Transcript Slide 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 The diurnal (daily) cycle in net radiation at the base of the atmos. Q* = K* + L* = K - K + L - L L* is typically negative unless there is low cloud cover -L* Surface energy budget Q* = QH + QE + QG Q* QE QH (shows sign convention only… each flux can have either sign) QG (= ground/lake/ocean heat flux) an arbitrary example of a duirnal cycle Understanding the diurnal (daily) cycle in temperature (similar principles apply to understanding the seasonal cycle) Fig. 3-22a Diurnal cycle in near-ground stratification Daytime near-ground temperature profile… “unstable stratification” z Upward heat flow, vertical mixing enhanced (p65) T=T(z) Night-time near-ground temperature profile… “stable stratification” Inversion … downward heat flow, mixing damped z T=T(z) The atmospheric boundary layer and the depth () of mixing “free atmosphere” • no friction • vertical velocities steady and of order cm s-1 except in clouds/over mountains “friction layer” or “boundary layer” z • friction reduces windspeed • 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 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 QH • stronger wind • rougher surface summer Order 1 km winter Order 100 m dawn dusk Nocturnal Radiation Inversion Cause … ground cooling: Q* < 0, ie. outgoing longwave radiation exceeds incoming longwave then air above cools by convection (stirring), QH < 0 Conditions for severest inversion … clear sky, dry air long night with light wind Result: radiation frost? Photo :Keith Cooley Figs. 3-21 Complexity of local (sitespecific) effects on local radiation and energy balance… producing “micro-climates” that can be manipulated (eg. windbreaks) Latitudinal variation in net allwave radiation Averaged over a long period, latitudinal heat advection by ocean (25%) and atmosphere (75%) rectifies the imbalance Fig. 3-15 Why do we consider earth’s global climatological temperature Teq to be at equilibrium (Sec. 3-2)? Because there is a stabilizing feedback... Let Teq be the change in Teq over time interval t. Then: Teq t area of earth’s surface area of earth’s shadow R (1 a) S0 4 R T Rate of change 2 gains 2 - 4 eq losses Where R is earth’s radius, S0 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 Teq ). At earth’s equilibrium temperature, there is balance... R (1 a) S0 4 R T 2 2 4 eq 0 Common factor cancels Set a =0.3 and =1 to obtain earth’s (radiative) equilibrium temperature (Sec. 3-2). 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 • latitudinal temperature gradient is greatest in the winter hemisphere • in summer (winter) temperature over land warmer (cooler) than over ocean Fig. 3-18a 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”