SO441 Synoptic Meteorology Extratropical cyclones Visible satellite image 26 Oct 2010. Low pressure 955.2 mb. Image courtesy NASA Cloud pattern typically.

Presentation on theme: "SO441 Synoptic Meteorology Extratropical cyclones Visible satellite image 26 Oct 2010. Low pressure 955.2 mb. Image courtesy NASA Cloud pattern typically."— Presentation transcript:

SO441 Synoptic Meteorology Extratropical cyclones Visible satellite image 26 Oct 2010. Low pressure 955.2 mb. Image courtesy NASA Cloud pattern typically associated with an extratropical cyclone (Image courtesy NASA)

Extratropical cyclones What is an extratropical cyclone? – Surface low pressure center associated with upper-level trough – Most commonly found between 15°-65° latitude Where are the extratropical cyclones here?

Where do extratropical cyclones exist? Cyclone days per winter. Figure from Ulbrich et al. (2009) Note: the extratropical cyclone track paths tend to follow the areas of greatest surface temperature gradients (land/sea contrasts). This is not by accident.

Increase of vorticity of an extratropical cyclone along a front From the vorticity equation: – A. Change in the absolute vorticity at a point on the Earth’s surface – B. Horiztonal advection of absolute vorticity – C. Vertical advection of absolute vorticity – D. Tilting of absolute vorticity – E. Stretching of absolute vorticity – F. Frictional dissipation of absolute vorticity Along a front: – Neglect the tilting and friction terms (D and F) (when we say “neglect” in meteorology, we really mean “realize that the term is usually small compared to the other terms”) – Re-write the equation in a Lagrangian framework (combine A, B, and C): Final result: the change in vorticity following an air parcel is the result of the stretching of vorticity Eqn. 1, without D & F (1) You learned this in SO335 (Methods) as the “material derivative”, !

Increase of vorticity of an extratropical cyclone along a front From the vorticity equation: – A. Change in the absolute vorticity at a point on the Earth’s surface – B. Horiztonal advection of absolute vorticity – C. Vertical advection of absolute vorticity – D. Tilting of absolute vorticity – E. Stretching of absolute vorticity – F. Frictional dissipation of absolute vorticity Along a front: – Neglect the tilting and friction terms (D and F) (when we say “neglect” in meteorology, we really mean “realize that the term is usually small compared to the other terms”) – Re-write the equation in a Lagrangian framework (combine A, B, and C): Final result: the change in vorticity following an air parcel is the result of the stretching of vorticity Eqn. 1, without D & F (1) You learned this in SO335 (Methods) as the “material derivative”, !

Increase of vorticity of an extratropical cyclone along a front Still along a frontal boundary: – Separate variables and integrate the vorticity equation, – to get: This says: the surface absolute vorticity at time t is equal to the absolute vorticity at the initial time multiplied by an exponential of convergence When surface air converges, surface vorticity increases exponentially with time Derive this equation for extra credit! Hint:

Role of temperature advection in the life cycle of an extratropical cyclone Initial trough/ridge pattern moves over a front Temperature advection causes the trough to “dig” (slow down), effectively shortening the wavelength and increasing the gradient of vorticity Ahead of the ridge: cold-air advection Ahead of the trough: warm advection Now, in the same space, there are 2 troughs and ridges (wavelength is now ½ of what it was originally)

Role of temperature advection in the life cycle of an extratropical cyclone Result of upper-level vorticity gradient is more rising motion – More rising motion at the low center leads to more convergence More convergence/rising motion increases surface vorticity (via stretching) – Increased surface vorticity causes even more temperature advection » Temperature advection feeds back to even larger upper level vorticity gradient

Example from 16-17 Feb 2003 0000Z 16 Feb 2003

Example from 16-17 Feb 2003 0000Z 16 Feb 2003 Weak frontal boundaries over Southeast U.S. Upper-level trough moves overhead

Example from 16-17 Feb 2003 L H

Example from 16-17 Feb 2003 1200Z 16 Feb 2003

Example from 16-17 Feb 2003 1200Z 16 Feb 2003 Surface low has translated northeast & deepened 1-2 mb Greater temperature advection causes upper- level trough to dig

Example from 16-17 Feb 2003 0000Z 17 Feb 2003

Example from 16-17 Feb 2003 0000Z 17 Feb 2003 Original low is now filling in over TN/AL Very large temperature gradient off U.S. East Coast promoting new low development there

Example from 16-17 Feb 2003 L H L Warm-air advection Cold-air advection Upper-level trough is now more intense (lowest height: 5550 m)

Example from 16-17 Feb 2003 1200Z 17 Feb 2003 New surface low has now deepened along the very large temperature gradient off U.S. East Coast

Example from 16-17 Feb 2003 L H L L Warm-air advection Cold-air advection Upper-level trough is deeper (lowest height 5510 m)

Example from February 2003: heaviest snow ever in Baltimore Image of Downtown Annapolis (Main St.) from http://www.weatherbook.com/snow2003.html http://www.weatherbook.com/snow2003.html