midterm 2: friday, february 28 - lecture material covering...

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

Because  of  land  masses,    subtropical  highs  and  polar  lows    

are  not  con7nuous  bands  

Simulated  Water  Planet   Reality  with  Land  Masses  

par7cularly  true  in  N.  Hem    where  there  is  more  land  mass    

Semi-permanent Subtropical Highs and Polar Lows - Pacific High is an important factor for California's climate

Bermuda High Pacific High

Aleutian Low Icelandic Low

In  Northern  Hemisphere:    

         Highs        Lows    

                         Pacific  High                    Aleu7an  Low                        Bermuda  High                    Icelandic  Low  

                                               weaken  and                                                      strengthen  and                                                    shiG  south                                                                shiG  south                                                strengthen  and                                                weaken  and    

                           shiG  north                                                              shiG  north  SUMMER  

WINTER  

34°N 34°N

U.S. West vs. East Coast Weather/Precipitation

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

~60°N N.Pole ~30°N

COLD AIR WARM AIR

300 mb

300 mb PGF

Polar Front

~60°N N.Pole ~30°N

COLD AIR WARM AIR

300 mb

300 mb PGF

Polar Front

JT

Coriolis Deflects air to the right => Polar Jet (into the screen = westerly) => Maximum speed near top of troposphere JET

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

~60°N N.Pole ~30°N

COLD AIR WARM AIR

300 mb

300 mb PGF

Polar Front

Stronger Temperature Difference: ==> Stronger PGF ==> Stronger Coriolis needed to balance ==> Stronger Winds

Polar Jet (aka Midlatitude Jet)

Subtropical Jet

Polar Front

Polar Easterlies

Mid-Latitude Westerlies

Horizontal Shear Zone - wind changes direction and/or speed - will induce air to rotate

Now, back to the Surface Winds.....

Polar Front

Polar Easterlies

Mid-Latitude Westerlies

Horizontal Shear Zone

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) (B) Anti-Cyclonically (CW in N. Hem) (C) Will vary back and forth

Polar Front

Polar Easterlies

Mid-Latitude Westerlies

Horizontal Shear Zone

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) (B) Anti-Cyclonically (CW in N. Hem) (C) Will vary back and forth

Anti-Cyclonic Flow clockwise in N. Hem

Cyclonic Flow counter clockwise in N. Hem

Polar Front

Polar Easterlies

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

Now, back to the Surface Winds.....

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

V2

V3

V4 HIGH

LOW

Waves in Jet Stream high above surface (roughly 300 mb height) assume equal isobar spacing and Northern Hemisphere

isobars V1

V2

V3

V4 HIGH

CW Flow

CF = PGF + V2/R

CF = PGF - V2/R CCW Flow

LOW

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

V2

V3

V4 HIGH

CW Flow

CF = PGF + V2/R

CF = PGF - V2/R CCW Flow

LOW

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

A

Waves in Jet Stream high above surface (roughly 300 mb height) assume equal isobar spacing and Northern Hemisphere

isobars V1

V2

V3

V4 HIGH

CW Flow

CF = PGF + V2/R

CF = PGF - V2/R CCW Flow

LOW

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

A B

JET STREAKS = REGIONS OF FASTEST WINDS IN JET STREAM

Jet Streaks

isobars

JET STREAKS = REGIONS OF FASTEST WINDS IN JET STREAM

Largest ∆T => Largest PGF => Smallest Isobar Spacing => Fastest Winds

isobars

LOW

HIGH

isobars

JET STREAKS = REGIONS OF FASTEST WINDS IN JET STREAM

Largest ∆T => Largest PGF => Smallest Isobar Spacing => Fastest Winds

isobars

A

At A: winds increase quickly => Coriolis takes time to adjust => Forces temporarily out of balance (too little Coriolis and wind veers to left)

LOW

HIGH

isobars

JET STREAKS = REGIONS OF FASTEST WINDS IN JET STREAM

Largest ∆T => Largest PGF => Smallest Isobar Spacing => Fastest Winds

isobars

A

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)

B

LOW

HIGH

isobars

JET STREAKS = REGIONS OF FASTEST WINDS IN JET STREAM

Largest ∆T => Largest PGF => Smallest Isobar Spacing => Fastest Winds

isobars

A

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)

B

Strong Convergence

Region

Weak Convergence

Region

Strong Divergence

Region

Weak Divergence

Region

LOW

HIGH

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