Jennifer Owen*, Tomek Trzeciak and Peter Knippertz*Contact: ee09jso@leeds.ac.uk; School of Earth and Environment, University of Leeds, LS2 9JT, UK

Severe European Cyclones: A Storm-Prone Situation Perspective

Introduction• Intense winter storms are the most damaging weather phenomenon to afflict

Europe. Therefore, forecasting them in weather and climate models is a priority. • The SEAMSEW project, financed through the AXA Research fund, aims to

assess uncertainties in future projections of severe storms. • We can separate sources of uncertainty using the idea of a large-scale ‘storm-

prone situation’ (SPS), and evaluating their representation in climate models. • First, we identified 31 severe European windstorms, based on the Storm Severity

Index, which calculates how unusual the wind speed is (Leckebusch et al., 2008).

Conclusions

• Selected 31 historic, severe European windstorms.• Identified 4 jet stream types. • Proposed new approach to storm-prone situations,

using existing idea of Eady Growth Rate.• Included effects of moisture.• Mixed success in detecting storms. • Success potentially related to jet stream type.

Storm-Prone Situations

Four Versions of Growth RateQG dry

•Shear•Dry stability•Coriolis parameter only•Lindzen & Farrell (1980).

QG moist•Shear•Moist stability•Coriolis parameter only•Whitaker and Davis (1994)

SG dry•Shear•Dry stability•Vorticity•Emanuel, Fantini & Thorpe (1987)

SG moist•Shear•Moist stability•Vorticity•Emanuel, Fantini & Thorpe (1987)

Emanuel, K., M. Fantini, and A. Thorpe, 1987: Baroclinic instability in an environment of small stability to slantwise moist convection. Part I: Two-dimensional models. Journal of the Atmospheric Sciences, v. 44, pp. 1559–1573.Leckebusch, G., D. Renggli, and U.Ulbrich, 2008: Development and application of an objective storm severity measure for the northeast Atlantic region. Meteorologische Zeitschrift, vol. 17 (5), pp. 575–587.Lindzen, R. and B. Farrell, 1980: A simple approximate result for the maximum growth rate of baroclinic instabilities. Journal of the Atmospheric Sciences, vol. 37, pp.1648–1654.Whitaker, J. and C. Davis, 1994: Cyclogenesis in a saturated environment. Journal of the Atmospheric Sciences, vol. 51, pp. 889–907.

MethodUse Eady Growth Rate to quantify baroclincity.

Use quasi-geostrophic (QG) and semi-geostrophic (SG) equations to describe atmosphere

Including effect of moisture means four versions identified.

Calculate the four versions everywhere

Average over the box (35-65oN, 40oE to 10oE) →

Search for high peaks, followed by sudden drops.

KlausQG dryQG moistSG drySG moist

KyrillQG dryQG moistSG drySG moist

XynthiaQG dryQG moistSG drySG moist

QG dryQG moistSG drySG moist

Emma

Figure 2: Showing 4 versions of growth rate (σ) for 1 October 2006 to 31 March 2007.

Figure 3: Showing 4 versions of growth rate (σ) for 1 October 2007 to 31 March 2008.

Figure 4: Showing 4 versions of growth rate (σ) for 1 October 2008 to 31 March 2009.

Figure 5: Showing 4 versions of growth rate (σ) for 1 October 2009 to 31 March 2010.

Indentifying Storms• Moisture accelerates growth• SG moist is an upper limit, assuming

atmosphere totally saturated everywhere in the domain

• Expect σ to peak just before the storm develops, and drop as the baroclinic energy is removed.

• Objectively identify peaks (shown as stars on plots) as exceeding 98th percentile once in 48 hour period.

• Here, one storm of each jet stream type is presented.

• e-folding times typically 0.5-1.0 days. • Klaus: behaves as expected.• Emma: peak apparent in some

versions, not all. • Kyrill: large peak slightly after. • Xynthia: no peak. Very different jet

stream configuration to the others.• Overall, mixed results as to whether

there is a peak before every storm. • Might be related to jet stream type.

Storm Jet Type

Kyrill Cross early

Emma Edge

Klaus Cross late

Xynthia Split

σ

σ

σ

σ

Wind speed (m/s) at 300hPa

KlausKyrill

XynthiaEmma

Jet Stream Types• Once 31 storms selected, examined analysis

data and tracked each storm. • Plotted jet stream in sections that move along

with the track of each storm. • Four categories emerged.• Here, one example of each is presented