An illustrative case: 3 January 2005
A prominent DI occurred during 1–5 January 2005, in which air descended slantwise from the upper troposphere in eastern Canada and reached the lower troposphere in the north western Atlantic Ocean, positioned behind a trailing front that obtained its maximum length during 3 January (Fig. 1). The front is connected to a deep cyclone to its north east, between southern Greenland and Iceland. The cyclone hosts a bent-back warm front as well, and an additional front feature within the cyclone area, regarded as a ‘central front’ in Catto and Raveh-Rubin (2019). Being by construction outside the cyclone area, the trailing front stretches further between two anticyclones to its east and west, on both sides of the Atlantic basin (Fig. 2a). The low-level DI object lies outside of the cyclone area, behind the front where its intensity is strongest, reaching 2.5 K/100 km (Fig. 2a). In the upper troposphere, the tropopause is lowered in a trough, below which a cold anomaly is located at low levels (Fig. 2b).
The relative location of the full DI (i.e., not only the low-level DI object) relative to the front at 00 UTC 3 January, is shown in Fig. 3 in a vertical cross section along the dashed line in Fig. 2. The location of the front is evident from the strong \(\theta _{e}\) and \(\theta\) gradients at \(40^{\circ }\hbox {N}\), indicating a strongly baroclinic environment. The prominent upper-level trough reaches the 500-hPa level within the 40–45\(^{\circ }\hbox {N}\) band. At this time, DI air spans from the area of the upper-tropospheric ridge at \(55^{\circ }\hbox {N}\), below the trough and towards the front, at 900 hPa in the baroclinic zone. The moisture distribution shows the long vertical extent of the strong horizontal moisture gradients across the front, from 900 hPa to the upper troposphere ahead of the trough. The DI air is found in the driest regions, overriding the relatively moist boundary layer in the cold sector.
In the cold sector of the cyclone, intense surface fluxes into the atmosphere coincide with the DI object (Fig. 2c), consistent with the cyclone-relative location in Winschall et al. (2012), Aemisegger and Papritz (2018). Strong wind gusts occur in three main locations: in the cold conveyor belt jet (Hewson and Neu 2015), in the bent-back warm front region and in the northern portion of the trailing front, coinciding with intense precipitation there (Fig. 2c). Large-scale and convective precipitation are organized along the southwest-northeast oriented part of the front. The convective precipitation to the south of the front is seemingly related to the warm southerly advection (Fig. 2b, c).
To put this case study in a climatological perspective, the environment of similar trailing fronts with and without the presence of DIs behind them is examined in the next section.
Trailing fronts composite
In order to generalize the characteristic environment of trailing fronts matching with DIs, compared to similar fronts without DIs, composite maps incorporate all trailing fronts in a confined geographical location in the central North Atlantic (Figs. 4, 5, 6) and Pacific (Supplementary Figures S1, S2, S3) Oceans. Since fronts occurring with DIs are significantly stronger (Catto and Raveh-Rubin 2019), here we control for the front intensity using separate composites for different intensities (see Sect. 2). By construction of the trailing front identification criterion and the compositing approach, a clear cyclonic field is indeed present with its centre around \(-\,5^{\circ }\hbox {E}\) relative longitude and \(5^{\circ }\hbox {N}\) relative latitude. The mean field for the 3 different front strength categories indicates that the stronger the front intensity, the larger and deeper the cyclone is in terms of central SLP, and the stronger the pressure gradients near [\(0^{\circ }\hbox {E},0^{\circ }\hbox {N}\)]. The front strength is directly visible as the area of maximal gradients of the \(\theta _{e}\) field, indicating the mean position of the trailing fronts to the south and southwest of the cyclone. The equivalent potential temperature field also indicates that weak fronts occur on average in warmer regions at lower latitudes, consistent with the global climatological results in Catto and Raveh-Rubin (2019). The accompanying upper-tropospheric trough, shifted west of the cyclone centre by 5–10\(^{\circ }\), is more pronounced the stronger the front. Considering the co-occurrence of DIs and trailing fronts, it is evident that fronts that match with DIs are associated climatologically with deeper cyclones, for a given front intensity. In these cases, the upper-level trough is enhanced as well (Fig. 4c, f, i). Although the difference in cyclone intensity is less pronounced in the weak fronts set, the anticyclone to its north east is deepened by more than 5 hPa on average when a DI is present (Fig. 4i), compared to weaker anticyclone differences for stronger fronts (Fig. 4c, f). Another anticyclone south west of the cyclone is enhanced in the presence of DIs. The overall stronger dipole structure of the SLP field in the vicinity of the front indicates stronger pressure gradient forces and stronger low-level northwesterly winds towards the front, in the presence of DIs. This finding is consistent with Tilinina et al. (2018), who recently highlighted the importance of the North American high located southwest of the turbulent heat fluxes maxima in the Western North Atlantic, in addition to a cyclone to their north east. In contrast, when DIs occur with fronts over the Pacific (Fig. S1), the northeastern anticyclone is enhanced mainly for the strongest fronts, while the south western anticyclone is enhanced in particular for weak fronts.
