Can we trust climate models to realistically represent severe European windstorms?

Abstract

times, and horizontal and vertical resolutions. These runs are then compared to re-analysis data. The main conclusions from this work are: (a) objectively identified cyclone tracks are represented satisfactorily by most hindcasts; (b) sensitivity to vertical resolution is low; (c) cyclone depth is systematically under-predicted for a coarse resolution of T63 by both climate models; (d) no systematic bias is found for the higher resolution of T127 out to about three days, demonstrating that climate models are in fact able to represent the complex dynamics of explosively deepening cyclones well, if given the correct initial conditions; (e) an analysis using a recently developed diagnostic tool based on the surface pressure tendency equation points to too weak diabatic processes, mainly latent heating, as the main source for the under-prediction in the coarse-resolution runs. Finally, an interesting implication of these results is that the too low number of deep cyclones in many freerunning climate simulations may therefore be related to an insufficient number of storm-prone initial conditions. This question will be addressed in future work.

Drivers for the deepening of severe European windstorms and their impacts on forecast quality

“…Averaged over all 39 storms, the deterministic forecasts are 2.4 hPa, −0.2° latitude and −0.5° longitude different from the analysis at 24 h lead time. These results are consistent with the work of Froude et al (), who examined a larger set of storms including many weak storms.The results from Trzeciak et al () also indicate that storm intensity is generally underpredicted, with a strong dependence on model resolution.…”

“…The storms were tracked using the method described in detail by Trzeciak et al (). The first steps are to identify a minimum in mean sea‐level pressure (MSLP) and an associated maximum in the vorticity at 850 hPa.…”

“…To close the equation, correction terms for mass loss (gain) through precipitation (evaporation) ( EP ) and for geopotential tendencies at the upper boundary (d Phi ) need to be taken into account; these are usually small (see discussion in Knippertz et al , ). Any remaining Residual is an indication of errors due to vertical integration, using finite differences or model errors (Knippertz et al , ; Trzeciak et al , ). Finally, the d Temp term can be split into contributions from horizontal temperature advection ( horiz ), vertical motion ( vert ) and diabatic heating ( diab , diabres ).…”

“…rainfall along fronts) and their impacts on the generation of potential vorticity are not well represented (Jung et al ; Willison et al ). However, diagnosing this effect quantitatively is challenging and requires new approaches, as, for example, shown by Trzeciak et al (), who applied the PTE tool developed by FPPK12 to NWP experiments with climate models.…”

Attribution of Extreme Weather Events in the Context of Climate Change

“…However, both van Oldenborgh et al (2015) and Wild et al (2015) use observational analysis to challenge the suggestion (e.g., Huntingford et al, 2014) that the PREPUBLICATION COPY intense storminess over the UK in winter 2013/2014 was driven by anomalously warm Pacific SSTs, which might have an anthropogenic component. Trzeciak et al (2014) suggest that although current global climate models generally under-represent the intensity of extratropical cyclones due to insufficient latent heat release, once the horizontal resolution is finer than about 100 km they should be adequate, and that the systematic biases will then mainly involve storm track location. Seiler and Zwiers (2015b) found that resolution is not correlated with explosive storm intensity across the CMIP5 ensemble, but note that competing effects of vertical resolution and model physics inhibit strong interpretation of that result.…”