UW-Madison ATMOCN 100 - Extratropical and Tropical Cyclones (5 pages)

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Extratropical and Tropical Cyclones

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Extratropical and Tropical Cyclones


A lecture about the fundamental differences between extropical cyclones and tropical cyclones.

Lecture number:
Lecture Note
University of Wisconsin, Madison
Atmocn 100 - Weather and Climate
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Atm Ocn 100 Edition 1nd Lecture 31 Outline of Last Lecture I Reminders II Weather of the day III Tropical Cyclone vs Midlatitude or Frontal Cyclone IV Hurricane Tracks V Parts and Weather of a Hurricane VI Wind and Storm Movement VII Initiation of Tropical Cyclones VIII Easterly Waves Outline of Current Lecture II Reminder III Current Weather IV Basic reasons why Extratropical cyclones and Tropical cyclones exist V Extratropical and Tropical Cyclone Configurations VI Main Summary Points Current Lecture Reminder Homework due Wednesday Current Weather Now it is a 75 chance that there will be an El Nino There is heavy rain developing in the south near Texas There are many subtropical jets not polar jets which is odd Vorticity is the curl of the wind It represents how much spin there is in the wind On the map we are looking at the red means positive cyclonic vorticity This is because curvature of the flow and because there is a little shear in there The blue is low vorticity its positive curvature it probably has negative shear which would make it turn the other way We will get about an inch of rain this weekend More Tropical Cyclones Basic reasons why Extratropical cyclones and Tropical cyclone exist The difference for their existence is that they are two fundamentally different phenomena in the atmosphere The sun when it shines on the earth heats the surface and moistens the surface in the tropics This is more than when it heats the surface at the middle latitudes because it shines more directly on the surface of the tropics In the tropics this creates a thermal gradient between the supper atmosphere and the lower atmosphere By heating the surface it becomes conditionally unstable The result of the shine shining on the surface is to create thermals that go up and down that transfer heat to the upper levels When all the thermals like cumulus clouds get together and have a party that s a tropical cyclone It is a swirl of clouds releasing latent heat into the atmosphere The swirl can be interpreted like one cumulous cloud Can think of a swirl of clouds of a tropical cyclone is a warm anomaly driven by latent heating And the air is moving along the surface of the ocean where it picks up vapor latent heat and it rises up in a swirl fashion into the tropical cyclone where it condenses and puts the heat into the atmosphere and then extends outward The circulation in a tropical cyclone mimics that of the TC It converges into the eyeball and the eye ball rises up on either side and we can think of it like an ITCZ When it goes out it can form jetstreams With a tropical cyclone we are looking at warm moist air at the surface rising up through this cloud and overturning and releasing its heat and then reaching another level of warm moist air up above At the tropopause in the tropics the air is warm The air in between is cold the air rises and circulates out and then sinking motion makes it warm in the middle of the cyclone An extratropical cyclone is different An extratropical cyclone is like a front Sometimes the extratropical cyclone has different names such as frontal cyclone If the atmosphere was divided into cold verse warm which is like north verse south And then a barrier separating the two and then removed it What happens is that the cold air goes underneath and the warm air goes over It looks like a slanted surface separating the cold and warm air We live on the planet that is spinning If there was a pan of water and it was spun the water would go up against the side It would be on a slant That is like what is happening on the earth The earth is spinning around so the cold air can never fall all the way it s the result of the rotation of the earth The surface between the warm and cold air would represent a cold front When there are jet streams moving around you may start off with cold air and warm air next to each other but at some point they become like a wave into a circulation Cold air would be lower and warm air would be higher Extratropical and Tropical Cyclone Configurations An extratropical cyclone configuration is that there would be low pressure in the middle with cold air on either side The relatively warm air would be trapped in the middle In a tropical cyclone the core is warm It will produce strong winds at the surface because the convergence of the circulation is focused at the surface where they are being drawn together to rise For extratropical cyclones why is there low pressure at all The cold air is sinking which drives the system It sucks air from the ground and its low pressure because the cold air is sinking The strong flow tends to result from the jetstreams that are actually part of the system at the top of the cold air The wind in the situation of the extratropical cyclone actually increases as you go up And the surface is secondary to the whole process But in the tropical cyclone all the energy is coming from the surface The energy is coming from the warm air where the sun heated it rising in a fashion that forms the tropical cyclone Main Summary Points The bottom line is that tropical cyclones have potential energy from position of warm air under the cold air This is an unstable situation The tropical cyclone is like a lava lamp You heat the plastic and it rises in bubbles Extratropical cyclone energy is from position of cold air next to warm air And the cold air then falls underneath the warm air This is like the wave machine They are two fundamentally different things Tropical cyclones go by a lot of different names they tend to have local names based on history They are hurricanes in the Atlantic typhoons etc They are all under the general category of tropical cyclones that drive their energy from the rising motion of condensing currents of air

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