Regional Finite-element Model Research Articles

Wind Forcing of Ice Cover in the Labrador Shelf Marginal Ice Zone

Anemometer‐measured winds for the period 5–13 March 1994 were used to study the coherence of observed and forecast coastal winds along the mid‐Labrador shelf. The reliability of these variables in predicting the response of the ocean and ice to wind forcing is an important issue for ice forecasting in this area. Two anemometer‐equipped 2‐m ice beacons were deployed on pack ice north of Wolf Island and a third beacon was deployed on Grady Island. The results indicate that due to the influence of local topography, 10‐m winds observed at the meteorological station in Cartwright, Labrador provide a poor estimate (r2 = 0.2) of wind conditions over the offshore sea‐ice. In contrast, the σ = 1 level (∼10 m) winds from the Canadian Meteorological Centre's Regional Finite Element (RFE) model provided a better correlation with anemometer beacon winds (0.90 for the 6‐hour forecast down to 0.45 at 36 hours). However, the RFE model overestimates the magnitude of the winds by 10–40%. The response of the ocean and ice cover to wind forcing was measured by an ocean bottom‐mounted acoustic Doppler current proþler (ADCP). Relative to the 2‐m beacon winds, the ice moved at 2.5% of wind magnitude and turned 0.6° to the left of the wind. The ocean response decreased with depth until it reached a constant value of 0.9% of the wind speed. The turning angle increased from 0.3° to the right of the wind at 3.5 m to 50° at the lowest level measured by the ADCP (73 m depth). Approximately 57% of the variance in the ocean currents at 3 m below the surface can be attributed to the 2‐m winds; at 73 m the explained variance decreases to 27%.

A numerical investigation of a moderate coastal storm with intense precipitation

In this study, the development of a moderate coastal storm with intense precipitation that occurred during 12–14 February 1993 is examined using a high‐resolution version of the Canadian Regional Finite‐element (RFE) model with more realistic physical representations. It is shown that the improved RFE model predicts well the coastal cyclogenesis events and also the distribution and intensity of heavy mixed precipitation (rain and snow) associated with the storm. It is found that the cyclogenesis takes place in response to the low‐level inshore advection of high‐θe air from the maritime boundary layer, and the approach of a mid‐level shortwave trough with a warm pool above that is previously associated with a decaying cyclone upstream. More rapid deepening of the cyclone ensues as intense precipitation falls along the warm and cold fronts near the cyclone centre. Diagnosis of the control and sensitivity simulations reveals that the low‐level inshore warm advection and the propagation of the stratospheric warm pool contribute more significantly to the surface pressure falls during the incipient stage, whereas the mid‐level shortwave trough plays an important role in the cyclogenesis at later stages. Overall, latent heat release accounts for about 50% of the cyclone's total deepening, in agreement with the presence of a moderate baroclinic environment and the generation of intense precipitation. The diabatic and kinematic structures near the rain‐snow boundary are examined to gain insight into the influence of melting snow on the cyclogenesis. It is shown that the improved RFE model reproduces well the rain‐snow boundary structures as previously observed. Moreover, a thermally indirect circulation (perturbation) can be seen in the vicinity of the rain‐snow boundary. It is found, however, that melting of snow tends to produce a weak negative or negligible impact on the cyclogenesis, as opposed to previous hypotheses.

A Diagnostic Analysis of the Superstorm of March 1993

Abstract In this study, the synoptic evolution of the March 1993 superstorm is documented using conventional observations, and the predictability and deepening mechanisms of the storm are investigated using a newly implemented mesoscale version of the Canadian Regional Finite-Element (RFE) Mode. It is found that although the RFE model can predict well the large-scale background flow associated with the storm, it fails to predict various important mesoscale elements when it is initialized with fields that contain weak signals of the low-level circulations. However, when the storm is located close to the data-rich region, the model has considerable skill in predicting those mesoscale elements, such as a prefrontal squall line, upper- and low-level jets, as well as the quantitative aspects of their associated precipitation. Observational analysis reveals that the storm developed in a convectively unstable prestorm environment with ample moisture content. It first experienced an antecedent surface vorticity g...

