* Your assessment is very important for improving the work of artificial intelligence, which forms the content of this project
Chapter 8 – Part 1 The Dynamic Development of High and Low Pressure Systems Extratropical cyclones are the parent storms of many types of hazardous weather •These cyclones (low pressure systems) result from accelerations created by imbalances between the pressure gradient force and the Coriolis force, primarily at the level of the jet stream. •Today we will look at how the jet stream can spawn high and low pressure systems at the surface Extratropical Cyclone near Iceland Geostrophic Balance and the Jet Stream Jetstream – a narrow band of strong winds that encircle the earth in the midlatitudes. Produced by a strong PGF and balanced by Coriolis force. The jetstream is typically 200 to 300 miles wide and can be located between 250 mb and 500 mb (typically ~300mb) Polar Up to 3 jetstreams can exist between the equator and the pole (they can interact as well): •Subtropical jetstream •Polar jetstream •Arctic jetstream We will generally consider one main jet for this class The large Equator to Pole temperature gradient is what produces large pressure gradient that leads to the jet stream. The exact location of the jet stream is influenced by large temperature gradients found near the surface of the midlatitudes WARM COLD COLD COLD WARM WARM The variations in surface temperature gradients worldwide help make the jets wavy! Convergence – a net inflow of air molecules into a region of the atmosphere Convergence within an air column is always associated with increasing surface pressure, since the mass per unit area, or weight of the column, will increase with time. Divergence – a net outflow of air molecules from a region of the atmosphere (the opposite of convergence) Divergence within an air column is always associated with decreasing surface pressure, since the mass per unit area, or weight of the column, will decrease with time. Faster Slower Faster OK, so now we’ve seen what convergence and divergence does to surface pressure Let’s now look first at how the upper atmosphere can cause convergence and divergence at upper levels, which leads to changes in surface pressure https://youtu.be/hW57tLMcqt4 What force is responsible for throwing the guy off the merrygo-round? The centrifugal force! Gradient Wind Balance A 3 way balance between Coriolis, PG, and centrifugal forces For flow around a high (clockwise in NH), the Coriolis force has to balance both PG & centrifugal forces. The only way to increase Coriolis at one location is to increase the windspeed! HIGH flow Gradient Wind Balance A 3 way balance between Coriolis, PG, and centrifugal forces For flow around a low (counterclockwise in NH), the Coriolis force gets the help of the centrifugal force to balance the PGF. This means Coriolis force can be relatively small, and thus the windspeed slows! flow LOW Its important to note that any cyclonic flow (counterclockwise for NH) will be slower than the geostrophic value and any anticyclonic flow (clockwise for NH) will be faster than the geostrophic value. If the jet turns counterclockwise, it will slow down (NH) If the jet turns clockwise, it will speed up (NH) HIGH LOW Flow around a trough is cyclonic flow and flow around a ridge is anticyclonic flow. HI LOW LOW •Air flows slow around trough and fast around ridge •That means air must speed up moving from trough and ridge (just like the start of a marathon) •That means air must slow down moving from ridge to trough (just like cars at a stop light) •Thus you get air converging moving from ridge to trough, and air diverging moving from trough to ridge Flow around a trough is cyclonic flow and flow around a ridge is anticyclonic flow. •Jetstreak – regions of exceptionally strong winds in the jetstream •Jetstreaks occur where the isobars (or height contours) are very closely spaced. •At A, PGF=CF, and air travels in straight line at constant speed •From A to B, PGF > CF, so air accelerates and heads north a bit. • Since speed increases, CF increases, which will again balance the PGF at C. •From C to D, PGF decreases but wind is already moving fast, which allows CF to be larger than PGF. Flow will turn to right and slow down as a result. •At E, CF decreases since wind decreased. PGF=CF and flow is geostrophic and straight again. PGF=CF PGF > CF CONVERGENCE DIVERGENCE PGF < CF DIVERGENCE CONVERGENCE PGF=CF The net result of these changes in the balances of PGF and CF is the pattern of convergence and divergence around a jetstreak shown below Combined Effect of Curvature and Jetstreaks What happens when a jetstreak and curved flow occur in the same location? Clicker Session 3 points per question for correct answer 1 point per question for participation Where would you expect the surface pressure to decrease most rapidly? Where would you expect the surface pressure to increase most rapidly? Discuss with your Learning Assistants and/or neighbors! Clicker 1 Where would you expect the surface pressure to decrease most rapidly? A. Upper left (two C’s) B. Upper right (two D’s) C. Lower right (C and D) D. Lower left (D and C) Clicker 2 Where would you expect the surface pressure to increase most rapidly? A. Upper left (two C’s) B. Upper right (two D’s) C. Lower right (C and D) D. Lower left (D and C) Where would surface pressure rise most rapidly in this scenario? Where would surface pressure fall most rapidly in this scenario? Discuss with your Learning Assistants and/or neighbors! Jet streak NH 1 3 9700m 2 4 9750m Ridge 9800m 9850m Trough 9900m Trough Where would surface pressure rise most rapidly in this scenario? A. B. C. D. Section 1 Section 2 Section 3 Section 4 Jet streak NH 1 3 9700m 2 4 9750m Ridge 9800m 9850m Trough 9900m Trough Where would surface pressure fall most rapidly in this scenario? A. B. C. D. Section 1 Section 2 Section 3 Section 4 Jet streak NH 1 3 9700m 2 4 9750m Ridge 9800m 9850m Trough 9900m Trough Jet streak NH 3 1 D C 2 C D 9700m 4 9750m D D Ridge C C 9800m 9850m Trough 9900m Trough Explanation: •From curvature you have divergence left of ridge and convergence right of ridge. •From the jet streak, you have divergence at the right entrance and left exit, and convergence at the left entrance and right exit. Often, the jet stream splits and you can get 2 jet streaks adjacent to one another Just considering the jet streaks (not curvature), where would you expect to find the surface pressure dropping most rapidly? A B C D E Explanation D C C D D C C D Have a great weekend!