Radiation transfer in numerical models of the atmosphere

ECMWFRadiation Transfer in NWP : Introduction 1

Radiation transfer in numerical models of the atmosphere

Jean-Jacques MorcretteRoom 110

extension 2733


LecturesLecture 1: IntroductionLecture 2: Radiation Transfer in the atmosphere:

Basic concepts and approximations Lecture 3: The ECMWF shortwave and longwave

radiation schemesLecture 4: Validation, recent radiation-related

results


ExercisesExercise 1: LW and SW heating rate profiles and

fluxesExercise 2: Diurnal cycle of the longwave window

brightness temperature Exercise 3: Impact of a revised set of radiation

schemes on the behaviour of a GCM


Liou, K.-N., 1992: Radiation and Cloud Processes in the Atmosphere. Oxford University Press, 487 pp.

Liou, K.-N., 1980: An Introduction to Atmospheric Radiation. International Geophysics Series, Vol. 25, Academic Press, 392 pp.

Fouquart, Y., 1987: Radiative transfer in climate models. NATO ASI May 1986, M.E. Schlesinger, Ed., Kluwer Academic Publ., 223-284.

Goody, R.M., and Y.L. Yung, 1989: Atmospheric Radiation - Theoretical basis, 2nd ed., Oxford University Press.

References


Introduction: Outline

Need for parametrisation

Radiation and the general circulation

Global energy balance

Time and space variations of the solar zenith angle

A Top of the Atmosphere (ToA) view of the components of the Earth’s radiative budget


The parametrisation problem - 1

Adiabatic processes

Winds Temperature Humidity Cloud FractionCloud Water

Diffusion Radiation Cumulus convection

Stratiform precipitation

Friction Sensibleheat flux

Evaporation

Groundroughness

Groundtemperature

Snow Ground humidity

Snowmelt



Assumptions Assuming an accurate partition of the poleward transport of heat

between oceans and the atmosphere (i.e., fixed SST as in operational FC), we need a good estimate of the pole-equator radiative imbalance.

Horizontal radiative fluxes are negligible (Independent Column Approximation) so vertical profile of radiative fluxes can be computed from local vertical distributions of the relevant parameters as well as the boundary conditions at the surface and top of the atmosphere



In the ECMWF model, the 3-D distributions of T, H2O, cloud fraction (CF), cloud liquid water (CLW), cloud ice (CIW) are given for every time-step by the prognostic equations.

Other parameters, i.e., O3, CO2 and other uniformly mixed gases of radiative importance (O2, CH4, N2O, CFC-11, CFC-12 and aerosols) have to be specified (prognostic O3 soon interactive with rad).

Radiation black box

Efficient radiation transfer algorithms

Profiles of T, q, CF, CLW, CIW, O3

Climatological data:other tracegases, aerosols

OUTPUTupdated from time to time

to be used in the thermodynamic equation

FLW, FSW to beused in the surface(soil) energy balance equation

Radt

T


Radiation and the general circulation - 1What is the time scale?

(traditionally) Large characteristic time scale (but for clear-sky and the stratosphere). Radiation within stratosphere ~ 50-120 days

10 to 20 times slower than effects of other physical processesSynoptic situations Temperature change Frequency of occurrence

Chinook, foehn 10 K / hour sporadicallyCold wave 20 K / day monthlyCyclone at 850 hPa 8 - 10 K /day weeklyMid-latitude summer cyclone 7 - 8 K / day weeklySubsiding air in high pressure 2 - 3 K / day once / twice a weekEquatorial waves 0 - 3 K / day once / twice a week

Since the main part of the atmosphere (troposphere - stratosphere ?) is far away from state of radiative equilibrium, radiative effects (which are permanent) are generally cumulative and therefore non negligible. They also occur everywhere (NP to SP, surface to ToA).Of course, there are feedbacks! Those between clouds and radiation have a time scale similar to that of the cloud processes, and are therefore much faster.


Radiation and the general circulation - 2

Differences with other physical processes

There exists a well known theory (from Quantum Mechanics to Spectroscopy to

Radiation Transfer).

Radiation is exchanged with the outside space: radiative balance determines the

climate.

