Lecture 31Outline of Last Lecture I. RemindersII. Weather of the dayIII. Tropical Cyclone vs. Midlatitude or Frontal CycloneIV. Hurricane TracksV. Parts and Weather of a HurricaneVI. Wind and Storm MovementVII. Initiation of Tropical CyclonesVIII. Easterly WavesOutline of Current Lecture II. ReminderIII. Current WeatherIV. Basic reasons why Extratropical cyclones and Tropical cyclones existV. Extratropical and Tropical Cyclone ConfigurationsVI. Main Summary PointsCurrent LectureReminderHomework due Wednesday Current WeatherNow 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. Atm Ocn 100 1nd EditionWe 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 movingalong 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. Thatis 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, butat 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 ConfigurationsAn extratropical cyclone configuration is that there would be low pressure in the middle with cold air on eitherside. 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