Chapter 4 The Energy Balance of The Surface Kiehl and Trenberth (1997) 1.Why The SEB? 2.What and How? a.SEB components (Rn, SH, LE, G, B, Tskin, ε, α, examples) b.ABL (neutral, stable, unstable, Ri, z/L, entrainment, LCL, eddy covariance, bulk formulations, examples) c.SEB measurements d.SEB remote sensing e.SEB modeling (LSMs) f.International Programs (GEWEX)

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

The Atmospheric Boundary Layer 1.Definition: 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. 2.Structure: free atmosphere, entrainment zone, mixed layer (where U, θ, q almost constant with height), surface layer (where vertical fluxes of momentum, heat, and moisture are almost constant with height) 3.Thickness: typically 1 km; varying from 20 m to several km; deeper with strong solar heating, strong winds, rough surface, or upward mean vertical motion in the free troposphere. 4.Both structure and thickness have a strong diurnal cycle. 5.Turbulent motions (opposite to laminar flow) i.chaotic swirls; rapid chaotic fluctuations in winds, temperature, moisture, other mass ii.generated mechanically (in the presence of strong near surface mean winds), or iii.generated thermally (strong solar heating  high buoyancy  vertical motion) (mostly daytime, land; also common over the oceans) 6.ABL clouds: fog, fair weather cumulus, stratocumulus,

Potential Temperature The potential temperature ( θ ) of a parcel of air at pressure P is the temperature that the parcel would acquire if adiabatically brought to a standard reference pressure P 0 (= 1000 millibars). where T = the current absolute temperature (in K) of the parcel, R = the gas constant of air, and c p = the specific heat capacity at a constant pressure. See GPC Appendix C for derivations. θ is a more dynamically important quantity than T. Under almost all circumstances, θ increases upwards in the atmosphere, unlike T which may increase or decrease. θ is conserved for all dry adiabatic processes, and as such is an important quantity in the ABL (which is often very close to being dry adiabatic). The dry adiabatic lapse rate: Γ d = g/c p = 9.8 °C/km θ is a useful measure of the static stability of the unsaturated atmosphere. stable, vertical motion is suppressed; unstable, convection is likely

Stüve diagram Stüve diagram (Thermodynamic Diagram) Isotherms are straight and vertical, isobars are straight and horizontal and dry adiabats are also straight and have a 45 degree inclination to the left while moist adiabats are curved (see also GPC Appendix C, Fig. C.1). T=20°C, P=1000 mb  θ= 20°C T=20°C, P=900 mb  θ= 28.96°C A parcel with P, T, q  T d =? q*=?, RH=?, LCL=? Δq=?

Thermodynamics

Air Flow and Turbulent Vortices Air flow can be imagined as a horizontal flow of numerous rotating eddies, a turbulent vortices of various sizes, with each eddy having 3D components, including vertical components as well. The situation looks chaotic, but vertical movement of the components can be measured from the tower.

Determine Vertical Fluxes

Reynolds Decomposition and Eddy Covariance

Bulk Aerodynamic Formulas (Parameterizations) τ = ρ C DM U r 2 SH = c p ρ C DH U r [T s – T a (z r )] LE = L ρ C DE U r [q s – q a (z r )] C DN = [κ / ln(z r /z 0 )] 2 C DM = C DN,M f M (R iB ) C DH = C DN,H f H (R iB ) C DE = C DN,E f E (R iB )

Global Distribution of Sensible Heat Flux

Global Distribution of Latent Heat Flux

Regional Patterns of The Surface Energy Balance Yuma, AZ energy balance (ly/day) At the other extreme is Yuma, Arizona, a warm and dry climate. The most noticeable characteristic of this place is the lack of latent heat transfer. Though ample radiation is available here, there is no water to evaporate. Nearly all net radiation is used for sensible heat transfer which explains the hot dry conditions at Yuma. West Palm Beach, Fl energy balance (ly/day) West Palm Beach, Florida is located in a warm and moist climate. Latent energy transfer into the air is greatest during the summer time which is the wettest period of the year, and when net radiation is the highest. During the summer, sensible heat transfer decreases as net radiation is allocated to evaporation and latent heat transfer.

Modeling of The Surface Energy Balance NCAR CLM: for global climate modeling and projections NCEP Noah LSM: for numerical weather predictions

NCAR CLM 3.5 Biogeochemistry Ecosystem Dynamics Hydrology Biogeophysics Niu, Yang, et al., 2007 Niu, Yang, et al., 2005 Yang et al., 1997, 1999 Niu & Yang, 2003, 2006 Yang & Niu, 2003 Collaborators: UT (Z.-L. Yang, G.-Y. Niu, R.E. Dickinson); NCAR (G.B. Bonan, K. Oleson, D. Lawrence) 2008 CCSM Distinguished Achievement Award

Collaborators: UT (Z.-L. Yang, G.-Y. Niu, D. Maidment), NCAR (Fei Chen, Dave Gochis); NCEP (Ken Mitchell) 1-D ‘Noah’ Community Land Surface Model Dynamical Routing Methodologies Explicit diffusive wave overland flow Explicit saturated subsurface flow Groundwater discharge, reservoir routing & Explicit channel routing fully distributed flow/head reservoir levels distributed soil moisture distributed land/atmo fluxes distributed snow depth/SWE Noah LSM with hydrological enhancements

Observing The Surface Energy Balance FLUXNET See also other flux measurement networks (e.g., Ameriflux, CarboEurope, Fluxnet Canada, and iLEAPS).Ameriflux CarboEuropeFluxnet CanadaiLEAPS

International Programs GEWEX Many others