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Lecture 11

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McGill University
Atmospheric & Oceanic Sciences
ATOC 184
Eyad Atallah

Tuesday, February 11, 2014 Cold Front Recap - Cold air mass displacing a warm air mass - Counterclockwise motion, drawing in cold, dry air in the Northwest flow behind he storm, and drawing in warm, moist air in the Southerly flow ahead of the storm - It’s advancing to the East and bringing with it cold, dry air Warm Front - Warm air pushing Northward in to the cold air - Gray area represents the area of cloud and precipitation – that area is much larger ahead of the warm front, then ahead of the cold front o This is because our cold air is more dense. Because the warm, less dense air is trying to displace the cold air, what generally happens is the warm air rides up and over the cold air o Eventually it will push the cold air out of the way, but it’s a very slow process. It’s much easier for a cold air mass to displace a cold air mass – simply because of the difference in density o As the air rises, it starts to expands and cool and once it reaches it dew point, condensation forms and clouds starts to farm. o As the cold air gets deeper, the level of the clouds of the warm air that’s being lifted gets higher and higher (follows the slope). The closer the clouds are to the ground, the closer you are to the warm front. - If you want to know if air is stable or unstable, just have to look at the type of clouds that are forming - If the clouds are pancake (mmm) like, the atmosphere is stable – smooth clouds - If the clouds are puffy and cottony, the atmosphere is likely unstable Warm Fronts - Start off cool and as you come in contact with the front and you have a sudden warming - Every front, whether cold or warm, will always have the pressure falling as it approached you - When the front passes you, the level will level off, and then will generally increase as the front goes by. The difference is that behind a warm front the pressure rises a little, but behind a cold front the pressure rises a lot! (due to density) - The winds tend to be East to Northeast (change in table!) - In the summer a lot of precipitation comes with cold fronts, but in the winter we are dominated with warm front precipitation (snow) Two distinct examples of stationary fronts and the associated clouds - The boundary between two air masses that is stationary at the surface - If we’re going to have precipitation with a stationary boundary, the side of the boundary that the precipitation is going to occur on depends on the winds: o When the air flow is fairly parallel to the front on either side, any precipitation that occurs with the stationary front tends to be on the warm side of the boundary o When the warm air is actually blowing more perpendicular to the front, most of our precipitation tends to be on the cold side Two examples of cross sections through stationary fronts each part corresponds to either A and B in previous figure - Stationary boundaries, but A behaves more like a cold front, and B behaves more like a warm front Occluded Front - Occluded fronts are produced when a fast moving cold front catches and overtakes a slow moving warm front - Cold front generally moves faster because of its density - There’s not warm air left at the surface, and all of the warm air has been lifted higher up in to the atmosphere The development of an occluded front - The point of occlusions is exactly when the cold front reaches up to the warm front - Since the warm air is less dense, it just gets lifted higher up in the to atmosphere – pinching the warm air off - Difference between cold occlusion and a warm occlusion is dependent on whether the air behind the warm front is colder than the air approaching the warm front (don’t spend a lot of time worrying about this) Cyclones - Our boundaries start to move when they’re associated with a significant low pressure system - Our stationary boundaries become mobile (warm fronts and cold fronts) when we get a low pressure system developing in their vicinity Norwegian Cyclone Model - In this model, there will initially be a boundary, or front, separating warm air to the south from cold air to the north - Because of what’s happening at jet stream level, we can get a low pressure starting to form at the surface, and we get a counterclockwise circulation around our low which starts to move the air masses - A wave on the front will form as an upper level disturbance embedded in the jet stream moves over the front. The front develops a “kink” where the wave is developing. Precipitation will begin to develop with the heaviest occurrence along the front (dark green) - As the circulation of the low develops, warm air is drawn northward east of the cyclone and cold air is
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