Funding for this research was provided by NSF Grant ATM9912075. The ECMWF and the Data Support Section of the Computational and Information Systems Laboratory at NCAR provided the data used in this research. Comments from two anonymous reviewers greatly improved the quality of this paper.
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A typical midlatitude Rossby radius of deformation Nhm/f is 200 km, based on a 2.0-km mountain height hm, a 2.0 × 10−2 rad s−1 Brunt–Väisälä frequency N, and a 1.0 × 10−4 s−1 Coriolis parameter f. This value increases with decreasing latitude.
The Froude number is defined as U/Nhm, where U is the wind speed component normal to the mountain ridge and N and hm are defined as above.
The 2.5° × 2.5° National Centers for Environmental Prediction–National Center for Atmospheric Research (NCEP–NCAR) reanalysis showed qualitatively similar results, but was found to be less realistic near high terrain.
Temperature thresholds higher than 16°C yielded less distinct signatures of terrain-channeled cold surge pathways, while thresholds lower than 16°C masked the meridional extent of the pathways indicated from analyses of individual cold surges (not shown).
Climatological means at each point are weighted by the proportion of ECE analysis times that fall within each month. For example, if 30% of all ECE analysis times at a given point are from June, 40% from July, and 30% from August, then the weighted climatological mean would be [(0.3 × June climatological mean) + (0.4 × July climatological mean) + (0.3 × August climatological mean)].