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

Abstract

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. Electronic supplementary material The online version of this article (10.1007/s00382-019-04793-2) contains supplementary material, which is available to authorized users.

Climatology and dynamics of the link between dry intrusions and cold fronts during winter. Part I: global climatology

“…It may be that the stronger fronts are also associated with stronger cyclones, and therefore are more likely to be associated with DIs. The compositing methodology used in Part 2 (Raveh-Rubin and Catto 2019) will go some way to addressing these interrelationships between strength and DIs.…”

“…Identifying fronts on different levels shifts the maximum front frequencies to different locations (Figures S9–S10), and at 925 hPa gives stronger fronts without DIs than with DIs (Figure S12), likely due to the strong temperature contrasts over orography. There are some differences to the isolated front precipitation when identifying the fronts on 700 hPa, which may be associated with differences in the vertical structure of these features (see Raveh-Rubin and Catto 2019). While the exact values change for different parameter choices, overall the conclusions of the study are strengthened by this sensitivity analysis.…”

“…In this part of this two-part study, we have examined the statistical properties of the three types of cold fronts, with a particular focus on trailing and isolated fronts. Part II of this study (Raveh-Rubin and Catto 2019) will address the dynamical aspects of the trailing and isolated fronts using composites that show the spatial pattern of variables of interest associated with the environment of the fronts and their impacts.…”

A ranking of concurrent precipitation and wind events for the Iberian Peninsula

“…In the midlatitudes, it is common to experience concurrent precipitation and wind events: they are typically associated with the same mesoscale and/or synoptic features like extratropical cyclones (e.g., Reale and Lionello, 2013;Pfahl, 2014;Raveh-Rubin and Wernli, 2015), which are enhanced by local features such as the orography or the presence of water bodies (Veals and Steenburgh, 2015;Martius et al, 2016). Within extratropical cyclones, concurrent wind and precipitation are likely to occur along with frontal structures (Catto and Pfahl, 2013;Schemm et al, 2016) and their intensity scales with the frontal strength (Schemm et al, 2017;Raveh-Rubin and Catto, 2019). For the Iberian Peninsula (IP), extreme windstorms and extreme precipitation are among the costliest natural hazards (Lorente et al, 2008;Liberato and Trigo, 2014;Pereira et al, 2018).…”

Role of moist and dry air advection in the development of Mediterranean tropical‐like cyclones (medicanes)

“…Another important aspect related to the dry intrusion is the direct link with the surface latent‐heat fluxes. It is known that, when the dry air reaches the lower troposphere, the sharp humidity contrast with the sea surface and the associated strong winds typically trigger very intense ocean evaporation (Raveh‐Rubin, 2017; Raveh‐Rubin and Catto, 2019). This effect potentially intensifies the cyclone by enhancing surface fluxes and enhancing convection in some locations, and opposes the drying of the mid‐troposphere, which prevents its intensification.…”