Chapter 4 The Energy Balance of The Surface

1. Chapter 4 The Energy Balance of The Surface • Why The SEB? • What and How? • SEB components (Rn, SH, LE, G, B, Tskin, ε, α, examples) • ABL (neutral, stable, unstable, Ri, z/L, entrainment, LCL, eddy covariance, bulk formulations, examples) • SEB measurements • SEB remote sensing • SEB modeling (LSMs) • International Programs (GEWEX) Kiehl and Trenberth (1997)

2. Earth’s Global Energy Budget Trenberth et al. (2009)

3. SEB Components Rn = H + LE + G Available net radiation is used to do work in the Earth system. The principal use of this energy is in the phase change of water (latent heat, LE), changing the temperature of the air (sensible heat, H), and subsurface (ground heat or the storage of energy, G)

4. Sensible Heat Transfer Sensible heat is heat energy transferred between the surface and air when there is a difference in temperature between them. A change in temperature over distance is called a "temperature gradient". Heat is initially transferred into the air by conduction as air molecules collide with those of the surface. As the air warms it circulates upwards via convection. Thus the transfer of sensible heat is accomplished in a two-step process. When the surface is warmer than the air above, heat will be transferred upwards into the air as a positive sensible heat transfer. If the air is warmer than the surface, heat is transferred from the air to the surface creating a negative sensible heat transfer.

5. Latent Heat Transfer When energy is added to water it will change states or phase. The phase change of a liquid to a gas is called evaporation. The heat added during evaporation breaks the bonds between the clusters of water molecules creating individual molecules that escape the surface as a gas. The heat used in the phase change from a liquid to a gas is called the latent heat of vaporization. We say it is "latent" because it is being stored in the water molecules to later be released during the condensation process. We can't sense or feel latent heat as it does not raise the temperature of the water molecules.  When evaporation is taking place we say there is a positive latent heat flux (transfer). A positive latent heat flux is illustrated with an arrow pointing up away from the surface of the earth. Evaporation is a cooling process for a surface because energy is removed from the water as molecules escape the surface. This causes the surface temperature to decrease. You've probably experienced this cooling when water or sweat evaporates from your skin. Condensation is the phase change from a gas to a liquid (negative latent heat flux).

6. Ground Heat Transfer The third major use of radiant energy is to warm the subsurface of the Earth. Heat is transferred from the surface downwards via conduction. Like in the case of sensible heat transfer, a temperature gradient must exist between the surface and the subsurface for heat transfer to occur. Heat is transferred downwards when the surface is warmer than the subsurface (positive ground heat flux). If the subsurface is warmer than the surface then heat is transferred upwards (negative ground heat flux).   

7. Type of Energy Balances The differences between how energy is used in moist and dry locations. Water is available at the surface for evaporation and latent heat transfer into the air at moist locations. Without available water, no transfer of latent energy occurs, hence the absence of an LE flux for the dry surface. Most of the available energy, Q*, is allocated to sensible heat transfer creating warm air temperatures.

8. The Bowen Ratio The Bowen ratio (B) = H / LE B <1, a greater proportion of the available energy at the surface is passed to the atmosphere as latent heat than as sensible heat, B > 1, the converse is true LE→0, B → ∞, thus a poor choice of variable for arid surfaces. For this reason, use the evaporative fraction (EF): EF = LE/(H+LE) = 1/(1+B) In arid zones, B values are much greater than unity; in humid zones they are much below unity (see Table 4.6 in GPC).

9. Radiative Forcing of the Surface Net Radiation Rn = S↓ – S↑ + F ↓ – F ↑ Net Shortwave radiation S↓ – S↑ = S↓ (1 – α) Net longwave radiation F↓ – F↑ = ε ( F↓ – σTs4) where F↑ = ( 1 – ε ) F↓ + εσTs4

10. The Albedo http://www.eoearth.org/image/Table_1.JPG

11. The Albedo (October 1986) http://www-surf.larc.nasa.gov/surf/pages/bbalb.html

12. The Annual Mean Surface Albedo http://www.eoearth.org/image/Mean_Annual_albedo.JPG

13. The Seasonal Mean Surface Albedo http://www.eoearth.org/article/Albedo

14. Broadband Emissivity http://www-surf.larc.nasa.gov/surf/pages/ems_bb.html

15. Window (8–12 micron) Emissivity http://www-surf.larc.nasa.gov/surf/pages/ems_wn.html

16. Global Land Surface Skin Temperature (July 2003 Data from MODIS sensor) http://www.nasa.gov/centers/goddard/news/topstory/2004/0315skintemp.html

17. The Atmospheric Boundary Layer ABL = The part of the troposphere that is directly influenced by the presence of the earth’s surface, and responds to surface forcings with a time scale of about an hour or less. See http://lidar.ssec.wisc.edu/papers/akp_thes/node6.htmhttp://apollo.lsc.vsc.edu/classes/met455/notes/section9/1.html