the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 15 Apr 2026
Future Changes in Severe Frontal Precipitation Events over Europe and Their Drivers
Abstract. Atmospheric fronts are closely linked to extreme precipitation across the mid-latitudes, which is projected to intensify in many regions under global warming. Understanding the physical drivers of these changes is essential to improve confidence in climate projections. Here, we analyze projected changes in seasonal heavy and extreme frontal precipitation events over Europe using the CMIP6 and EURO-CORDEX ensembles, combining event frequency analysis with frontal composite cross-sections to assess the changes of the underlying thermodynamic and dynamic processes. We find that the number of heavy frontal precipitation events increases by up to 50 % per degree of global warming, while extreme events are projected to more than double per degree. Large-scale circulation changes account for most regional reductions in frontal extremes, but contribute only weakly to the widespread increases. Thermodynamic changes, however, dominate the intensification of extremes. Increases in specific humidity are the primary driver of more intense events, while changes in the frontal circulation are minimal, likely because a more stable atmosphere counteracts potential strengthening from enhanced latent heat release.
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RC1: 'Comment on egusphere-2026-1712', Anonymous Referee #1, 23 May 2026
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AC1: 'Reply on RC1', Armin Schaffer, 08 Jun 2026
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We thank the reviewer for the constructive and thoughtful comments. We agree that several aspects required clearer wording and discussion in the manuscript. We have revised the manuscript accordingly to better clarify the scope and interpretation of our analysis.
Concerning the definition of dynamic change, we evaluate changes in the dynamical drivers in two complementary ways. First, large-scale circulation changes are quantified through changes in frontal frequency. These changes reflect modifications in the mean synoptic environment over Europe, including shifts in storm tracks and regions of cyclogenesis. Second, within the composite analysis, dynamical changes refer to changes in the resolved frontal circulation, quantified through the mesoscale convergence and vorticity diagnostics.
Our intention was not to interpret these metrics as a complete dynamical diagnosis of frontogenesis tendencies or QG forcing, although they are physically related to such processes. We agree that a comprehensive attribution of frontal intensification would require explicit analysis of the frontogenesis equation, including deformation, tilting, and diabatic heating terms. However, such an analysis is beyond the scope of the present multi-model climatological study and would be difficult to apply consistently across CMIP6 and EURO-CORDEX simulations with differing spatial resolutions and only 6-hourly output available.
We further note that the focus of this study is on dynamical changes directly relevant for projected precipitation changes. While frontogenesis quantifies the strengthening of horizontal temperature gradients and is closely linked to frontal dynamics, it is not necessarily a direct measure of the circulation changes governing precipitation intensity itself. Our diagnostics therefore focus on resolved frontal circulation characteristics that are more directly connected to frontal ascent and precipitation formation. Our intention was not to argue that diabatic processes are dynamically unimportant, but rather that the resolved mesoscale circulation metrics analyzed here do not exhibit a robust strengthening comparable to the very consistent thermodynamic amplification seen in humidity-related fields.
Concerning the discussion of atmospheric stability, we agree that moist effective stability is particularly relevant in the warm sector of frontal systems and that dry static stability alone does not fully characterize frontal ascent dynamics. Our analysis is based on front-relative composite cross-sections rather than zonal-mean quantities and therefore retains the frontal structure, i.e. the warm and cold sectors. Nevertheless, we agree that our diagnostics do not permit a complete assessment of effective moist stability changes. We therefore softened the interpretation and now state that the comparatively weak circulation changes are “consistent with” a stabilizing influence from enhanced upper-tropospheric warming, rather than demonstrating a direct causal suppression of ascent.
We agree that substantial event-to-event variability exists within frontal systems and may obscure localized circulation responses in composite means. However, the purpose of the composite framework is to identify robust and systematic changes that emerge across many frontal events and models. While individual events may exhibit strong dynamic signals, the absence of a coherent strengthening in the ensemble composites suggests that such responses are not a dominant feature of the projected changes. We further note that the mesoscale circulation changes shown in Figs. 6 and 7 are qualitatively consistent across ensembles, particularly the weak increases in DJF and the tendency toward reduced mesoscale circulation in JJA. While the magnitude differs between CMIP6-LR, CMIP6-HR, and EURO-CORDEX, such quantitative differences are expected because mesoscale convergence and vorticity are strongly resolution dependent, as discussed in Schaffer et al. (2025). Our interpretation is therefore intentionally qualitative and focuses on the robustness of the sign and spatial structure of the response rather than quantitative agreement in amplitude.
