Lecture 21 Outline of Last Lecture I. RemindersII. Weather of the dayIII. ReviewIV. VorticityV. Why do tornados form?Outline of Current Lecture II. Weather of the dayIII. Tornado Formation Mechanismsa. Top-Downb. Bottom-upIV. Two Major Types of TornadosV. Daisy ChainVI. Dry SlotsVII. Hook Echo Big PictureVIII. Successive tornado genesis Current LectureRemindersHomework due this Friday 10/31Test 2 is next Friday 11/7This test will be entirely on deep convections like thunderstorms. This means chapters 18, 19, 20, 21, and 22Weather of the dayThe white line we can see is the 0 degree Celsius line at 850 mb. The white line is the rain, snow line. This map we are looking at is running into next Tuesday. We can see cold air moving towards us. On Friday afternoon at 850 mb it will be -10 degree Celsius so at the surface it will be upper 30 degrees. It will be cold on Halloween! Winter is on its way. Back to lecture:Tornado: Rotating cloud that reaches the surface Atm Ocn 100 1nd EditionKey thing is how do we get rotation to the surface? Tornado Formation MechanismsA primary area of current research is to better understand tornado genesis near the surfaceTwo categories of formation are studied:Top down genesis: carry down cloud rotation from supercell. Or a storm updraft is being pulled from above. This means a vertical upward motion at the top of the vortex, stretching vortex.Bottom up genesis: build rotation at surface independently. Or this is known as surface vortex, which has a given amount of rotation or angular momentum. There is an overall rate of spin, related to the spin of the environment converging into the vortex. Ratio of total spin to total vertical motion at top is swirl ratio– Swirl ratio too big => multiple vortices form– Swirl ratio too small => just updraft, vortex (tornado) washed out– Swirl ratio just right => long-lived single tornado vortexFormation Mechanism 1: Top-Down1. Dynamic Pipe EffectLow pressure from storm projects downward to below cloud from vortex inside cloudInstigates spin-up of wind below cloud producing convergence fieldThis diagram is showing the constricted flow. The air speeds up into a vortex and when it extends downward, the low pressure also does, same process occurs again. It is hard to say rotation beneath the cloud, where it comes from. You can only descend a dynamic pipe into rotation. If there is no rotation already where does it come from? This is a big problem.Idea is that you produce a pressure and the wind adjusts to it. By constraining rotation that has to already be there…2. Wind rotation moves down from the mesocyclone above, causing pressure dropsInteracting vortex dynamics(2nd way that the top down mechanism might work)Formation Mechanism 2: Bottom-up Processes 1. From forward flanking gust front Rotation from circulation driven by thermal contrast We can see circulation along the boundary, which is driven by buoyance difference.There is rotation from the circulation produced by a thermal contrast. Even in the supercell we can get a bottom up formation. Basically what happens is that there is a blast of cold air from the storm that goes toward the surface…cold air moves against the flow and wind goes over it and produces a strong wind shear. This cold layers from the downdrafts are very close to the ground. Then horizontal rotating air turns the air upward, which produces an enhanced rotation near the surface. This may be a way to have rotation at the ground. There is still a dynamic from above to role it into a strong vortex. 2. Tilting upward vorticity from Rear Flanking Gust front: Arching of vortex tubesRotating tube of air can be tilted by the updraft. And then you can have a column of vertically rotating air from the storm. The updraft is created by the downdraft hitting the surface. 3. Convergence line or vortex sheet, such as a land breeze front, produces wind shear line that rolls up into vorticesThere is a frontal contrast between wet and cold ground. This may produce flows that have some vorticity associated with them. The contrast in the winds means that the intersection between the two colors is a positive cyclonic shear and if you take a point and pull it upward it would spin up into a vortex. It could alsobreak down into a series of vortex features. Often along the line where this is happening, there are rising currents of air in low-pressure areas that rise up and form clouds. There might be clouds from contrast of thermal contrast. There might also be wind shear and vorticity, which would make clouds into spinning clouds of air which would pull and stretch to make air spin up into a smaller vortex. Mostly tornados are formed by shear line disturbances like this. Shear line tornados are all bottom up tornados, which are formed by wind shear at the surface. They are morethe rule than the exception. Two Major Types of TornadosSupercell tornadosAssociated with mesocyclone in a supercell resulting from tilting of vertical wind shearMost violent account for almost all tornados EF3 and higherAbout 50% of all supercells with spawn a tornadoAccount for less than 30% of all tornadosShear Line tornadosAssociated with a cumulus cloud forming along a wind shear (or wind shift) lineDerive their “spin” from the wind shift lineMay occur in a supercell or squall lineOther names: Land spout, cold air funnelWater spoutIntensity EF0-EF2Most common type of tornadoSupercell PictureTornado in a supercell is most likely to form at the triple point having the forward flank gust front and rear flank gust front. On the picture there is a T and it says mesocyclone and tornado.In the recent year we can see bottom up processes showing an arching near the surface. It is expected to arch upward which produces the rotation that creates a tornado. Our professor does not entirely believe this theory. The dynamic pipe theory lives on. This is the original theory of tornados, especially for supercells, seems to be the best explanation.Up in storm there is a rotating column and you can create rotation at the surface for many reasons. For one reason, there is cold air interacting with the warm air rising aloft and sinking to the surface.Daisy Chain Cold dry midlevel air from the back of the storm comes towards the warm air and interacts with it and forms avortex affect. The warm air already has formed a vortex. So we see “daisy chain”. The cold air coming around isthe same affect of the warm air coming up. One is a mushroom