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Lecture 4

Wind Lecture 4.doc

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Department
Environmental Science
Course
EESA09H3
Professor
Tanzina Mohsin
Semester
Summer

Description
Wind Lecture 4: Midlatitude Cyclones Hurricanes in the Great Lakes: Analysis - Relatively rare – return rate to Great Lakes - Data readily available on Web (i.e., Environment Canada) - Examine records of hurricanes in North America (Atlantic Basin) since 1951 to 2000: o 10 hurricanes in Great Lake region: 1 every 5 years o Never more than 1 per year o Clustering:  0 hurricanes from 1969-1987  5 hurricanes from 1988-2000  According to Gray’s index, this is maybe linked to North Atlantic Oscillation (NAO) o Some examples:  1954 Hazel, 5-18 October • 121.4mm at Toronto Pearson Airport  The 6 examples occurred approx. in September and October, and over the great lakes o 2006-2007  no great lake hurricanes o 2008  Hurricane Ike’s effect  The major difference between when storms pass oceans and lakes is that Great lakes are not affected by strong currents o Snow storm in 2011: U of T was closed for the day (midlatitude cyclone, “gulf low”)  Originated form gulf of Mexico  Huge damage in Texas  From Texas, it went to 2-3 more states of US, then over to Canada  Forecasted 30cm of snow  What actually happened was: wasn’t as bad as what the forecast was Midlatitude Cyclones: - Commonly referred to as a “low” or “low pressure system” or “frontal system” - Major weather maker in midlatitudes (35-60deg) - Most storms in Southern Ontario in the fall, winter, spring are midlatitude cyclones (not summer!) - Occur approx. every 4 to 7 days - Characteristics: o 100s to 1000s km in extent (larger than hurricanes!) o Less intense winds o Can have thunderstorms and tornadoes associated with them (cold front) o Stays only 4-7 days then goes away, whereas hurricanes have a longer time- span o Differences between hurricanes and midlatitude cyclones: Intense wind in hurricanes than cyclone, hurricanes longer time-span, cyclones are larger Large Scale Flow: - Polar front region between Polar and Ferrel cells (30-60deg – air rises at 60 and sinks at 30 for Ferrel cell) o Air Masses:  Midlatitudes – battle ground between cP (continental polar) and mT (maritime tropical air masses (air masses cover a huge area therefore anything can change at any time)  Toronto: dominant air masses are cP or DP and mT or MP and DT  For hurricanes  sea surface temperatures, and convergence and divergence Fronts: division between air masses - Stationary front: o Stable o Low pressure trough o Horizontal wind shear o Alternating blue triangles and red semi-circles o Stable because it doesn’t’ have any disturbances  lack of latent heat to fuel storm - Cold front: o Cold air pushing into a warm air mass (cP (cold) & mT (warm)) o Designated by blue line with triangles facing warm air o Frontal slope 1:50 o 15-25 knots (7-13m/s) o Heavy precipitation along the front where mT air is forced up o Criteria for Identification:  Strong temp. gradient  Change in moisture (dew point)  Shift in wind direction  Pressure change  Cloud and precipitation pattern (the line shows this, where the line is there, that is the place of the most cloud and precipitation) - Warm front: o Warm air pushing into a cold air mass – battle ground between the cP and the mT air o Designated by red line with semi-circles pointing toward cold air o Slope 1:150 -1:300 o Gentle precipitation (drizzle) - Occluded front: most intense storm!!! o Cold front catches up with warm front o Warm air forced above surface, so you start to get latent heat (fuel for the storm) o Warm front-style precipitation o Alternating blue triangles and red semi-circles Polar Front Theory: mechanism of the formation of the midlatitude cyclone - Low pressure or cyclone is the principal weather maker at midlatitudes - Development of a low pressure begins with a small perturbation or disturbance along the polar front (division between polar and Ferrel cells) - Step ONE: o Disturbance  stable stationary front becomes unstable o cP (northeast) and mT (south west) colliding with each other  disturbance  unstable o huge pressure gradient between cP and mT air that creates this disturbance  low pressure system develops o Stationary front with a strong horizontal wind shear o Wind shear is horizontal gradient of wind direction and can be unstable - Step TWO: o Under certain conditions a kink or small disturbances forms along the polar front o A “cold front” of cold air pushes to the south and warm air “warm front” pushes to the north  interaction between cP and mT = PIVOT POINT o The pivot point is the lowest local pressure and is the low pressure centre  temperature drops and low pressure system is getting intense (storm is building) o Precipitation begins - Step THREE: o Fully developed wave o The wave moves east or northeast – it takes 12 to 24 hours to reach this stage of development o The centre pressure continues to drop o Large bands of precipitation have formed (fuel for the storm) o A “warm sector” has formed in the region between fronts - Step FOUR: o The faster moving cold front catches up with the warm front, reducing the size o the warm sector - Step FIVE: o Occlusion occurs as cold front catches up with warm front  This is the most INTENSE part of the storm o There is widespread precipitation  the storm is losing its fuel, therefore starting to weaken - Step SIX: o Storm dissipates after occlusion  the source of the energy (mT air) has been cutoff (i.e., no more latent heating) o Storm gradually weakens and dissipates Midlatitude Cyclones in the Great Lakes Region: - Great lakes storms: o Major precipitation events – lake levels, happening because of the midlatitude cyclones o Major cause of erosion, sediment transport o Ship and property damage - Great lakes storm climatology: ***LOWER PRESSURE = STRONGER STORM*** o Reference: Angel & Isard (1998);
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