Chapter 4 The Energy Balance of The SurfaceChapter 4 The Energy Balance of The Surface

Kiehl and Trenberth (1997)Kiehl and Trenberth (1997)

1.1. Why The SEB?Why The SEB?2.2. What and How? What and How? a.a. SEB components (Rn, SH, LE, G, B, Tskin, SEB components (Rn, SH, LE, G, B, Tskin, εε, , αα, examples) , examples) b.b. ABL (neutral, stable, unstable, Ri, z/L, entrainment, LCL, eddy ABL (neutral, stable, unstable, Ri, z/L, entrainment, LCL, eddy

covariance, bulk formulations, examples) covariance, bulk formulations, examples) c.c. SEB measurementsSEB measurementsd.d. SEB remote sensing SEB remote sensing e.e. SEB modeling (LSMs)SEB modeling (LSMs)f.f. International Programs (GEWEX)International Programs (GEWEX)

The Atmospheric Boundary LayerThe 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.htm http://apollo.lsc.vsc.edu/classes/met455/notes/section9/1.html

Kiehl and Trenberth (1997)Kiehl and Trenberth (1997)

Earth’s Global Energy BudgetEarth’s Global Energy Budget

Earth’s Global Energy BudgetEarth’s Global Energy Budget

Trenberth et al. (2009)Trenberth et al. (2009)

Earth’s Global Energy Budget (2000–2010)Earth’s Global Energy Budget (2000–2010)

Stephens et al. (2012), Nature GeoscienceStephens et al. (2012), Nature Geoscience

Earth’s Global Energy BudgetEarth’s Global Energy Budget

Earth’s Global Energy BudgetEarth’s Global Energy Budget

SEB ComponentsSEB Components

Rn = H + LE + GRn = H + LE + G

Available net radiation is used to do work in the Earth Available net radiation is used to do work in the Earth system. system.

The principal use of this energy is in the phase change of The principal use of this energy is in the phase change of water (latent heat, water (latent heat, LELE), changing the temperature of ), changing the temperature of the air (sensible heat, the air (sensible heat, HH), and subsurface (ground ), and subsurface (ground heat or the storage of energy, heat or the storage of energy, GG))

Sensible Heat TransferSensible Heat Transfer

Sensible heatSensible heat is heat energy transferred between the surface and air when there is heat energy transferred between the surface and air when there is a difference in temperature between them. A change in temperature over is a difference in temperature between them. A change in temperature over distance is called a "distance is called a "temperature gradienttemperature gradient". ".

Heat is initially transferred into the air by conduction as air molecules collide 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 with those of the surface. As the air warms it circulates upwards via convection. Thus the transfer of sensible heat is accomplished in a convection. Thus the transfer of sensible heat is accomplished in a two-step two-step processprocess. .

When the surface is warmer than the air above, heat will be transferred When the surface is warmer than the air above, heat will be transferred upwards into the air as a upwards into the air as a positive sensible heat transferpositive sensible heat transfer. If the air is warmer . If the air is warmer than the surface, heat is transferred from the air to the surface creating a than the surface, heat is transferred from the air to the surface creating a negative sensible heat transfernegative sensible heat transfer..

Latent Heat TransferLatent Heat Transfer

When energy is added to water it will change states or phase. The phase change When energy is added to water it will change states or phase. The phase change of a liquid to a gas is called of a liquid to a gas is called evaporationevaporation. The heat added during evaporation . The heat added during evaporation breaks the bonds between the clusters of water molecules creating breaks the bonds between the clusters of water molecules creating individual molecules that escape the surface as a gas. The heat used in the 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 phase change from a liquid to a gas is called the latent heat of vaporizationlatent heat of vaporization. . We say it is "latent" because it is being stored in the water molecules to 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 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.  latent heat as it does not raise the temperature of the water molecules. 

When When evaporation is evaporation is taking place we say there is a taking place we say there is a positive latent heat flux positive latent heat flux (transfer). A positive latent heat flux is illustrated with an arrow pointing (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 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 a surface because energy is removed from the water as molecules escape the surface. This causes the surface temperature to decrease. You've probably surface. This causes the surface temperature to decrease. You've probably experienced this cooling when water or sweat evaporates from your experienced this cooling when water or sweat evaporates from your skin. skin. Condensation is the phase change from a gas to a liquid (negative Condensation is the phase change from a gas to a liquid (negative latent heat flux).latent heat flux).

Ground Heat TransferGround Heat Transfer

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

Type of Energy BalancesType of Energy Balances

The differences between how energy is used in moist and dry locations. Water is 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 available at the surface for evaporation and latent heat transfer into the air at moist locations. Without available water, no transfer of latent energy 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 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 available energy, Q*, is allocated to sensible heat transfer creating warm air temperatures.temperatures.

The Bowen RatioThe Bowen Ratio

The Bowen ratio (B) = H / LEThe 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).

Radiative Forcing of the SurfaceRadiative Forcing of the Surface

Net RadiationNet RadiationRRn = S = S↓ – S – S↑ + F + F ↓ – F – F ↑

Net Shortwave radiation

SS↓ – S – S↑ = S = S↓ (1 – (1 – αα))

Net longwave radiation

FF↓ – F – F↑ = = εε ( F ( F↓ – – σσTTs4))

wherewhere

FF↑ = ( 1 – = ( 1 – εε ) F ) F↓ + + εσεσTTs4

The AlbedoThe Albedo

http://www.eoearth.org/image/Table_1.JPG

The Albedo (October 1986)The Albedo (October 1986)

http://www-surf.larc.nasa.gov/surf/pages/bbalb.html

The Annual Mean Surface AlbedoThe Annual Mean Surface Albedo

http://www.eoearth.org/image/Mean_Annual_albedo.JPG

The Seasonal Mean Surface AlbedoThe Seasonal Mean Surface Albedo

http://www.eoearth.org/article/Albedo

Broadband EmissivityBroadband Emissivity

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

Window (8–12 micron) EmissivityWindow (8–12 micron) Emissivity

http://www-surf.larc.nasa.gov/surf/pages/ems_wn.html

Global Land Surface Skin Temperature (July 2003 Global Land Surface Skin Temperature (July 2003 Data from MODIS sensor)Data from MODIS sensor)

http://www.nasa.gov/centers/goddard/news/topstory/2004/0315skintemp.html

SummarySummary

Kevin Trenberth, GEWEX Meeting, The Hague, The Netherlands, 2014