1. A kinetic-energy budget of the windstorm « Klaus » (January 24, 2009)‏
G. Rivière, A. Joly
CNRM/GAME, Météo-France/CNRS

2. Presentation of « Klaus » and its damages
Trajectory
Image from Meteosat 9 (01/24/2009, 06 UTC)
km/h
Wind speed maximum
Strongest windstorm in France since Christmas 1999 storms « Lothar » and « Martin ».
« Klaus » hit Southwestern France as well as Northern Spain. Trajectory close to the second storm of 1999 « Martin ».

3. Comparis° between « Klaus » & « Martin » (Dec 26, 1999)‏
Data: operational analysis of the 4DVAR Météo-France ARPEGE system
Shading: low-freq (>6d) wind speed (300 hPa) blue: high-freq (<6d) relative vorticity (300 hPa) black: high-freq (<6d) relative vorticity (900 hPa)
« Klaus »
« Martin »
Even though they have similar trajectories, the timing of the interaction with upper-level structures is quite different.
High-frequency disturbances have a barotropic structure over the whole troposphere at 25°W for « Klaus » and at 0°W for « Martin »

4. « Klaus »
« Martin »
EKE ( hPa)
Baroclinic conversion
Rel vor maximum (900 hPa)
For « Klaus », the baroclinic interaction occurs 18H before reaching the coasts whereas for « Martin », this happens at the same time.

5. High-freq kinetic-energy redistribution within “Klaus”
Blue: HF wind speed
: LF wind speed : HF wind speed  total wind speed
At the end of the life cycle of « Klaus », a large part of the high-freq kinetic energy is redistributed south of the vortex maximum !
shading: LF wind speed
Shading: total wind speed

6. High-frequency kinetic-energy budget
Each variable decomposed into a high- (primes) and low- (subscript m) frequency part
High-frequency kinetic energy per unit mass (EKE)
Decomposition similar to Orlanski and Katzfey (1991) and Rivière and Joly (2006)
Residu
Pressure Work
Internal baroclinic conversion
Horizontal and vertical ageostrophic geopotential fluxes
Reynolds stress

7. Kinetic-energy budget (18 UTC Jan 23, 500-900hPa)
Internal convers° + vert ageos fluxes
Horizontal ageostrophic geop fluxes
Rel Vor (900hPa)
Pressure Work
Pressure Work+Reynolds stress
The pressure work dominates over the Reynolds stress term
Horiz ageos geopotential fluxes redistribute EKE southwestward

8. Kinetic-energy budget (06 UTC Jan 24, 500-900hPa)
Internal convers° + vert ageos fluxes
Horizontal ageostrophic geop fluxes
Rel Vor (900hPa)
Pressure Work
Pressure Work+Reynolds stress
The pressure work dominates over the Reynolds stress term
Horiz ageos geop fluxes redistribute EKE south of the vortex and are the only term that may explain EKE increase south of it !

9. zonal average in a band of longitude 10° centred on the vortex
Vertical cross section of the ageostrophic vertical geopotential fluxes
zonal average in a band of longitude 10° centred on the vortex
Ageostrophic vertical geopotential fluxes tend to redistribute EKE downward from 500 hPa to 900 hPa !

10. Meteosat infrared image
Conclusions / outlook
Strong winds at the end of « Klaus » life cycle are due to a favorable combination of low-freq westerlies and high-freq kinetic-energy redistribution south of the system.
Horizontal and vertical ageost geop fluxes tend to redistribute within the system eddy kinetic energy southward and downward !!!
Meteosat infrared image
Existence of multiple bands of cold air within the cloud head (similar to sting jets properties)
Is this EKE redistribution by ageost geop fluxes a synoptic-scale feature of a sting jet ?
Confirmation of the results by looking at other data (e.g., ecmwf)