Class Notes: Week 10. April 9 th,2019. Topics: Front and Mid-Latitude Wave Cyclones El Niño / La Niña Mid-term 2 review

Transcription

1 Class Notes: Week 10 April 9 th,2019 Topics: Front and Mid-Latitude Wave Cyclones El Niño / La Niña Mid-term 2 review Front- is a boundary between 2 different types of air masses Air masses with different densities and different temperatures. (Density differences tend to arise from the difference in the temperature) Cold fronts have cold air behind generally moving from West to Eat. Warm fronts have warm air behind as it moves towards colder temperatures. Occluded front is when cold front catches up with the warm front. (represented in purple When the front doesn t move it is a stationary front, the boundary doesn t actually move then it will look like the picture above of the cold front and the warm front together. There is still a front between the masses with different temperatures but the front itself doesn t move.

2 Cold Front: Cold air (more dense) air mass will push and move underneath that warm (less dense air) ahead. Causes that air to rise and then you get clouds and precipitation. Largest temperature differences usually relate to cold fronts Moves from West to East Average speed about 30mph (typically the cloud and rain fall associated with them would be over in about 6 hours). Temperatures drop quickly e.i. In Denver sometimes temperatures can be at 70ºF and can drop to 40ºF in an hour. The steepest leading edge is where you get the strongest uplift and the heaviest rain fall. The heavy rain cold storms associated with the cold front, is typically convective rain which is associated with tall clouds, cumulus or cumulonimbus.

3 Warm Front: (opposite of the cold front) Colder (more dense) air is heavier than warmer (less dense), so the cold air comes in and the warm air rises. Temperature doesn t warm up as quickly as the temperatures drop behind the cold front (changes slowly, maybe one or two degree in about 4-5hrs). Average Speed is of about 16 mph (about half of the speed of the cold front) e.i. cold fronts move almost twice as fast so they always catch up to the warm fronts Temperatures slowly rises The slope for the warm front is not as steep as the one we saw for the cold front. Since it is not a very steep slope the warm air is slowly rising up and notice that most of the type of the clouds associated with it. Most of that cloud is stratus which is a layering type of cloud, so the precipitation related with the warm front is light. Look at the figure on the left. Here we have a warm front with 3 types of precipitation here represented in different color streaks. Green: liquid rain Pink: mix of ice White: snow

4 Occluded Front: The cold front moves fast so when it catches up with the warm front and takes the warm front that is when we get an occluded front. It is like a mixture of both cold and warm fronts. Occluded fronts do not intersect to the ground (you can see that on image C bellow) If warm air replaces cold air than it s a warm front and if cold air replaces warm air then it s a cold front. *******(This is question on the midterm) If the front does NOT move, then it is a Stationary Front.

5 Polar Front Theory Around 1930 there was a Norwegian that was like the father of the theory of wave cyclone model o He said that about around 60 degrees North, we have West winds to the south and Eat winds coming from the polar region and converging there. You either have a long waves meaning, they tire from low pressure to low pressure.

6 Wave cyclone Formation Low 1: stationary front Low 2: Forming (developing stage) Low 3: Developed stage Cyclogenesis - development of the low pressure cyclone. Starts by the stationary boundary, then low pressure forms, it deepens then it falls up and dies. Conditions: _ Cyclone forms at the surface _ Cyclonic flow is in counterclockwise in the North of the hemisphere Cyclogenesis- development of a region of low pressure and can also refer to the formation of a hurricane, it just means we have a low pressure forming.

7 Inside the cyclones if the pressure inside drops more than 24 millibars in 24hrs that is regarded as explosive cyclogenesis, also referred to as bomb-agenesis. Formation: Convergence at the surface must be supported by the divergence aloft. So that low pressure on that boundary will not deepen and form a mid-latitude wave if there is no strong divergence above it. A really strong Jetstream that will pull the air up from the surface. If not sustained it will die out. These are cold core lows, unlike the tropical cyclone that forms a hurricane which is a warm core and there is air seeping in the middle, that doesn t happen. Air is rising at all levels and the upper level, divergence, is stronger than the surface convergence. If not the surface at that level is going to deepen. When divergence is stronger aloft than convergence from the surface, it means that it will continue to sucking up air and that low pressure is going to continue getting deeper and stronger. And the bomb-agenesis start cranking up in the upper levels, it is literally like a jet engine sucking up all that air. So more is taken out of the top than from the bottom and the surface pressure will drop. Convergence relates to low pressure you get convergence between a ridge and a trough. Divergence relates to high pressure you get divergence between a trough and a ridge. Troughs are V shaped and Ridges are the opposite live a V upside down. Low CONVERGENCE DIVERGENCE

8 Notice the Comma shape. That is a characteristics of a well-developed wave cyclone. Warm front takes cold front Cold air is all around the cyclone On late occlusion it slowly dies.