The sun providing the energy input, radiation undergoes regular forcings: seasonal,

diurnal.

Radiation at ToA has been globally measured since the 60’s (by operational

satellites), with real flux measurements from ERB (1978), ERBE (1985), ScaRaB

(1993), CERES (1998).

Surface radiation has been (roughly) measured at points over almost 40 years.

Present programs like ARM, BSRN, SURFRAD measure it with high accuracy. Also

satellite-derived SW (and LW) radiation is becoming available.

Therefore, there exist some relatively extended possibilities of validation/verification

(radiation in the SW visible and near-IR, in the LW, … in the W).


The global radiative balance - 1

S = 1370 +/- 4 Wm-2

dm = 1.5 +/- 0.03 1011 m

Earth as a black body would give

R2 S = 4 R2 Tg4 => Tg = 278 K

Earth with an actual albedo = 0.30 R2 S (1-) = 4 R2 s Te

4 => Te = 255 K

S

Difference with actual surface temperatureTs (=288 K) is due to the GREENHOUSE EFFECT:The atmosphere is (almost) transparent in the shortwave range (0.2 - 4.0 m), and more opaque in the longwave range (4-100 m)

=> there exists a temperature gradient between the surface and the ToA


The global radiative balance - 2

237

390

327

343

237

68

169

106

atmosphere

H2O, CO2, O3, ...

90

16

Solar

Terrestrial

Latent heat

Sensible heat

All fluxes in Wm-2


Time and space variations of the solar zenith angle - 1

o = f( latitude, longitude day of the year, time of the day)o = cos ( o )

Two influences:

- amount of energy incident at ToA above of given point of the Earth

S = So ( d / dm )2 0

- the atmospheric mass encountered by a solar beam is proportional to 1 / 0

=> Need to account for the daily cycle, and the yearly seasonal cycle



ImplicationsBetter insolation of the equatorial belt than of polar regions (not

compensated by the terrestrial/thermal/longwave output)=> equatorial regions are warmer than the polar ones=> same pressure layers are thicker at Equator than at the poles=> since sea level pressure is uniformised by friction in the PBL,

given the rotation of the Earth and Buys-Ballot’s law, there should be westerlies

=> there is a need to transport heat from Equator to polar regions oceanic transport disturbances in the zonal mean flow



Implications for modelling In the tropics, the maximum net SW heating of 50 to 60 Wm-2 is only

about 20% of the total absorbed SW radiation.The other 80% contributes to the warming of the tropical surface,

which in turn radiates the energy back to space in terms of LW radiation.

Therefore, any systematic error on the determination of the SW column net heating in the tropics can potentially induce an error 5 times larger in the required poleward transport of heat (by the ocean and the atmosphere)

If SST is specified, the whole error goes into the atmospheric contribution, with direct impact on the atmospheric structure, the stability, and the resultant convection (intensity and temporal characteristics)


Other considerations … “Anomalous SW absorption”

Overall, 70 % of the absorbed SW radiation is deposited at the surface: 169 or is it 150 Wm-2 vs. 237 Wm-2 at ToA.

The controversy is not solved yet. Arguments include: for clear-sky, lack of water vapour absorption, lack of or improper

consideration of aerosol effects for cloudy sky, “anomalous” absorption in clouds, linked to

inhomogeneity in condensed water (horizontal, i.e., sub-grid variability, and vertical, i.e., overlap-related, distributions)

Cess et al., 1995; Arking, 1996; Li et al., 1997; Wild et al., 1998; Cairns et al., 2000

ARM meeting 2002: “When all measurements are considered and space-time scales are properly matched, the unexplained absorption at the SGP site is at most around 10%”

Role of few “huge” drops among a given DSD (Marshak et al., 2003)


A Top-of-the-Atmosphere view

IInter Tropical Convergence Zone:high-level cloudiness: Tcloud << Tsurf

Outgoing Longwave Radiation: OLR

Subtropics:clear-sky or low-level cloudiness

Permanent cloudiness

Polar latitudes: Tcloud not very different from Tsurf

Clear-sky OLR obtained by averaging over the clear-sky situations (based on thresholds)


A Top-of-the-Atmosphere view : ASW = S (1 - )



Absorbed Shortwave Radiation

ASW = Sxy ( 1 - xy )

Highly reflecting stratocumulus cloud decks are seen in the SW, not in the LW

ITCZ



Cloud forcing: Total - Clear-Sky

In the SW: ASWtotal - ASW clear-sky

is negative: Clouds cool the atmosphere-surface system

In the LW: OLRtotal - OLR clear-sky

is generally positive: Clouds heat the atmosphere-surface system

Clouds show large SW and LW cloudforcing in the tropics, which largely cancel out. Overall |SWCF| > |LWCF|, clouds have a cooling effect.