We further agree that CAPE and CIN may provide additional insight into convective instability changes. However, our focus here is on large-scale frontal precipitation and resolved frontal circulation characteristics. In frontal systems, precipitation is often strongly controlled by dynamically forced ascent along the frontal surface, which is not fully captured by traditional CAPE and CIN diagnostics. CAPE and CIN diagnostics are additionally sensitive to temporal and vertical resolution, parcel selection, and convective parameterization, which complicates a consistent comparison across CMIP6 and EURO-CORDEX ensembles. We therefore consider such diagnostics beyond the scope of the present process-oriented climatological analysis, but agree that they would be valuable for future dedicated studies of frontal convection in a warming climate.
Finally, we expanded the discussion on precipitation efficiency and moisture availability. In particular, we now emphasize that while increases in specific humidity robustly favor stronger precipitation, future changes in precipitation efficiency and moist dynamical feedbacks remain more uncertain. Reductions in relative humidity in some regions and seasons may influence condensation efficiency and moisture availability within frontal systems, potentially contributing to the comparatively weak circulation responses.
Citation: https://doi.org/10.5194/egusphere-2026-1712-AC1
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AC1: 'Reply on RC1', Armin Schaffer, 08 Jun 2026
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Title: Future Changes in Severe Frontal Precipitation Events over Europe and Their Drivers
Authors: Schaffer et al.
Manuscript submitted to Weather and Climate Dynamics
This article analyses changes in frontal precipitation using CMIP6 and CORDEX data. Front detection is based on equivalent potential temperature and the 850-hPa TFP parameter, with precipitation linked to identified fronts using a 300 km distance criterion. The definitions of 'strong' and 'extreme' precipitation are based on percentiles and vary between models and periods.
The changes appear to align with previous findings and expectations. The change patterns depict the well-known projected poleward shift of the storm tracks, specifically a triple pattern of change with intensification in an elongated band across the UK, at least during some season. This band is flanked by regions with reduced changes to the north and south of it.
The frontal composites show that the changes align with the hypothesis that relative humidity changes are minimal, and that CC scaling suggests a disproportionate increase in specific humidity in the warm sector compared to the cold sector, which ultimately causes the observed increase in horizontal gradients of equivalent potential temperature. The authors conclude that the observed changes are driven by thermodynamic rather than dynamic changes and hypothesis that changes in the stability offset changes in increased vertical motion from latent heat release.
The results are solid and supported by the findings. The methods are accepted, although the numerous thresholds could have been discussed more broadly given the associated uncertainties. The debate surrounding the definition of fronts is hinted at and could be expanded upon, particularly since the dynamically more relevant potential temperature fronts appear to remain unchanged in a warmer climate.
My main concern is the distinction between thermodynamic and dynamic changes, and the hypothesis that dynamic changes are suppressed by changes in tropospheric stability.
Firstly, the definition of 'dynamic change' is somewhat open to interpretation. Does it refer to a potential change in large-scale deformation that drives frontogenesis? Or is it simply a change in frontal frequency? Or is it vertical motion obtained from inverting the QG omega equation? If the authors argue that dry dynamic changes do not underlie the observed changes, I suggest computing the leading-order tendencies of frontogenesis and compare those to the frontogenesis tendency due to latent heating to quantitatively substantiate this argument. Previous studies argue that frontogenesis due to latent heating can be in the same order of magnitude as that due to tilting and deformation (Igel and Heever, QJ, 140, 139-150, 2013) and to show that dynamic changes are indeed small (assuming that you refer to dynamics in terms of large-scale deformation and tilting) than a more detailed analysis of their changes in the frontogenesis equation would be helpful.
Second, the hypothesis that static stability changes suppress dynamic changes is interesting, but moisture seems not considered in this argument. The dry static stability matters most for the cold sector, where an increase would weaken downward motion. In the warm sector what matters is the effective stability that takes moisture into account and it has been shown that this asymmetry in vertical motion across fronts indeed increases because of a reduction in effective static stability in the warm sector, which tends to increase vertical motion in this quadrant of the cyclone (O'Gorman, JAS, 68, 75-90, 2011). (Dry) static stability alone, in particular if zonally averaged, might obscure changes in the individual sectors of the cyclone.
Composites:
Fig. 4–9: Is it possible that substantial variability within each sample masks any clear circulation signal in the composite mean? Section 2.5 could benefit from an example illustrating how the cross sections are computed.
Is the increase in specific humidity resulting in increased CAPE? Is CIN reducing? Both fields could be added to the composites with a small lag relative to the precipitation event.
Results:
L265: Due to the model inconsistency in mesoscale changes (see Figures 6 and 7), I would not conclude that dynamic changes are of secondary importance. The change is much more uncertain than the increase in humidity and no conclusion seems possible. This fits into the bigger picture that circulation changes are often much more uncertain in projections than changes in temperature or humidity.
A few more thought could be added about unclear changes in precipitation efficiency in the future and moisture availability.