AOSS 401, Fall 2006 Lecture 19 October 26, 2007 Richard B. Rood (Room 2525, SRB) Derek Posselt (Room 2517D, SRB)
Class News October 26, 2007 Homework –Homework 5 posted today –Includes a programming assignment that will be posted this afternoon/evening –Focus your attention on question 1
Today Bring together physical concepts and preview the rest of the course Material from Chapter 6 –Middle Latitude Structure –Quasi-geostrophic theory
Flow over a mountain range West to East
What is happening with planetary vorticity? (In the (east-west, north-south) plane) Depth, H Depth, H + ΔH west east s n Depth, H - ΔH Depth, H + ΔH f is greater for deflections to north f is less for deflections to south f + ζ is less than earth’s vorticity and wants to turn north. Arrives here wanting vorticity. “Overshoots”
Flow over a mountain range East to West
What is happening with planetary vorticity? (In the (east-west, north-south) plane) Depth, H Depth, H + ΔH west east s n Depth, H - ΔH Depth, H + ΔH Flow from east planetary and relative vorticity interact together, no overshoot or undershoot.
Wind and geopotential 200 hPa Note: Troughs associated with mountain ranges, continents
Observations of the Atmosphere Vorticity –Small scale flow –Large-scale flow Large scale flow and the climate system –Heat transport –Jet streams –Development of mid-latitude cyclones
Vorticity on Small Scales From the southern California fires: What is the cause?
Vorticity on Large Scales Remember, vorticity is caused by –Wind shear –Rotation in the flow Can we identify these on weather maps? (The following maps come from
300 mb Wind Speed
Where is there positive vorticity?
500 mb Vorticity
Thermal Wind Remember, thermal wind relates –Vertical shear of geostrophic wind –Horizontal temperature gradients Can we identify these on weather maps?
Where are the strongest ?
850 mb Temperature
Convergence/Divergence Remember, vertical motion on large scales directly related to –Convergence/divergence of ageostrophic wind –Curvature in the flow Can we identify these on weather maps?
Where are surface lows/highs?
Surface Precipitation
850 mb Temperature
Concepts Vorticity: shear and curvature –Why is curvature vorticity (as opposed to shear vorticity) usually associated with developing low pressure systems? Divergence and convergence and location of surface high and low pressure systems Thermal wind—vertical shear of the horizontal wind and horizontal temperature gradients
Concepts Features commonly found together –Jet stream –Upper level positive vorticity –Fronts –Midlatitude cyclones (low pressure systems) Coincidence? More on this later…
Large scale flow and the climate system
Transfer of heat north and south is an important element of the climate at the Earth’s surface. Redistribution by atmosphere, ocean, etc. SURFACE Top of Atmosphere / Edge of Space ATMOSPHERE CLOUD heat is moved to poles cool air moved towards equator This is a transfer. Both ocean and atmosphere are important! Large scale weather systems transport large quantities of thermal energy from equator toward the poles
Hurricanes and heat
Mid-latitude cyclones
Mid-latitude cyclones & Heat
Mid-latitude Cyclones & Jet Stream
An estimate of the January mean temperature north winter south summer tropopause stratopause mesosphere stratosphere troposphere note where the horizontal temperature gradients are large
An estimate of the January mean zonal wind north winter south summer note the jet streams
An estimate of the July mean zonal wind north summer south winter note the jet streams
Wind and geopotential 200 hPa Note: Variability in east-west of the wind field. Note: Troughs associated with mountain ranges, continents Note: Time variability of the wind field.
Waves in the atmosphere 300 mb Jet Stream Animation
Short summary We have strong mean zonal winds. We have latitudinal and time variability of the zonal winds –Quasi-stationary long waves. On these quasi-stationary long waves, mid-latitude cyclones form and propagate.
Mid-latitude cyclones What we know: –Low pressure systems –Form through spinup of low-level positive vorticity –Divergence/convergence is key This is just the beginning… –Always closely associated with fronts—why? –Sometimes develop rapidly, sometimes not at all—why?
The mid-latitude cyclone
Mid-latitude cyclones: Norwegian Cyclone Model
Fronts and Precipitation CloudSat Radar Norwegian Cyclone Model
Relationship between upper troposphere and surface note tilt with height
Idealized vertical cross section
What’s at work here?
Mid-latitude cyclone development
Mid-latitude cyclones: Norwegian Cyclone Model ptic/cyclone.htmhttp:// ptic/cyclone.htm
Cold and warm advection cold warm
Lifting and sinking
Increasing the pressure gradient force
Relationship between upper troposphere and surface divergence over low enhances surface low // increases vorticity
Relationship between upper troposphere and surface vertical stretching // increases vorticity
Modern education at its best. /tools/swf/ /tools/swf/
Analysis Tools We have used many of the concepts and tools that we have introduced and explored. –Observed characteristics of the atmosphere –Conservation principles –Scale analysis: Geostrophic and hydrostatic –Thermal wind –Divergence and convergence These ideas are integrated into quasi- geostrophic theory (analysis and prediction)
Programming Exercise Gain experience writing programs to –Read data –Analyze data –Plot data Tools for research/analysis
Remember the vertical structure of the atmosphere Hydrostatic Eq. of State
If we assume T is constant with height (Isothermal)
If we assume T varies with height (Realistic)
If we assume T varies linearly with height (Not a bad assumption, in general)
Programming Exercise Read in data from two sounding files –Height –Potential temperature Compute pressure on each level –Isothermal atmosphere –Varying temperature –Constant lapse rate Use this information –Geostrophic wind –Temperature gradients
Programming Exercise Goals: programming concepts –Reading data –Arrays –Loops –Iteration Materials posted to ctools this afternoon/evening –Skeleton MatLAB program –Data –Instructions
Next Week Programming exercise in class Monday Start looking at quasi-geostrophic system –Scale analysis of equations in pressure coordinates –Quantify wave movement and development