Lecture 8: Thermal Wind, Ozone, Precipitation, Rainshadow Effect

1. Lecture 8 • Thermal wind (consistency requirement between change in geostrophic wind with height and change in temperature in the horizontal) • Ozone • Precipitation • Rainshadow effect

2. The thermal wind (not a wind!) • Hydrostatic balance in the vertical together with geostrophic balance in the horizontal puts constraints on the horizontal temperature field. • If there is a horizontal temperature gradient (in a hydrostatic atmosphere), there must be a change in the geostrophic wind in the vertical

3. Vertical structure in the atmosphere • What about pressure? • Hydrostatic equation: balance between pressure gradient force and gravity. • dp/dz = - rho g • Ideal gas law: • p = rho R T Remember! z = - H ln (p/p0), where H is scale height and is only constant if T is constant. In other words, p = p0 exp(- z/H)

4. Forces that move the air • Gravitational force (g=9.8 m/s2) • Pressure gradient force -1/rho x dp/dx -1/rho x dp/dy in x and y direction, respectively. PGF points toward lower p. The pressure gradients causing the wind are horizontal. • Coriolis force • Centrifugal force • Frictional force

5. Geostrophic wind, geostrophic balancePGF + CF = 0

6. When air between two pressure levels is warmed, the distance between the two pressure levels (the thickness) increases. This creates horiz. PGF

7. Notes on thermal “wind” (shear not wind) • Hydrostatic balance tells us that pressure must decrease more rapidly in the vertical in cold air than in warm air. • Cold air more compressed than warm air (denser) • 300mb pressure sfc is at a higher altitude at 30N than at the pole. A PGF must act (on constant pressure surfaces) from the south to the north. • The geostrophic wind is proportional to the slope of pressure sfc, the greater the slope the stronger the wind • Slope of pressure surfaces keeps increasing with altitude (therefore westerly wind increases in the vertical in the lower to mid troposphere.

8. Winds are more westerly as you go up where it’s colder toward the poles

9. Continuity at surface as air flows toward the center of the low. Air must go up! Rain or sun

10. Sea breeze

11. Scales of motion in the atmosphere • Microscale – less than a km • Mesoscale – from 1km to few hundred km. Thunderstorms, fronts are mesoscale systems. Coriolis force becomes important at longer scales. • Synoptic scale systems ~1000 km, geostrophic balance is important. • Planetary scale systems are greater than 1000 km

12. Ozone • Why do we care about ozone? • Harmful to organisms on the surface • Helpful – absorbs harmful solar radiation • Where is ozone? • Troposphere • Stratosphere • What is the ozone hole? • Up to 70% seasonal reductions • Halogen gasses (CFCs)- Cl, Br, I • Polar Stratospheric clouds (-80 C) • Montreal Protocol 1987

16. Hemispheric differences in land distribution

17. Precipitation growth in warm clouds • Collision – coalescence process • In cold clouds, ice crystals may collect super cooled droplets and grow fast (accretion)

18. Ice crystal aggregation

19. Saturation vapor pressure over ice is less than over water at same temperature Water vapor is preferentially attracted to ice vs. water

20. Ice crystals grow at the expense of the supercooled water drops

21. When air is saturated w.r.t water, it is supersaturated w.r.t. ice

22. Virga (if rain evaporating) Fallstreaks (shown here, ice)

24. Steps to forming precipitation

25. Rain shadow effect