Atm Ocn 100 Edition 1nd Lecture 20 Outline of Last Lecture I Weather of the day II Review III Diagram IV Lifecycle of a Supercell V Where Supercells Form VI Tornados Outline of Current Lecture II Reminders III Weather of the day IV Review V Vorticity VI Why do tornados form Current Lecture Reminders Next Test is 2 weeks from today Test November 7th It 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 3 Weather of the day This 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 Lecture 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 at two 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 they almost 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 it can 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 strong wind 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 Vorticity Vorticity 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 the north 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 So as 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 current Any vortex creates a cyclostrophic balance in the direction of the circulation Low 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 prevent it from being destroyed If really long may take a while for air to work way in to blow it apart Supercell protects its cyclone is that air