Lecture 18 Outline of Last Lecture I. Current WeatherII. CumulonimbusIII. ThunderstormsIV. Precipitation process in a CumulonimbusV. Wind ShearVI. What causes a thunderstorm to form? Outline of Current Lecture II. Weather of the dayIII. What causes a thunderstorm to break, weaken, or collapseIV. 3 Stages of an Air Mass ThunderstormV. Effects of Wind ShearVI. Introduce Environmental Wind Shear to Prevent BreakdownVII. Frontal Thunderstorm VIII. Squall Line Growtha. Other picturesIX. Three-Dimensional Effect of Wind ShearX. Supercell Soundinga. Pictureb. EchoCurrent LectureReminders:Homework due 10/31Weather of the day:We are expecting it to get a little warmer this week. From Monday to next Tuesday we can see that warm air is coming in. Warmer air is moving from the southern U.S. up towards us. On Saturday we can see that it will be quite warm at 850 mb temperatures. We are not going to hit the mid 80’s on Saturday. But we may expect it toget into the 70’s on Saturday. The rain snow line is well to our north. The seasons are coming. Siberia is about snowed in and it is moving intoMongolia. Canada the snow cover is moving south. We can see winter coming throughout the world. Atm Ocn 100 1nd EditionAround the Artic Circle there is open water it isn’t normally there. This is from climate change. That will eventually freeze up in the winter. Fall is a little longer then it usually is. Southern Hemisphere we see winter ending. We can see the surface temperature over the water; there is a tendency to move warm water toward the eastern pacific. El Niño year looks like it will occur. Higher temperatures will move toward Hawaii and Central American coast. These happen once every 3 or 4 years. Associated with moving of warm water, usually in WestPacific all the way to the East Pacific. Warm water gets warmer off the coast of Peru than it normally would. In West Coast, waters tend to be cool. But during El Niño warm water will be brought up the coasts. Thecurrent that brings warm water toward the coast is stronger. It affects the coast of California also. But for Peru the fishing industry needs cold water. El Niño warms water up and fish move out of the area, so they can’t fish anymore. Normally, there is some warming of the ocean every December. El Niño is an annual phenomenon along the west coast of Peru every year. But now El Niño means the whole warming of the Pacific. Especially the East Pacific that occurs. There are predictions from people who have ocean models that we should get the general big time El Niño this year. We are about 50% sure this will happen. We aren’t sure though. Creates more tropical convective weather, will produce stronger subtropical jets and plug into the northern hemisphere. It will bring moisture up towards the west coast. California is keeping fingers crossed it will happen. They have very strong draught conditions. If there is an El Niño there will be rain. Keeps enough moisture to get through typical dry years. Back to lecture:Review: Thunderstorm is a collection of cumulus cells. They are weakening often. They tend to form on one side. Multicellular thunderstorm…Average life of a cell is 20 minutes. Also supercells thunderstorms have one dominate cell that lasts a very long time. These cells can last indefinitely. But it is really how long these conditions can last in the environmental conditions. Usually this is not more than a few hours.What causes a thunderstorm to break, weaken or collapse• Of course the updraft stops at its equilibrium level, where the updraft is no longer buoyant, but why does the rest of the updraft also eventually die?• Evaporational Cooling. The air parcels of the updraft mix with environmental air– Air of the environment is relatively dry, so when mixed with the updraft parcel, it can lower the humidity of the parcel and cause condensed water to evaporate– This evaporation cools the parcel lowering the parcel’s buoyancy– The mixing also dilutes the parcel’s buoyancy• Water Loading. When precipitation forms, it falls from the upper part of the cloud and “loads” the updraft in the lower part of the cloud with the extra “weight” of the falling water droplets, further eroding the parcels’ buoyancy.• The combination of evaporational cooling and water loading will at some point reverse the parcel buoyancy to negative buoyancy causing a downdraft to form.• As the downdraft grows, it leads to the demise of the storm3 Stages of an Air Mass ThunderstormsThis is a picture that describes a 20-minute life cycle of a thunderstorm. Starts as a Cumulus stage, there is upward motion of air and cloud droplets. In these early stages there is no precipitation yet. It has to get cold enough for the Bergeron process or start an autoconversion process tocreate rain without ice. But lets say it does have ice. But it won’t get the ice until the cloud gets big and tall. In early stages when it is relatively small, there is only cloud droplets and upward motion. Upward motion at lower levels has to be forced. Some phenomenon as we have discussed will have to push air up like a front. It has to push it up to the level of free convection. Area in the lower region is cold; something needs to go in there to forcibly pick air up. It is the process of conditional instability. This is the same thing as saying a parcel is unstable, and something has to push it over the hill. To get thunderstorm started, something has to push air to the level of free convection. Once there, it is at top of the hill, or at the level of free convection, which means it will accelerate on its own. Once circulation reaches the level of free convection there is still no precipitation yet. It is formed in thenext stage, which is the mature stage. Mature stage or the 2nd stage, is when precipitation is formed and it reorganizes how a cloud is structured. Once high enough, with precipitation formed, the precipitation will fall to the bottom of a cloud. Forming against the updraft. It will cause precipitation to accumulate. If you keep having updraft, hold it off, collect and more and more water. Water adds weight or mass to the air. Initially the water is just mass of theair as water vapor, but once precipitation it then starts falling to the bottom of the cloud, the weight of the precipitation is transferred to the air by friction. But as the rain falls to lower part of cloud, collecting there in mass, the lower part of a