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Acceleration of high winds in idealised simulations of extratropical cyclones. Tim P. Slater Prof. David M. Schultz Prof. Geraint Vaughan 3 Jan 2012 UTC.

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Presentation on theme: "Acceleration of high winds in idealised simulations of extratropical cyclones. Tim P. Slater Prof. David M. Schultz Prof. Geraint Vaughan 3 Jan 2012 UTC."— Presentation transcript:

1 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

2 Widespread residential damage,... (BBC)

3 …. blown down trees and flooding. …. blown down trees and flooding. (presseurop) (The Great Storm of 1987: 20 year retrospective) www.ideal-homes.org.uk (presseurop)

4 What is the distribution of high winds in an extratropical cyclone? Why does this distribution exist?

5 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) Pressure (hPa)

6 ● 20 km grid-spacing ● Dry ● Where are the high winds? Start simple!

7 Day 4.75 Horizontal wind (shaded) Pressure (thick black) Potential temperature (thin black) ms -1 Distance (km) Distance (km)

8 Day 5.25 Horizontal wind (shaded) Pressure (thick black) Potential temperature (thin black) ms -1 Distance (km) Distance (km)

9 Day 5.75 Horizontal wind (shaded) Pressure (thick black) Potential temperature (thin black) ms -1 Distance (km) Distance (km)

10 Day 6.25 Horizontal wind (shaded) Pressure (thick black) Potential temperature (thin black) ms -1 Distance (km) Distance (km)

11 Windspeed increases before pressure gradient increases. Peak in windspeeds after peak in maximum pressure gradient. Minimum Central Pressure Maximum Pressure Gradient Maximum Horizontal Wind

12 The momentum equation tells us how high winds develop.

13 Can create acceleration/force vectors from model output. To understand increases in windspeed look at along-flow accelerations. Velocity Pressure gradient force Coriolis force Vertical advection Horizontal advection Friction

14 The momentum equation tells us how high winds develop.

15 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)

16 Day 4.75 System-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)

17 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)

18 Day 5.25 System-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)

19 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)

20 Day 5.75 System-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)

21 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)

22 Day 6.25 System-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)

23 Day 5.75 Horizontal wind (shaded) Pressure (thick black) Potential temperature (thin black) ms -1 Distance (km) Distance (km)

24 Day 5.75 Windspeed (ms -1 )Pressure (hPa) Pressure Gradient(ms -2 )Vertical Vel. (cms -1 ) Colours – Starting heights Red 850-900 hPa Pink 900-950 hPa Green 950-1000 hPa

25 Day 5.75 Horizontal wind (shaded) Pressure (thick black) Potential temperature (thin black) ms -1 Distance (km) Distance (km)

26 Day 5.75 Windspeed (ms -1 )Pressure (hPa) Pressure Gradient(ms -2 )Vertical Vel. (cms -1 ) Colours – Starting heights Green 950-1000 hPa Orange 1000-1050hPa

27 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

28 The Essential Model Stats Slide ModelWRF-ARW v3.4 (Skamarock et al. 08) Grid-spacing20 km Number of vertical levels40 Side boundary conditionsPeriodic in x, symmetric in y Lower boundary conditionMeridional variation with latitude Boundary layerYSU planetary boundary layer scheme (Hong et. al 2006) Surface layerMM5 similarity surface physics (Skamarock et. al 08) Surface fluxesHeat and momentum. MicrophysicsNone Radiation schemeNone Cumulus parametrisationNone Land-surface modelNone

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