Acceleration of high winds in idealised

simulations of extratropical

cyclones.

Tim P. Slater

Prof. David M. Schultz

Prof. Geraint Vaughan 3 Jan 2012 UTC 1114

Widespread residential damage,...

(BBC)

Windstorm Klaus (2009)> 1 billion euros

…. blown down trees and flooding.

(presseurop)

Windstorm Xynthia (2010)

65 deaths

(The Great Storm of 1987: 20 year retrospective)

www.ideal-homes.org.uk

(presseurop)

What is the distribution of high winds in an extratropical cyclone?

Why does this distribution exist?

Start simple!

● Initialisation by 1K perturbation to jet on a f-plane in thermal wind balance .

● Idealised, baroclinic wave using Weather Research and Forecasting (WRF) model.

Horizontal Windspeed (thick black)Temperature (thin black)

Distance (km)

Pres

sure

(hPa

)

● 20 km grid-spacing

● Dry

● Where are the high winds?

Start simple!

Day 4.75

Horizontal wind (shaded)

Pressure (thick black)

Potential temperature (thin black)

Potential temperature (thin black)

ms-1

Distance (km)

Distance (km)

Day 5.25

Horizontal wind (shaded)

Pressure (thick black)

Potential temperature (thin black)

Potential temperature (thin black)

ms-1

Distance (km)

Distance (km)

Day 5.75

Horizontal wind (shaded)

Pressure (thick black)

Potential temperature (thin black)

Potential temperature (thin black)

ms-1

Distance (km)

Distance (km)

Day 6.25

Horizontal wind (shaded)

Pressure (thick black)

Potential temperature (thin black)

Potential temperature (thin black)

ms-1

Distance (km)

Distance (km)

Windspeed increases before pressure gradient increases.Peak in windspeeds after peak in maximum pressure gradient.

MinimumCentralPressure

MaximumPressureGradient

MaximumHorizontalWind

The momentum equation tells us how high winds develop.

Can create acceleration/force vectors from model output. To understand increases in windspeed look at along-flow accelerations.

VelocityPressure gradient force Coriolis forceVertical advectionHorizontal advectionFriction

The momentum equation tells us how high winds develop.

Day 4.75

System-relative wind (shaded)

Potential temperature (thin black)

Vertical velocity (thick black, negative dotted)

Along-flow acceleration by pressure gradient (blue)

ms-1

L

Distance (km)

Day 4.75System-relative wind (shaded)

Potential temperature (thin black)

Vertical velocity (thick black, negative dotted)

Along-flow acceleration by pressure gradient (blue) and vertical advections (red)

ms-1

L

Distance (km)

Day 5.25

System-relative wind (shaded)

Potential temperature (thin black)

Vertical velocity (thick black, negative dotted)

Along-flow acceleration by pressure gradient (blue)

ms-1

L

Distance (km)

Day 5.25System-relative wind (shaded)

Potential temperature (thin black)

Vertical velocity (thick black, negative dotted)

Along-flow acceleration by pressure gradient (blue) and vertical advections (red)

ms-1

L

Distance (km)

Day 5.75

System-relative wind (shaded)

Potential temperature (thin black)

Vertical velocity (thick black, negative dotted)

Along-flow acceleration by pressure gradient (blue)

ms-1

L

Distance (km)

Day 5.75System-relative wind (shaded)

Potential temperature (thin black)

Vertical velocity (thick black, negative dotted)

Along-flow acceleration by pressure gradient (blue) and vertical advections (red)

ms-1

L

Distance (km)

Day 6.25

System-relative wind (shaded)

Potential temperature (thin black)

Vertical velocity (thick black, negative dotted)

Along-flow acceleration by pressure gradient (blue)

ms-1

L

Distance (km)

Day 6.25System-relative wind (shaded)

Potential temperature (thin black)

Vertical velocity (thick black, negative dotted)

Along-flow acceleration by pressure gradient (blue) and vertical advections (red)

ms-1

L

Distance (km)

Day 5.75

Horizontal wind (shaded)

Pressure (thick black)

Potential temperature (thin black)

Potential temperature (thin black)

ms-1

Distance (km)

Distance (km)

Day 5.75 Windspeed (ms-1) Pressure (hPa)

Pressure Gradient(ms-2) Vertical Vel. (cms-1)

Colours – Starting heights

Red850-900 hPa

Pink900-950 hPa

Green950-1000 hPa

Day 5.75

Horizontal wind (shaded)

Pressure (thick black)

Potential temperature (thin black)

Potential temperature (thin black)

ms-1

Distance (km)

Distance (km)

Day 5.75 Windspeed (ms-1) Pressure (hPa)

Pressure Gradient(ms-2) Vertical Vel. (cms-1)

Colours – Starting heights

Green950-1000 hPa

Orange1000-1050hPa

Momentum equations are a useful tool for studying the development of the high winds within an extratropical cyclone.

Pressure gradient creates momentum ahead of warm front.

Vertical advections transport high wind from aloft that is accelerated on descent as a result of the pressure gradient.

Conclusions

The Essential Model Stats SlideModel WRF-ARW v3.4 (Skamarock et al.

08)Grid-spacing 20 km

Number of vertical levels 40

Side boundary conditions Periodic in x, symmetric in y

Lower boundary condition Meridional variation with latitude

Boundary layer YSU planetary boundary layer scheme (Hong et. al 2006)

Surface layer MM5 similarity surface physics (Skamarock et. al 08)

Surface fluxes Heat and momentum.

Microphysics None

Radiation scheme None

Cumulus parametrisation None

Land-surface model None