-240

60°S60°S

30°S 30°S

0°0°

30°N 30°N

60°N60°N

135°W

135°W 90°W

90°W 45°W

45°W 0°

0° 45°E

45°E 90°E

90°E 135°E

135°E

TOA lw ejx2 September 2000 nmonth=12 nens=3 Global Mean: -246 50S-50N Mean: -257

[W/m2]

-300

-270

-240

-210

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-150

-125.3

60°S60°S

30°S 30°S

0°0°

30°N 30°N

60°N60°N

135°W

135°W 90°W

90°W 45°W

45°W 0°

0° 45°E

45°E 90°E

90°E 135°E

135°E

TOA lw CERES September 2000 nmonth=12 Global Mean: -239 50S-50N Mean: -250

[W/m2]

-300

-270

-240

-210

-180

-150

-128.0

60°S60°S

30°S 30°S

0°0°

30°N 30°N

60°N60°N

135°W

135°W 90°W

90°W 45°W

45°W 0°

0° 45°E

45°E 90°E

90°E 135°E

135°E

Difference ejx2 - CERES 50N-S Mean err -7.41 50N-S rms 12.4

[W/m2]

10

20

30

30.41

-80

-70

-60

-50

-40

-30

-20

-10

-240

60°S60°S

30°S 30°S

0°0°

30°N 30°N

60°N60°N

135°W

135°W 90°W

90°W 45°W

45°W 0°

0° 45°E

45°E 90°E

90°E 135°E

135°E

TOA lw ej89 September 2000 nmonth=12 nens=3 Global Mean: -248 50S-50N Mean: -261

[W/m2]

-300

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-240

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-150

-125.9

60°S60°S

30°S 30°S

0°0°

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60°N60°N

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135°W 90°W

90°W 45°W

45°W 0°

0° 45°E

45°E 90°E

90°E 135°E

135°E


[W/m2]

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-240

-210

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-128.0

60°S60°S

30°S 30°S

0°0°

30°N 30°N

60°N60°N

135°W

135°W 90°W

90°W 45°W

45°W 0°

0° 45°E

45°E 90°E

90°E 135°E

135°E

Difference ej89 - CERES 50N-S Mean err -10.6 50N-S rms 15

[W/m2]

10

20

22.43

-80

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-40

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-20

-10

-240

60°S60°S

30°S 30°S

0°0°

30°N 30°N

60°N60°N

135°W

135°W 90°W

90°W 45°W

45°W 0°

0° 45°E

45°E 90°E

90°E 135°E

135°E

TOA lw ektk September 2000 nmonth=12 nens=3 Global Mean: -243 50S-50N Mean: -255

[W/m2]

-300

-270

-240

-210

-180

-150

-128.3

60°S60°S

30°S 30°S

0°0°

30°N 30°N

60°N60°N

135°W

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90°W 45°W

45°W 0°

0° 45°E

45°E 90°E

90°E 135°E

135°E


[W/m2]

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-240

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-180

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-128.0

60°S60°S

30°S 30°S

0°0°

30°N 30°N

60°N60°N

135°W

135°W 90°W

90°W 45°W

45°W 0°

0° 45°E

45°E 90°E

90°E 135°E

135°E

Difference ektk - CERES 50N-S Mean err -5.1 50N-S rms 10.2

[W/m2]

10

20

29.43

-80

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-60

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-40

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-20

-10


60°S60°S

30°S 30°S

0°0°

30°N 30°N

60°N60°N

135°W

135°W 90°W

90°W 45°W

45°W 0°

0° 45°E

45°E 90°E

90°E 135°E

135°E

TOA sw ejx2 September 2000 nmonth=12 nens=3 Global Mean: 242 50S-50N Mean: 275

[W/m2]

