1. Lecture 7-8: Energy balance and temperature (Ch 3) the diurnal cycle in net radiation, temperature and stratification the friction layer local microclimates influences on regional temperature patterns

2. The diurnal (daily) cycle in net radiation at the base of the atmos. Q* = K* + L* = K  - K  + L  - L  -L* L* is typically negative unless there is low cloud cover

3. Surface energy budget Q* = Q H + Q E + Q G Q* QHQHQHQH QGQGQGQG QEQEQEQE (shows sign convention only… each flux can have either sign) (= ground/lake/ocean heat flux) an arbitrary example of a duirnal cycle

4. Understanding the diurnal (daily) cycle in temperature (similar principles apply to understanding the seasonal cycle) Fig. 3-22a

5. Night-time near-ground temperature profile… “stable stratification” z T=T(z) Daytime near-ground temperature profile… “unstable stratification” z T=T(z) Diurnal cycle in near-ground stratification Upward heat flow, vertical mixing enhanced (p65) Inversion Inversion … downward heat flow, mixing damped

6. The atmospheric boundary layer and the depth (  ) of mixing z  “free atmosphere” no friction no friction vertical velocities steady and of order cm s -1 except vertical velocities steady and of order cm s -1 except in clouds/over mountains “friction layer” or “boundary layer” friction reduces windspeed friction reduces windspeed variation of wind with height, instability (warm air underneath cold), and flow around obstacles variation of wind with height, instability (warm air underneath cold), and flow around obstacles produce turbulence vertical velocities fluctuate and are of order m s -1 vertical velocities fluctuate and are of order m s -1

7. Depth (  ) of mixing varies in time/space Depth of the ABL (i.e. magnitude of  ) depends on the turbulence, and increases with: stronger surface heating Q H stronger wind rougher surface dawndusk  Order 100 m Order 1 km winter summer

8. Nocturnal Radiation Inversion Cause … ‘ ground cooling: Q* < 0, ie. outgoing longwave radiation exceeds incoming longwave ‘ then air above cools by convection (stirring), Q H < 0 Conditions for severest inversion … ‘ clear sky, dry air ‘ long night with light wind Result: radiation frost? Photo :Keith Cooley

9. Complexity of local (site- specific) effects on local radiation and energy balance… producing “micro-climates” that can be manipulated (eg. windbreaks) Figs. 3-21

10. Latitudinal variation in net allwave radiation Averaged over a long period, latitudinal heat advection by ocean (25%) and atmosphere (75%) rectifies the imbalance a S 0 +L  ( 1-a ) S 0, a the albedo Fig. 3-15

11. Why do we consider earth’s global climatological temperature T eq to be at equilibrium (Sec. 3-2)? Because there is a stabilizing feedback... Let  eq be the change in  eq over time interval  t. Then: Where R is earth’s radius, S 0 is the solar constant, a (=0.3) is the planetary albeto,  (  1) is the planetary emissivity and  is the Stefan-Boltzmann constant. The proportionality constant involves the heat capacity of the earth-atmosphere system. (In reality a,  may depend on  eq ). Rate of change  gains - losses area of earth’s shadow area of earth’s surface

12. At earth’s equilibrium temperature, there is balance... Set a =0.3 and  =1 to obtain earth’s (radiative) equilibrium temperature (Sec. 3-2). Common factor cancels

13. Factors controlling temperature on regional & global time & space scales Latitude solar radiation distribution of land & water** surface thermal inertia, surface energy balance topographic steering/blockage of winds Ocean Currents advective domination (horizontal heat transport) Elevation

14. Fig. 3-18a latitudinal temperature gradient is greatest in the winter hemisphere in summer (winter) temperature over land warmer (cooler) than over ocean

15. Why are water bodies “more conservative” in their temperature? solar radiation penetrates to some depth so warms a volume much of the available radiant energy used to evaporate water mixing of the water in the ocean/lake “mixed layer” ensures heat deposited/drawn from a deep layer water has a much higher specific heat (4128 J kg -1 K -1 ) than “land”