Since strong surface wind gusts and heat fluxes into the atmosphere are expected to be influenced by the presence of DIs, here we examine their composites for the three trailing front intensities (Fig. 5). First, fronts without DIs show monotonic increase of 10-m wind gusts with front intensity (Fig. 5b, e, h), peaking near the northeastern part of the front, at the composite centre, and to its west, i.e., where the cold conveyor belt is typically located (Hewson and Neu 2015; Raveh-Rubin and Wernli 2016). In addition, near strong and medium fronts, strong wind gusts show a secondary maximum over relative coordinates \(10^{\circ }\hbox {E}, 10-15^{\circ }\hbox {N}\) (Fig. 5b, e). This secondary maximum is less pronounced for the Pacific front sets (Fig. S2). Sensible heat flux from the ocean to the atmosphere is expected to peak in the cold sector of the cyclone, as observed clearly in all the composite sets. A monotonic increase of sensible heat flux with front intensity is evident, possibly related to the colder air in the cold sector (Fig. 4). The composite latent heat flux does not follow linearly the front intensity. Considering the mean environmental conditions in the presence of DIs, a mean 10-m gusts increase of up to 3 m/s is evident west of the composite centre, roughly co-located with the increase in the magnitude of sensible heat flux into the atmosphere. Although these fields vary with front intensity on average, the additional DI occurs with the same mean increase in wind gusts independent of the front intensity. Rather differently, latent heat flux near weak fronts has a larger relative difference with the presence of DIs (of more than \(60\hbox { W m}^{-2}\)), compared to the climatological addition in the presence of DIs near strong fronts (reaching \(40\hbox { W m}^{-2}\)), possibly because of the warmer surface temperature near weak fronts.
The occurrence of precipitation in the frontal environment is expected to generally increase with front intensity (Catto and Pfahl 2013; Catto et al. 2015). In fact, the correlation between front intensity and precipitation was found to increase in the presence of DIs, in some regions more than two-fold, compared to fronts occurring without DIs (Catto and Raveh-Rubin 2019). It is still unclear to what degree the precipitation increase in the presence of DIs is a mere result of increased front intensity in such cases. To delineate these relationships, and learn about the spatial distribution of precipitation, here we examine composites of total 6-h precipitation and convective precipitation for the different front sets (Figs. 6, S3). Indeed, weaker precipitation is observed around the weak fronts, compared to stronger fronts in both ocean basins. Yet, no robust differences in mean precipitation amounts are seen between medium and strong front intensities. However, when occurring together with DIs, precipitation is enhanced for all front categories, with the most pronounced differences for medium front intensities. Interesting is that the convective fraction of the precipitation enhancement in the presence of DIs is higher for weaker fronts compared to medium, and further compared to strong fronts. Furthermore, the precipitation enhancement with DIs occurs to the north of weak trailing fronts, whereas for stronger fronts it has clear hook shape. South of the fronts, possibly on their warm side, a reduction of precipitation occurs in the presence of DIs, most notably south of weak fronts. This may be a result of the local subsidence conditions within the enhanced anticyclone.