à grande échelle ainsi que l'amélioration des prévisions à courtes et moyennes échéances, particulièrement en hiver. Il est démontré qu'au jour cinq de l'intégration du modèle, l'influence de la force de blocage appliquée près de la surface est en fait ressentie sur toute la profondeur de la troposphère ainsi que sur la basse stratosphère. Un mécanisme basé sur les perturbations du flux d'Eliassen-Palm induites par le forçage près de la surface est proposé afin d'expliquer la propagation verticale du signal induit par la force de blocage. ATMOSPHERE-OCEAN 41 (2) 2003, 155-170
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