Lecture 6: Energy balance and temperature (Ch 3)

• atmospheric influences on insolation & the fate of solar radiation

• interaction of terrestrial radiation with atmospheric gases

• global climatology of longwave and net (allwave) radiation

• convective energy transport and the surface energy balance in relation to the near-ground temperature profile

In Ch 2 we dealt with geometry of solar radiation: In Ch 2 we dealt with geometry of solar radiation: now, its interaction with atmosphere… we have to now, its interaction with atmosphere… we have to consider scattering and absorptionconsider scattering and absorption

Fig. 3-2

• absorption implies energy loss from the beam and direct heating of the air

• almost 100% absorption of u.v. by O3

• far greater fraction of IR is absorbed than of visible… NIR is strongly absorbed by water vapour

• scattering (same as “diffuse reflection”) does not heat the air

Fig. 3-3

Direct beam

solar radiation

Direct beam

solar radiation

Scattering out of the direct beam produces diffuse solar radiation… scattering by the air molecules is more effective in the blue than in the red, thus the blue sky

Total incident solar intensity at gnd K [W m-2] is sum of beam + diffuse components

Rayleigh scattering off agents with diameter d << is -selective but omni-directional; Mie scttrng off aerosols is less - selective but mostly forward… so polluted sky is dull

Global-annual (climatological) disposition of solar radiationGlobal-annual (climatological) disposition of solar radiation

Fig. 3-7

• sfc takes 45%• atmos takes 25%

70%

• clouds reflect 19%• atmos reflects 6%• gnd reflects 5%

30% (= planetary albedo)

(But a clear, dry atmosphere may deliver~ 80% of solar energy to the surface)

Clouds mostly scatter(little absorption)

(of this, 7% is u.v. abs by O3, balance mostly NIR abs. by water vpr)

You don’t have to remember all these numbers

Fig. 3-9

Longwave radiationLongwave radiation

Earth’s surfacebehaves like a black body

Shading represents capacity of the atmos. to absorb at that

Spectral absorptivity of atmos. gases

0%

100%• longwave rad’n emitted by sfc largely absorbed by atmos, which re-radiates (at same wavelengths) iosotropically

Atmos gases do not have strong absorption bands in the visible

“window” 8-12 m

Longwave radiationLongwave radiation

• longwave rad’n emitted by sfc largely absorbed by atmos, which re-radiates (at same wavelengths) isotropically

(i.e. in all directions equally)

• thus there is an “infinite cycle of exchange”(atmos-atmos, atmos-grnd,…)

• however the atmos gases are relatively transparent in the 8-12 m “window”

• which however is “closed” by clouds,which absorb virtually alllongwave and re-emitas a grey body

(gases emit “isotropically”, ie.equal probability in all directions, so ½ up, ½ down)

emit

abs/emit

abs/emit

abs/emit

Global-annual (climatological) disposition of longwave radiationGlobal-annual (climatological) disposition of longwave radiation

• atmos emits up & down

4% + 66% = 70%, balancing earth’s net solar gain

• most lngwv from sfc is absorbed

Fig. 3-8

Global-annual (climatological) allwave (net) radiation balanceGlobal-annual (climatological) allwave (net) radiation balance

This imbalance is rectified when we account for convective energy exchange between sfc and atmos

Fig. 3-10

You don’t have to remember all these numbers

Role of (turbulent vertical) convection in local energy balance… dayRole of (turbulent vertical) convection in local energy balance… day

• convection: transport by virtue of bulk motion of a fluid/gas• turbulent vertical convection: vertical transport by eddies• sensible heat: ordinary thermal energy resident in kinetic energy of molecular motion

ground

relatively cool parcel decends

relatively warm parcel ascends

(turbulent) vertical convection of (sensible) (turbulent) vertical convection of (sensible) heat, or “convective heat exchange”, causes heat, or “convective heat exchange”, causes “sensible heat flux” Q“sensible heat flux” QHH [ W m [ W m-2 -2 ]]

Upward flow of sensible heat on average, QH > 0

Daytime near-ground temperature “profile”

z

T=T(z)

Convective “mixing” may be spontaneous (buoyancy-driven, “free Convective “mixing” may be spontaneous (buoyancy-driven, “free convection”)…convection”)…

… … or may be “forced”or may be “forced”

Fig. 3-11

Fig. 3-12

Role of convection in local energy balance… nightRole of convection in local energy balance… night

ground

relatively warm parcel decends

relatively cool parcel ascends

In this case, forced convection causes In this case, forced convection causes downward flow of sensible heatdownward flow of sensible heat

Downward flow of sensible heat on average, QH < 0

Night-time near-ground temperature profile

z

T=T(z)

Role of convection of latent heat in local energy balance… Role of convection of latent heat in local energy balance…

ground

Near-ground humidity (v) profile

relatively dry parcel decends

relatively moist parcel ascends

z

Vertical convection of vapour implies Vertical convection of vapour implies a “latent heat flux” Qa “latent heat flux” QEE [ W m [ W m-2-2 ] ]

Upward flow of latent heat on average, QE > 0

• latent heat: energy that is recoverable upon phase change (condensation)

v=v(z)

Global-annual (climatological) energy balanceGlobal-annual (climatological) energy balance

Fig. 3-14

You don’t have to remember all these numbers

Surface radiation budgetSurface radiation budget

KK

KK LL LL

K* = KK* = K - K - K

L* = LL* = L - L - L

Q* = K* + L* = KQ* = K* + L* = K - K - K + L + L - L - L

Net shortwaveNet shortwave

Net longwaveNet longwave

Net allwaveNet allwave(“net radiation”)(“net radiation”)

(not to scale)

**during clear, dry skies K ~ 0.8 S0 sin , i.e. about 80% of solar beam reaches grnd ( the solar elevation angle)