9 What is El Niño? El Niño doesn t have anything to do with the atmosphere. It is all about sea surface temperature, an ocean phenomenon. Normal conditions: (over 2-3years) We have East winds at the surface and that wind tends to push some of that warm water surface over Indonesia. Warmer water allows more evaporation, so in normal conditions during Dec- Feb there is usually rain occurring in Indonesia. Air rises and on the upper level we have West winds and after being pushed by the wind air sinks on this region to the right (indicated by the arrow on the picture to the left). El Niño: (after 2-3 years you get an imbalance) The East winds reverse become West winds and all the warm water in the West pacific moves back towards South America and Peru. Where they usually have no rain because air was sinking in that region, not look at the picture to the left and see that the clouds have shifted away from Indonesia and now where the warmest water is. Sometimes it shifts all the way to the coast of South America and they can have flooding rains in Ecuador and Peru. During normal conditions that upwelling is bring rich nutrients for the fish industry and El Niño does the opposite and obliterates the fishing industry during the El Niño.

10 How does the circulation change? The jet stream on the upper level troughs steer the surface of mid-latitude cyclones. During El Niño the storm tracks, these southern jet streams instead for going up it comes in towards California and Southern parts of the United States. Worldwide El Niño Impacts: (not only U.S. is impacted look at the image below) A lot of other parts of the world are dry during El Niño.

11 Normal conditions: Normal conditions: (over 2-3years) We have East winds at the surface and that wind tends to push some of that warm water surface over Indonesia. Warmer water allows more evaporation, so in normal conditions during Dec- Feb there is usually rain occurring in Indonesia. Air rises and on the upper level we have West winds and after being pushed by the wind air sinks on this region to the right (indicated by the arrow on the picture to the left). La Niña: During La Niña this normal circulation intensifies. We get East winds at the surface almost twice as strong. The circulation is stronger and it really pushes the warm water further West into Indonesia and South China Sea. By having all this warm water, it enhances the convection of Indonesia. So during La Niña it is even rainier than it is during normal conditions. We can see colder than average into the sea temperatures in the region of the tropics Easter pacific (completely the opposite of what we see in El Niño). La Niña Storm Track pattern: (what happens to the air circulation here?) The main Jet stream does this big North dive toward Alaska and the dives back down through the central parts of the country. We have this big High pressure here (represented by the H on the image below) and the weaker subtropical jet varies but it usually towards the north of the America and sometimes it does reach Northern California and get a lot of rain during La Niña when we don t expect to. La Niña is just the opposite of El Niño.

12 Worldwide La Niña Impacts: (not only U.S. is impacted look at the image below) A lot of other parts of the world are wet during La Niña This pattern reversal impression (look at the image below) called the Southern Oscillation Index (SOI) In 1924 Gilbert Walker discovered this pattern reversal impression Tropical See-saw between Darwin, Australian and Tahiti over the southern parts of the pacific ocean. He noticed that it coencided with El Niño and La Niña. This related to the atmosphere and how it is doing this pressure differnce between the two. During El Niño pressures are higher and during La Niña the pressures are lower over the Northern part of Australia. SOI: Air Pressure Index (blue orange image below) ONI : Sea surface Temperatures ONI (Oceanic Niño Index) 3 month Running (e.i. Dec-Feb) El Niños (0.5ºC) and La Niñas (-0.5ºC) are defined and a warning or watch goes out when either the temperature in that box for 3 months is a row is above 0.5ºC or negative -.5ºC *Not all El Niños are the same All La Niñas are the same

13 A large area of high pressure positioned off the west coast of South America produces southerly winds that cause cool surface water to drift northward as the Peru/Humboldt Current (dark blue). As southerly winds blow parallel to coastal margins, the Coriolis effect bends moving surface water to the left (Southern Hemisphere), which induces upwelling the rising of cold water from below. ( not exactly the same sea temperatures in these El Niños below)