Lecture 20Outline of Last Lecture I. Weather of the dayII. ReviewIII. DiagramIV. Lifecycle of a SupercellV. Where Supercells FormVI. TornadosOutline of Current Lecture II. RemindersIII. Weather of the dayIV. ReviewV. VorticityVI. Why do tornados form?Current LectureReminders:Next Test is 2 weeks from today. Test: November 7thIt will mostly be on the convections. We may get to tropical cyclones.Homework due Friday, October 31, 2014• TYU Ch 20: 1, 3, 4, 7, 20 ; TYPSS 3 • TYU Ch 21: 1, 5, 6, 9, 11 ; TYPSS 2• TYU Ch 22: 1, 2, 3, 4 ; TYPSS 3Weather of the dayThis is a 850 mb map we are looking at today. The map is for Monday afternoon. We are getting into the slightly orange yellow color, which is 15 degrees Celsius, and if we get some mixing we could see that the temperatures will probably get into the 70’s. It looks like there is a relative dry air coming from the map so it seems like there will be mostly clear skies for the next 24 hours. Next few days will be low moisture content, not much chance of precipitation. Atm Ocn 100 1nd EditionLecture:Review:Why is air swirling around in a supercell thunderstorm?We talked about wind shear in the environment in which a supercell forms. A favorable type of wind shear is helicity for supercells. Wind shear occurs because wind speed and direction changes with height. It is changing perpendicular to direction it is flowing. Basically what is happening here is that wind is going at different speeds and it is going next to each other. We can define it as a vector difference between the winds attwo different points.The vector difference between the winds blowing the same way is convergence or divergence. If you go up the wind is blowing at a different speed and direction with height that is vertical shear. We more often are talking about wind shear in the vertical. When airplanes are worried about wind shear theyalmost always are worried about vertical shear. Vertical shear of the wind is important for tornados. Vertical shear can either be straight or 2 dimensional. Or itcan involve turning of the wind with height. 2 dimensional shears would be the wind going both East and West directions. Directional shear could be the wind going different directions and could change in speed with height. If the wind is turning clockwise is called a veering wind profile. Normally won’t have instability when the winds are backing with height. For tornados we want to see jetstreams we want wind to change dramatically with height, which means strongwind shear. Also changing direction with height, this is even better for a tornado.We want wind to increase strongly with height that is the main reason why tornado activity goes away in the summertime. When summer comes jetstreams weakens and disappears and there is no wind shear anymore. By June most wind shear associated with jetstreams are gone. VorticityVorticity is local spinning motion…Vorticity is defined with the right hand rule. The direction is positive if you define positive in the vertical direction as up. When you have wind shear right hand will curl around the vortex and thumb points toward thenorth. North is considered positive; South is considered a negative direction. Cyclonic flow thumb points up and vertically up is the positive direction. Cyclonic is a positive vorticity. Anticyclonic thumb is point down and that is negative. So anticyclonic is negative vorticity. Supercell is the splitting of the two vortex’s one on the right and one on the left. The supercell normally forms with the right mover. The left could form a supercell but there is strong helicity so it usually dies off. If there is no helicity there could be an anticyclonic and cyclonic storm. If there is helicity the right one survives and that is the one that is going cyclonically or counterclockwise.Coriolis would create a different balance between the two; it doesn’t matter here because they are so small.So a supercell is an effective wind shear creating a horizontal rotation because of tilting of horizontal vortex role associated with westerly flow aloft and easterly flow in the surface. Up in the storm, which is up above the ground, the air is rotating with a fairly intense low in the middle. It is rotating because of the tilting producing the initial rotation, the air is accelerating up, if there is a lot of CAPE the storm will have a lot of energy and will accelerate up quickly. It will make vortex quite strong above. It doesn’t go to the ground. The vortex can’t go to the ground because it isn’t rotating down there. At cloud base the air becomes saturated and as you cool the air or lower the pressure it becomes saturated. Soas column vortex forming, saturated at lower levels so cloud base comes down. As the funnel grows, the pressure is lowing and condensing air at the cloud base. It then grows right to the ground, has to interact with the ground at first for the whole process to happen. Why do tornados form?Vortex features occur throughout atmospheric circulations, and spin in all directions throughout a thunderstorm and its attendant circulations such as a density currentAny vortex creates a cyclostrophic balance in the direction of the circulationLow pressure not balanced perpendicular to plane of rotation (direction of rotation vector)The low pressure in vortex draws air towards the middle. This slowly destroys vortex. Wall is curving, can’t go through the wall, you want to go in straight line feel as though pushed against the wall,but wall is pushing us actually. The pressure gradient pulls air towards the middle that’s going in different direction and that creates a balance or a spin type effect. The cylinder protects the low pressure in the middle of the cylinder. You form a balance between the low pressure and air trying to grow into it. When you have air spinning the effect of spinning will balance the low pressure and it won’t just go away. There is another dimensional, another low-pressure gradient. Pressure gradient into the end of the cylinder, which sucks air in, fill it up with mass and blow whole thing apart. That is a problem to maintaining the balance. Other balances though why does it not blow apart? Reason is because there is stability in the air. Air doesn’t easily rise unless it’s unstable. If stable air goes up it just comes back down because it is colder than the environment. So you can form a vortex with low pressure in the middle because stability will