Structure and predictive skill of strong northeasterly wind events using a limited area numerical weather prediction model at Iqaluit, Canada

摘要

Strong northeasterly wind events are infrequent over Baffin Island, but are potentially hazardous for aviation and the local community of Iqaluit (the capital of Nunavut, Canada). Three strong northeasterly wind events in this region are examined in this study, using the Canadian Global Environmental Multiscale-Limited Area Model (GEM-LAM) with a horizontal grid spacing of 2.5 km; in-situ observations; and reanalysis data. The skill of the GEM-LAM in simulating these events is examined. With the exception of one event, the GEM-LAM was successful at predicting the large-scale flow in terms of the circulation pattern, timing of the synoptic set-up and the low-level flow over the Hall Peninsula. The onset and cessation of strong winds and timing of major wind shifts was typically well handled by the model to within ~3 h, but with a tendency to underestimate the peak wind speed. The skill of the surface wind forecasts at Iqaluit is critically dependent on the predicted timing and location of the hydraulic jump and the grid point selected to represent Iqaluit. Examination of the observed and modelled data suggest that the strong northeasterly wind events have several features in common: (1) strong gradient-driven flow across the Hall Peninsula, (2) mean-state critical layer (or reverse shear) over the Hall Peninsula, (3) a low-level inversion, typically above the maximum barrier height immediately upstream of the Hall Peninsula, (4) subcritical flow, typically present upstream of the Hall Peninsula and (5) a hydraulic jump in the vicinity of Frobisher Bay. The modelled atmospheric conditions upwind of the Hall Peninsula immediately prior to the formation of the hydraulic jump (and acceleration of winds over the lee slope) are largely consistent with the prediction of propagating hydraulic jumps presented in the literature.