Climatology and dynamics of the link between dry intrusions and cold fronts during winter, Part Ⅱ: Front-centred perspective

摘要

The conceptual picture of an extratropical cyclone typically includes a cold front and a dry intrusion (DI) behind it. By objectively identifying fronts and DIs in ECMWF ERA-Interim data for 1979-2014, Part I quantified the climatological relationship between cold fronts and DIs. Driven by the finding that front intensity and frontal precipitation are enhanced in the presence of DIs, here we employ a front-centred perspective to focus on the dynamical and thermodynamical environment of cold fronts with and without DIs in the Northern Hemisphere winter. Distinguishing between trailing fronts (that connect to a parent cyclone) and isolated fronts, examples of DIs behind each type illustrate the baroclinic environment of the trailing front, and the lack of strong temperature gradients across the isolated front. Composite analyses of North Atlantic and North Pacific fronts outline the major differences in the presence of DIs, compared to similar fronts but without DIs in their vicinity. The magnitude and spatial structure of the modification by DIs depends on the front intensity. Yet, generally with DIs, trailing fronts occur with stronger SLP dipole, deeper upper-tropospheric trough, stronger 10-m wind gusts, enhanced ocean sensible and latent heat fluxes in the cyclone cold sector and heavier precipitation. Isolated weak fronts exhibit similar behaviour, with different spatial structure. This study highlights the central role of DIs for shaping the variability of fronts and their associated environment and impact.