56.89

100

150

200

250

300

350

361.0

250

60°S60°S

30°S 30°S

0°0°

30°N 30°N

60°N60°N

135°W

135°W 90°W

90°W 45°W

45°W 0°

0° 45°E

45°E 90°E

90°E 135°E

135°E

TOA sw CERES September 2000 nmonth=12 Global Mean: 244 50S-50N Mean: 280

[W/m2]

50

100

150

200

250

300

350

370.4

60°S60°S

30°S 30°S

0°0°

30°N 30°N

60°N60°N

135°W

135°W 90°W

90°W 45°W

45°W 0°

0° 45°E

45°E 90°E

90°E 135°E

135°E


[W/m2]

15

45

75

-130

-115

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-40

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-15

350

60°S60°S

30°S 30°S

0°0°

30°N 30°N

60°N60°N

135°W

135°W 90°W

90°W 45°W

45°W 0°

0° 45°E

45°E 90°E

90°E 135°E

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TOA sw ej89 September 2000 nmonth=12 nens=3 Global Mean: 240 50S-50N Mean: 274

[W/m2]

56.43

100

150

200

250

300

350

360.4

250

60°S60°S

30°S 30°S

0°0°

30°N 30°N

60°N60°N

135°W

135°W 90°W

90°W 45°W

45°W 0°

0° 45°E

45°E 90°E

90°E 135°E

135°E


[W/m2]

50

100

150

200

250

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350

370.4

1560°S60°S

30°S 30°S

0°0°

30°N 30°N

60°N60°N

135°W

135°W 90°W

90°W 45°W

45°W 0°

0° 45°E

45°E 90°E

90°E 135°E

135°E

Difference ej89 - CERES 50N-S Mean err -6.69 50N-S rms 18.4

[W/m2]

15

45

75

-130

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30°S 30°S

0°0°

30°N 30°N

60°N60°N

135°W

135°W 90°W

90°W 45°W

45°W 0°

0° 45°E

45°E 90°E

90°E 135°E

135°E

TOA sw ektk September 2000 nmonth=12 nens=3 Global Mean: 246 50S-50N Mean: 280

[W/m2]

56.40

100

150

200

250

300

350

358.8

150

250

60°S60°S

30°S 30°S

0°0°

30°N 30°N

60°N60°N

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135°W 90°W

90°W 45°W

45°W 0°

0° 45°E

45°E 90°E

90°E 135°E

135°E


[W/m2]

50

100

150

200

250

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350

370.4

15

60°S60°S

30°S 30°S

0°0°

30°N 30°N

60°N60°N

135°W

135°W 90°W

90°W 45°W

45°W 0°

0° 45°E

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Difference ektk - CERES 50N-S Mean err -0.184 50N-S rms 16

[W/m2]

15

45

75

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-15


60°S60°S

30°S 30°S

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135°W 90°W

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90°E 135°E

135°E

TOA lwcf ejx2 September 2000 nmonth=12 nens=3 Global Mean: 17.6 50S-50N Mean: 18.4

[W/m2]

15

30

45

53.17

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30°S 30°S

0°0°

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60°N60°N

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TOA lw cf CERES September 2000 nmonth=12 Global Mean: 27.3 50S-50N Mean: 28.5

[W/m2]

15

30

45

60

75

82.28

60°S60°S

30°S 30°S

0°0°

30°N 30°N

60°N60°N

135°W

135°W 90°W

90°W 45°W

45°W 0°

0° 45°E

45°E 90°E

90°E 135°E

135°E


[W/m2]

10

20

21.90

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45°E 90°E

90°E 135°E

135°E

TOA lwcf ej89 September 2000 nmonth=12 nens=3 Global Mean: 15.2 50S-50N Mean: 15.5

[W/m2]

15

30

38.90

60°S60°S

30°S 30°S

0°0°

30°N 30°N

60°N60°N

135°W

135°W 90°W

90°W 45°W

45°W 0°

0° 45°E

45°E 90°E

90°E 135°E

135°E


[W/m2]

