Lecture 22: Midlatitude Cyclones (Ch 10)

1. Lecture 22: Midlatitude Cyclones (Ch 10) • more about divergence • its connection with vorticity: the vorticity theorem • vorticity plots on the upper charts… divergence downstream from troughs • ideal upper support for surface storm • shortwaves • rough correlation of map type (zonal/meridional) and weather conditions • the Alberta clipper

2. Divergence aloft can sustain or deepen sfc low despite low-level convergence • recall horiz. divergence can be thought of as area expansion, horiz. convergence as area contraction • mathematically, con/divergence is defined in terms of velocity gradients • physically, two contributions: speed divergence diffluence

3. Vorticity tendency & horiz. divergence aloft: “Vorticity Theorem” • rate of change of absolute vorticity following an air parcel is: (p286) • now ordinarily so • decreasing vorticity (l.h.s. negative)  positive divergence aloft • happens in trough exit region • increasing vorticity (l.h.s. pos.)  negative divergence convergence • happens in ridge exit region

4. Changing relative vorticity as parcel moves through transition zones Fig. 10-6 (we’ll neglect changes in earth vorticity – valid if the wave amplitude is small) Vorticity decreases following parcel Vorticity increases following parcel

5. Vorticity of the mid-tropospheric flow so important meteorologists like to display it on upper winds analysis… Fig. 10-8

6. Illustrates the ideal support of upper wave for sfc storm • sfc convergence + upper divergence implies ascent… cloud + precip • sfc pressure trend result of a subtle imbalance • this pattern reliably valid for intense storms • topography complicates the pattern • Rossby waves not the only upper waves – short waves too Fig. 10-7

7. Short (Baroclinic) Waves (“impulses”) - - - - • shorter waves associated with strong T & p gradients and strong temperature advection • cold advection aloft  sink • warm advection aloft  lift • move more rapidly eastwards • deepen when approach longwave trough con div (Sec. 10-3) “baroclinic instability”

8. Here a wave in the flow aloft configured in relation to isotherms such that temperature advection is occurring (strongest around shortwaves)… cold advection aloft  sink warm advection aloft  lift

9. so the existence of the upper waves play a role in initiating and maintaining or deepening the surface storm • this role being exerted via the locations of the divergence & convergences zones • but conversely surface conditions, esp. the temperature distribution, exert an influence on the pressure pattern aloft, by controlling the pressure lapse rate • for the troughs in pressure aloft tend to develop “behind” (ie. upstream from) surface cold fronts (ie. low-level baroclinic zones) where the cold air guarantees p/z is numerically large, Fig. 10-10 (see also Fig. 10-9)

10. Here a mature cyclone is well supported by upper divergence, and moving approximately in the direction of the upper stream

11. But the motion has taken it away from the zone of upper divergence and the upper convergence will be tending to weaken the storm (This is an idealized…in the case shown the upper flow pattern was static)

12. Zonal pattern Meridional pattern

13. Generalized inferences about large scale weather in relation to pattern of the upper winds • upper patterns often persist, sometimes for longer than weeks, and are associated with distinct anomalies (eg. a persistent drought) • a zonal flow over a wide region lessens the likelihood of storm development and is indicative of relatively uniform temperature • conversely, highly meridional flow reflects the existence of temperature non-uniformity and implies favourable circumstances for storms • the upper flow also gives us a useful idea for storm motion: storms tend to move along the direction of the 500 mb flow, but at about half the speed

14. The storm that developed early Saturday is an example of the “Alberta Clipper”… here it is 3 days later

15. … chilling off the Great Lakes region • notice the uniformity of temperature across the prairies in its wake 12Z Tues 31 Oct 2006

16. its wrong to take a snapshot of the upper flow and think it gives trajectory of storm (because upper pattern may change) • but look how nicely this 500 mb snapshot matches the path of the Alberta Clipper • and the 850 mb baroclinic zone (cold front) is positioned relative to the 500 mb trough in about the way stated in the text • meridional pattern • main stream well south of border 12Z Tues 31 Oct 2006

17. its wrong to take a snapshot of the upper flow and think it gives trajectory of storm (because upper pattern may change) • but look how nicely this 500 mb snapshot matches the path of the Alberta Clipper • and the 850 mb baroclinic zone (cold front) is positioned relative to the 500 mb trough in about the way stated in the text • (this chart shows the vorticity pattern, highlighting the trough axis to make it easier to check the above claim) 12Z Tues 31 Oct 2006

18. Here’s the storm at 12Z today Note the “warm” moist air advecting off Hudson’s Bay 12Z Wed 1 Nov. 2006

19. A new paper (in press) states: “These cyclones generally move SE from the lee of the Cdn Rockies toward or just north of Lk. Superior… with less than 10% of cases tracking S. of the Great Lakes” Winter Ab. Clipper cold, therefore dry, therefore light snowfall

20. From: “A synoptic-climatology and composite analysis of the Alberta clipper”, by Thomas & Martin. Submitted to Weather & Forecasting, July 2005