AOS 101 Cyclone Structure April 22/24 April 29/May 1

Cyclone COLD WARM OCCLUDED STATIONARY Symbols: Point in direction of front movement COLD WARM OCCLUDED STATIONARY

Warm Front WARM COOL

Associated Weather (WF) Gradual Slope Stratiform rain long lasting light rain occurs on cool side of front Temperature increases prior to frontal passage Wind becomes southerly after passage

Cold Front WARM COOL

Associated Weather (CF) Much Steeper Slope More intense (convective) rain Thunderstorms for a shorter period occurs on warm side of front Temperature decreases after frontal passage Wind becomes northerly after passage

COOL AIR L LIGHTER RAIN COLD AIR WARM AIR HEAVIER RAIN

Finding a Front Temperature (dewpoint) Gradient Change in wind direction Converging winds at the front “Kink” or “trough” in isobars (lower pressure) Banded precipitation

Horizontal Cyclone Structure

Cyclone Life Cycle Stationary Front Wave Cyclogenesis Maximum Intensity Occluding Dissipation

Stationary Front COLD WARM North-to-south temperature gradient Winds parallel to the front COLD WARM

Wave COLD WARM Winds gain a northerly/southerly component Wave forms on the front Usually caused by an upper-level trough (next week) COLD WARM

Cyclogenesis COLD L WARM Closed cyclonic circulation froms Surface pressure minimum is observed Cold fronts and warm fronts become distinct COLD L WARM

Maximum Intensity L COLD WARM Warm air overruns cold air at warm front Cold air quickly moves around to the south Cold front moves faster, “catches” warm front Storm deepens to lowest pressure L COLD WARM

Comma Head

Occlusion Cold front catches warm front leaving warm air aloft Cold air encircles cyclone center Another cyclone may form at triple point L L COLD WARM

Occluded Front WARM COLD COOL

Dissipating Cold air completely encircles low All warm air is pushed aloft Cyclone can no longer feed off of temperature differences

Vertical Cyclone Structure

Upper-level terminology TROUGH: area of lower heights RIDGE: area of higher heights L H

L H Vorticity Advection VORTICITY: how much the flow curves Trough = positive; Ridge = negative L NEGATIVE VORTICITY H POSITIVE VORTICITY

L H Downwind of trough axis = Positive V.A. Downwind of ridge axis = Negative V.A. L NEGATIVE VORTICITY NEGATIVE VA POSITIVE VA H

Mechanisms for Deepening Cyclones Warm air advection (WAA) near the ground Look at surface temperature and winds Positive vorticity advection (PVA) at upper levels Look at upper-level (e.g. 500 hPa) winds and trough/ridge axes. Both promote upward vertical motion (ascent).

Vertical Structure: Beginning STATIONARY FRONT PVA POSITIVE VORTICITY

Vertical Structure: Cyclogenesis WAA L Westward tilt with height POSITIVE VORTICITY

Vertical Structure: Occlusion WAA L PVA POSITIVE VORTICITY

EXAMPLE 0000 UTC 10 November 1998 300 mb flow which resulted in a massive cyclone development over the midwest. PVA downwind of trough PVA TROUGH AXIS 300 hPa Height, Wind Speed

Surface Pressure, 500-1000 Thickness EXAMPLE 0000 UTC 10 November 1998 Weak cyclone downwind of trough Fronts are relatively weak L TROUGH AXIS Surface Pressure, 500-1000 Thickness

EXAMPLE 1200 UTC 10 November 1998 12 hours later… Trough moves west, strong jet rotates around trough PVA TROUGH AXIS

EXAMPLE 1200 UTC 10 November 1998 Cyclone has deepened to 977 hPa, has moved closer to trough axis. Intense WAA north of cyclone WAA L TROUGH AXIS

EXAMPLE 0000 UTC 11 November 1998 12 hours later… Trough is now cutoff PVA TROUGH AXIS

EXAMPLE 0000 UTC 11 November 1998 Surface cyclone practically under trough axis i.e. The cyclone is “vertically stacked.” No PVA or WAA = no more intensification L TROUGH AXIS

Main Points Cyclones will tend to form downwind of an upper-level trough in an area of PVA. As the cyclone develops, WAA will occur on the warm front further deepening the cyclone Whole system will tilt westward with height While developing, cyclone will move closer to trough axis While occluding, cyclone will move nearly under trough axis in an area void of PVA/WAA.