15

30

45

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75

82.28

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0°0°

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90°W 45°W

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135°E

Difference ej89 - CERES 50N-S Mean err -13.1 50N-S rms 16.7

[W/m2]

[W/m2]

10 - 16.754 -80 - -70 -70 - -60 -60 - -50 -50 - -40 -40 - -30 -30 - -20 -20 - -10

60°S60°S

30°S 30°S

0°0°

30°N 30°N

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135°W

135°W 90°W

90°W 45°W

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135°E

TOA lwcf ektk September 2000 nmonth=12 nens=3 Global Mean: 21.8 50S-50N Mean: 22.2

[W/m2]

15

30

45

60

61.59

60°S60°S

30°S 30°S

0°0°

30°N 30°N

60°N60°N

135°W

135°W 90°W

90°W 45°W

45°W 0°

0° 45°E

45°E 90°E

90°E 135°E

135°E


[W/m2]

15

30

45

60

75

82.28

10

-20

60°S60°S

30°S 30°S

0°0°

30°N 30°N

60°N60°N

135°W

135°W 90°W

90°W 45°W

45°W 0°

0° 45°E

45°E 90°E

90°E 135°E

135°E

Difference ektk - CERES 50N-S Mean err -6.36 50N-S rms 10.4

[W/m2]

10

20

24.74

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-30

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-10


60°S60°S

30°S 30°S

0°0°

30°N 30°N

60°N60°N

135°W

135°W 90°W

90°W 45°W

45°W 0°

0° 45°E

45°E 90°E

90°E 135°E

135°E

TOA swcf ejx2 September 2000 nmonth=12 nens=3 Global Mean: -48.7 50S-50N Mean: -50.8

[W/m2]

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0°0°

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135°W 90°W

90°W 45°W

45°W 0°

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90°E 135°E

135°E

TOA swcf CERES September 2000 nmonth=12 Global Mean: -48.7 50S-50N Mean: -48.7

[W/m2]

-160

-145

-130

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-55

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-25

-15

15 60°S60°S

30°S 30°S

0°0°

30°N 30°N

60°N60°N

135°W

135°W 90°W

90°W 45°W

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0° 45°E

45°E 90°E

90°E 135°E

135°E


[W/m2]

15

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75

-130

-115

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30°S 30°S

0°0°

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135°W 90°W

90°W 45°W

45°W 0°

0° 45°E

45°E 90°E

90°E 135°E

135°E

TOA swcf ej89 September 2000 nmonth=12 nens=3 Global Mean: -50.4 50S-50N Mean: -52.7

[W/m2]

-160

-145

-130

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-15

60°S60°S

30°S 30°S

0°0°

30°N 30°N

60°N60°N

135°W

135°W 90°W

90°W 45°W

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0° 45°E

45°E 90°E

90°E 135°E

135°E


[W/m2]

-160

-145

-130

-115

-100

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-55

-40

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-15

60°S60°S

30°S 30°S

0°0°

30°N 30°N

60°N60°N

135°W

135°W 90°W

90°W 45°W

45°W 0°

0° 45°E

45°E 90°E

90°E 135°E

135°E

Difference ej89 - CERES 50N-S Mean err -3.99 50N-S rms 17

[W/m2]

15

45

75

-130

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-40

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30°S 30°S

0°0°

30°N 30°N

60°N60°N

135°W

135°W 90°W

90°W 45°W

45°W 0°

0° 45°E

45°E 90°E

90°E 135°E

135°E

TOA swcf ektk September 2000 nmonth=12 nens=3 Global Mean: -44.5 50S-50N Mean: -46.3

[W/m2]

-160

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0°0°

30°N 30°N

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135°W 90°W

90°W 45°W

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[W/m2]

-160

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30°S 30°S

0°0°

30°N 30°N

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135°W

135°W 90°W

90°W 45°W

45°W 0°

0° 45°E

45°E 90°E

90°E 135°E

135°E

Difference ektk - CERES 50N-S Mean err 2.41 50N-S rms 15.2

[W/m2]

15

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-130

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-15

Radiation transfer in numerical models of the atmosphere

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