Changes to the Canadian regional forecast system: Description and evaluation of the 50‐km version

Abstract On 3 November 1993 a new higher‐resolution version of the regional forecast system was implemented into operations at the Canadian Meteorological Centre. The changes include modifications to the regional data assimilation system and to the regional finite‐element (RFE) forecast model. The main features of the new version of the RFE model include an increase in resolution from 100 to 50 km and to 25 σ levels in the vertical. The fields that describe the surface characteristics are generated directly on the 50‐km grid of the model from high‐resolution global geophysical datasets, yielding more details and a much better definition of the orography and coastlines. The new RFE model also includes an improved package of physical parametrizations, notably for condensation and radiation processes. The major changes to the data assimilation are a higher‐resolution analysis, and the assimilation of humidity profiles retrieved from satellite imagery. The new system is evaluated using performance statistics,...

Numerical Prediction of the 10–11 June 1985 Squall Line with the Canadian Regional Finite-Element Model

Abstract In an effort to improve operational forecasts of mesoscale convective systems (MCSs), a mesoscale version of the operational Canadian Regional Finite-Element (RFE) Model with a grid size of 25 km is used to predict an intense MCS that occurred during 10–11 June 1985. The mesoscale version of the RFE model contains the Fritsch–Chappell scheme for the treatment of subgrid-scale convective processes and an explicit scheme for the treatment of grid-scale cloud water (ice) and rainwater (snow). With higher resolution and improved condensation physics, the RFE model reproduces many detailed structures of the MCS, as compared with all available observations. In particular, the model predicts well the timing and location of the leading convective line followed by stratiform precipitation; the distribution of surface temperature and pressure perturbations (e.g., cold outflow boundaries, mesolows, mesohighs, and wake lows); and the circulation of front-to-rear flows at both upper and lower levels separated...

Sensitivity experiments for polar low forecasting with the CMC mesoscale finite‐element model

Abstract High‐resolution versions of the Canadian operational regional finite‐element model (RFE) have been developed to assess their potential in simulating mesoscale, difficult‐to‐forecast and potentially dangerous weather systems commonly referred to as polar lows. The operational (1989) 100‐km version and a 50‐km version of the model have been run for two different polar low cases: one over Hudson Bay and one over Davis Strait. More integrations have also been performed on the Hudson Bay event both at 50 and 25 km to assess the model sensitivity to ice cover. As expected, the reduction in spatial truncation errors provided by the increase in resolution results in a better simulation of the systems. Moreover, when run at higher resolutions the model shows a significant sensitivity to ice cover. The results of the ice‐cover experiments also put into perspective the interaction between the heat and moisture fluxes at the surface, the low‐level wind structure, and the relation of these to the development of the polar low. This study suggests that the improved forecast accuracy obtained from increased resolution is limited by the correctness of the analysis of the ice cover, which acts as a stationary forcing for the entire forecast period.

Variability in Successive Operational Model Forecasts of Maritime Cyclogenesis

The level of variability present in operational model simulations of marine cyclogenesis was examined. Successive forecasts valid for the same 12-h time period of analysed maximum cyclone central pressure fall from the National Meteorological Center (NMC) nested grid model (NGM) and the Canadian Meteorological Centre (CMC) regional finite element model (RFE) were examined for one cold season (November 1988–March 1989). All cyclones with an analysed 12-h maximum central pressure fall occurring within the Western Atlantic region were included in the study, comprising a total of 52 sets of 0–12-,12–24-, 24–36-, and 36–48-h forecasts. Analyzed cyclone development spanned a wide range of intensities from no development (one case) to extraordinarily rapid development (greater than 24 mb in 12 h, 14 cases). The primary storm track was located offshore, resulting in reduced precipitation along the coastal regions. Both models tended to under-forecast cyclone development, and under-represent the clustering of cyclone activity within a narrow band offshore. This tendency became progressively stronger in the NGM with forecast range, while the behavior of the RFE was complicated by an apparent short-range (0–12-h) spin-up problem. The degree of variability in a sequence of forecasts was not well related to the overall prediction error for either model, belying the view that consistency in successive model forecasts indicates reliability. Each model exhibited forecast of low variance with high error (consistently poor forecasts of development) and sets of high variance with low error (inconsistent, but within range of the analyzed development). The models performed more similarly with regard to their mean error characteristics for a given set of forecasts than in terms of the dispersion of that error within the forecast sequences.