Surface Energy Exchange or Surface Heat Balance
Background “…The Arctic is considered to be particularly sensitive to climate change…” (Arrhenius, 1896) The complex interactions between the surface and the overlying atmosphere are important aspects of the surface heat budget
Meteorological Measurements – Weather stations – Research vessels/icebreakers – Drifting stations – Drifting buoys – Satellite measurements – Meteorological forecast models and re-analyses
NABOS Summer School 2006 Kapitan Dranitsyn © University of Miami, RSMAS
NABOS Summer School 2006
NABOS Summer School 2006 Kapitan Dranitsyn © University of Miami, RSMAS
Surface heat budget The surface heat budget has two components: • Radiative fluxes: – Short wave insolation (summer, only), dependent on solar geometry (sun above horizon) and state of the atmosphere (moisture, aerosols and clouds); – Long wave emission from the atmosphere (year round) dependent on the state of the atmosphere (moisture, aerosols and clouds). • Turbulent fluxes: – Sensible heat flux – dependent on air sea temperature difference, wind speed, surface roughness; – Latent heat flux – dependent on humidity of the atmospheric surface, wind speed, surface roughness.
Surface and radiative exchanges • The fraction of sunlight (both direct and diffuse) that a surface reflects is called its albedo. • Snow have very high albedos; open ground has a low albedo. Transitions can be on time and spatial scales. • The albedo is also a spectrally varying quantity
What more important? • For the summer season, turbulent fluxes are secondary to radiative fluxes, in terms of heat budget. • For the winter season (polar night), long-wave radiation and turbulent fluxes are dominates
Radiative fluxes generally more important than sensible fluxes • Summertime conditions. • Large standard deviation in SW is caused by diurnal range of solar zenith angle. • Turbulent fluxes ≈ 0 Minnett, 1995
Mathematical description of heat balance (HB) HB – defined by equation, consist of Σ(+) and Σ(-): incoming and out going heat and radiation fluxes (surface, layer…) HB equation – private form of energy saving law t=t 1 t=t 2 Σ (Qsw + Qlw+ Qshf + Qlhf) = ∆Qa-s (=0, ҂0) ∆Qa-s = срρV(T 2 – T 1) – changes of heat content of V=1 during ∆t=1
Physical meaning Radiation Balance (RB) • RB – difference between incoming (↓) and out going (↑) radiation • If RB = 0, SAT (ST) does not change! Radiation equilibrium!
Definition_1 RB = 0 : ↓ = ↑ !!! Typically for Earth as a planet on average per year Heat/Radiation Budget - ↓ ≠ ↑ Heat Content (HC) – heat enclosed in the test column of ground (snow, ice, water)
Definition_2 • Heat Storage (HS) – difference between the moment (current) heat content and a smallest (minimum) heat content during year ±∆Q Layer
Составляющие радиационного баланса подстилающей поверхности Q=(1 -α)Rs – солнечная радиация Тs 4 - излучение Земли RT - противоизлучение атмосферы Eэф = RT – εgσTs 4 эффективное излучение Rп – радиационный баланс подстилающей поверхности
Definiton_3 Reflection of solar radiation. Albedo R 1 Q R 2 α β Backscattering Albedo = (R 1+R 2)/Q
Mirror albedo Frenel formula (“mirror” surface, no backscattering) A = 0. 5[sin 2(f-r)/sin 2(f+r) + tg 2(f-r)/tg 2(f+r)] f – incidence angle; r - refraction angle, sin(f) = 1. 34 sin(r)
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Lecture_2 (Surface heat balance).ppt