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Download 02-24-2014 Jet Streams, Mid
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REMINDERS: Midterm 2: Friday, February 28 - lecture material covering chapters 6, 7, and 15 (since first midterm and through Wed lecture) - same Format as first midterm UPCOMING REVIEW SESSIONS: - Thursday, Feb 27, 6:30-8:00pm in HSS 1330 Simulated Water Planet Reality with Land Masses par7cularly true in N. Hem where there is more land mass Because of land masses, subtropical highs and polar lows are not con7nuous bands Semi-permanent Subtropical Highs and Polar Lows - Pacific High is an important factor for California's climate Aleutian Low Pacific High Icelandic Low Bermuda High In Northern Hemisphere: Highs Lows Pacific High Aleu7an Low Bermuda High Icelandic Low weaken and strengthen and WINTER shiG south shiG south strengthen and weaken and SUMMER shiG north shiG north U.S. West vs. East Coast Weather/Precipitation 34°N 34°N West US: Prevailing wind is from the Northwest bringing cool and moist air onto land. Because air is cool, still not much actual water vapor in air, so over land RH is low. => inhibits thunderstorm development East US: Prevailing wind is from the Southeast bringing warm and moist air onto land. Because air is warm there is large amount of water vapor in air, so over land RH is high. => good for thunderstorm development Also important: in summer ocean water off West coast (50-70°F) much colder than water off East coast (70-85°F) Polar Front Polar Front PGF 300 mb 300 mb COLD AIR N.Pole WARM AIR ~60°N ~30°N Polar Front JET JT Coriolis Deflects air to the right => Polar Jet (into the screen = westerly) => Maximum speed near top of troposphere PGF 300 mb 300 mb COLD AIR N.Pole WARM AIR ~60°N ~30°N Clicker Question Set Frequency to "BB" When do you think the Polar Jet Stream would be strongest? (A) Equal at all times of the year (B) Winter: When temperature contrast across front is largest (C) Summer: When temperature contrast across front is smallest Clicker Question Set Frequency to "BB" When do you think the Polar Jet Stream would be strongest? (A) Equal at all times of the year (B) Winter: When temperature contrast across front is largest (C) Summer: When temperature contrast across front is smallest Polar Front Stronger Temperature Difference: ==> Stronger PGF ==> Stronger Coriolis needed to balance ==> Stronger Winds PGF 300 mb 300 mb COLD AIR N.Pole WARM AIR ~60°N ~30°N Polar Jet (aka Midlatitude Jet) Subtropical Jet Now, back to the Surface Winds..... Polar Easterlies Polar Front Mid-Latitude Westerlies Horizontal Shear Zone - wind changes direction and/or speed - will induce air to rotate Clicker Question Set Frequency to "AB" The horizontal shear along the polar front will induce the air near the surface to rotate. Which direction will it rotate? (A) Cyclonically (CCW in N. Hem) Mid-Latitude Westerlies (B) Anti-Cyclonically (CW in N. Hem) (C) Will vary back and forth Polar Easterlies Horizontal Shear Zone Polar Front Clicker Question Set Frequency to "AB" The horizontal shear along the polar front will induce the air near the surface to rotate. Which direction will it rotate? (A) Cyclonically (CCW in N. Hem) Mid-Latitude Westerlies (B) Anti-Cyclonically (CW in N. Hem) (C) Will vary back and forth Polar Easterlies Horizontal Shear Zone Polar Front Anti-Cyclonic Flow clockwise in N. Hem Cyclonic Flow counter clockwise in N. Hem Now, back to the Surface Winds..... Polar Easterlies Polar Front Mid-Latitude Westerlies Horizontal Shear Zone - wind changes direction and/or speed - will induce air to rotate => Cyclonic Flow is induced here => Mid-latitude storms (low pressure) may form if conditions are right Divergence aloft can lead to low pressure forming/intensifying at surface and possible storm formation. When does divergence aloft occur?? 1) Waves in Jet Stream 2) Jet Streaks Waves in Jet Stream high above surface (roughly 300 mb height) assume equal isobar spacing and Northern Hemisphere isobars V1 HIGH V2 V4 LOW V3 Waves in Jet Stream high above surface (roughly 300 mb height) assume equal isobar spacing and Northern Hemisphere isobars V1 HIGH CW Flow CF = PGF + V2/R V2 CF = PGF - V2/R CCW Flow V4 LOW V3 Since Coriolis Force (CF) is proportional to velocity, flow around HIGH is faster than flow around LOW for the same isobar spacing (PGF=constant). Waves in Jet Stream high above surface (roughly 300 mb height) assume equal isobar spacing and Northern Hemisphere isobars V1 HIGH V2 CF = PGF - V2/R CCW Flow LOW CW Flow CF = PGF + V2/R V4 A V3 Since Coriolis Force (CF) is proportional to velocity, flow around HIGH is faster than flow around LOW for the same isobar spacing (PGF=constant). V2 > V3 and air in region A is converging (more entering than leaving) ==> Causes surface pressure to increase Waves in Jet Stream high above surface (roughly 300 mb height) assume equal isobar spacing and Northern Hemisphere isobars V1 HIGH V2 CF = PGF - V2/R CCW Flow LOW CW Flow CF = PGF + V2/R V4 A B V3 Since Coriolis Force (CF) is proportional to velocity, flow around HIGH is faster than flow around LOW for the same isobar spacing (PGF=constant). V2 > V3 and air in region A is converging (more entering than leaving) ==> Causes surface pressure to increase V3 < V4 and air in region B is diverging (more leaving than entering) ==> Causes surface pressure to decrease => surface low forms JET STREAKS = REGIONS OF FASTEST WINDS IN JET STREAM Jet Streaks JET STREAKS = REGIONS OF FASTEST WINDS IN JET STREAM Largest ∆T => Largest PGF => Smallest Isobar Spacing => Fastest Winds isobars LOW isobars HIGH JET STREAKS = REGIONS OF FASTEST WINDS IN JET STREAM Largest ∆T => Largest PGF => Smallest Isobar Spacing => Fastest Winds LOW isobars A isobars HIGH At A: winds increase quickly => Coriolis takes time to adjust => Forces temporarily out of balance (too little Coriolis and wind veers to left) JET STREAKS = REGIONS OF FASTEST WINDS IN JET STREAM Largest ∆T => Largest PGF => Smallest Isobar Spacing => Fastest Winds LOW isobars A B isobars HIGH At A: winds increase quickly => Coriolis takes time to adjust => Forces temporarily out of balance (too little Coriolis and wind veers to left) At B: winds decrease quickly => Coriolis takes time to adjust => Forces temporarily out of balance (too much Coriolis and wind veers to right) JET STREAKS = REGIONS OF FASTEST WINDS IN JET STREAM Largest ∆T => Largest PGF => Smallest Isobar Spacing => Fastest Winds LOW isobars Strong Divergence Region Strong Convergence Region A isobars B Weak Divergence Region Weak Convergence Region HIGH At A: winds increase quickly => Coriolis takes time to adjust => Forces temporarily out of balance (too little Coriolis and wind veers to left) At B: winds decrease quickly => Coriolis takes time to adjust => Forces temporarily out of balance (too much Coriolis and wind veers to right) Divergence aloft – creates/enhances low pressure at surface – leads to rising air Forecast of 300 mb winds (roughly Jet Stream level) for Friday a=ernoon Forecast of 300 mb winds (roughly Jet Stream level) for Friday a=ernoon base of trough jet streak
