8. Clouds and Climate

Transcription

1 8. Clouds and Climate 1. Clouds (along with rain, snow, fog, haze, etc.) are wet atmospheric aerosols. They are made up of tiny spheres of water from m which fall with terminal velocities of a few cm/sec. With a concentration of a few hundred droplets/cm 3, a small cubic cloud, 1km on a side, contains droplets.

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3 2. Cloud types: cirrus, cirrocumulus, cirrostratus, altocumulus, altostratus nimbostratus, stratocumulus, stratus, cumulus, cumulonimbus. Polar regions Temperate Regions Tropics High ( cirro ) 3-8 km 5-13 km 6-18 km Middle ( alto ) 2-4 km 2-7 km 2-8 km Low ( strato ) 0-2 km 0-2 km 0-2 km

4 Cirro clouds are composed of ice crystals; alto clouds are supercooled with water droplets existing at temperatures below 0 o C; strato implies layering, indicative of stable conditions (vertical velocities m/sec); cumulus literally means heap or mass and implies more unstable conditions (vertical velocities 5-50 m/s). Nimbus means rain or snow producing.

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7 Cloud Fraction - October 2009

8 Annual mean Cloud Cover Global mean cloud cover = 62%

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13 The particle sizes are on the order of 10 m (30 m for ice clouds), and therefore are comparable to the wavelength of terrestrial radiation r~, while for solar radiation r >>, thus Mie scattering results. But clouds tend to be optically thick ( ~ 5 for each 100m thickness of cloud) so multiple scattering occurs an average of interactions with droplets in a cloud with =5. If the total liquid water content in a cloud is: LWC = 4/3 r 3 N where N is the column number density of the drops, then =2 r 2 N If LWC=const. and we decrease drop size such that r 2 =r 1 /2 LWC 1 = 4/3 r 13 N 1 = 4/3 (r 1 /2) 3 N 2 = LWC 2 => N 2 = 8 N 1 and 2 = 2 (r 1 /2) 2 8N 1 = 2 1

14 So for a given water amount, smaller drops (smaller r) mean a greater N and larger. Low clouds have droplet concentrations ranging from s /cm 3 ; ice clouds have concentrations of /cm 3, thus having smaller. Larger will produce more multiple scattering and thus a higher albedo. Clean Air Less drops Lower Albedo Polluted Air More drops Higher Albedo

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18 4. Effect on short-wave radiation: Cloud type Low Middle High Cumuliform Albedo Absorptivity In general, absorption is very small, especially below 0.7 m. Taken by itself, the cloud albedo forcing tends to cool the surface of the earth.

19 5. With Mie scattering, forward scattering is favoured for each droplet. Multiple scattering, however, changes the direction of forward so the greater the thickness of the cloud, the greater the multiple scattering and the higher the albedo. Stratus clouds of 100m thickness would have an albedo of 0.2; for 200m, 0.45; and for greater than 300m, Cirrus clouds, being thin with smaller optical thicknesses thus have a much smaller albedo. The albedo of the planet depends on the amount of cloud cover as well as the cloud thickness. The total (or global) solar radiation (direct Q, and diffuse q) received at the surface on cloudy days is: Q + q = (Q + q) o [b + (1-b)(1-c)] where (Q+q) o = the global solar radiation for clear skies (no clouds) c = cloudiness (fraction of sky cover) b = coefficient depending on cloud type and thickness, and the depth of the atmosphere through which the radiation must pass. For midlatitudes b~0.35 => (Q + q) ~ (Q + q) o [1-0.65c]

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24 6. Effect of long-wave radiation: Due to liquid water absorption bands, water clouds are good absorbers, and the absorption dominates any scattering. At 10 m, water clouds reflect less than 3%, ice clouds less than 1%. Because or the lower water content, the ice clouds are somewhat poorer absorbers also (note that their larger particle size compared to 10 m would make them somewhat poorer scatterers). Absorption depends on thickness as well: a 10m thick cloud will absorb almost nothing, while a 1 km thick cloud absorbs 40%, and a 10 km thick cloud would absorb 100%. 7. The total effect of clouds on long wave radiation involves the reradiation in addition. The amount of radiation being sent up from the earth is proportional to T s4 ; it is almost all absorbed by the bottom of the cloud and then reradiated, half up and half down. That which goes down increases T s. That which goes up is smaller than what originally was absorbed by the cloud, and gets absorbed and reradiated at higher levels in the cloud, again half up and half down.

25 In this way the amount of long wave radiation going out is gradually reduced, and the amount absorbed in the cloud decreases with altitude, resulting in colder temperatures at cloud top. The amount of long wave radiation leaving the cloud top is proportional to the cloud top temperature T ct4. Taken by itself, the cloud greenhouse forcing tends to warm the earth s climate.

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27 March 2000

28 SW Flux LW Flux

29 8. Cirrus clouds transmit most of the incoming shorwave radiation, but trap significant amounts of longwave radiation. High clouds have infrared absorption which increases rapidly with increased thickness, but because of the difference in, albedo changes slowly. At = 1, albedo A=0.1, absorptivity for IR = 0.5 At = 2, absorptivity = 0.8 At = 10, albedo is only up to 42%. Their cloud-greenhouse-forcing is greater than their albedo-forcing, resulting in a net warming of the earth.

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36 9. Low clouds will reradiate at a temperature close to that of the surface, so although they absorb long wave energy as well, there is not much difference in the long wave flux to space. But low clouds have a much higher albedo than the surface, especially at low latitudes over the oceans. They will thus decrease the short wave energy reaching the surface net effect is to produce cooling, although the effect will be small over ice and snow. If the low clouds were to thicken, the reradiation would come from a cloud top at a higher altitude, and if this represented a colder temperature, then it would minimize the cooling. Generally, low clouds have a cloud-albedo-forcing that is larger than their cloud-greenhouse-forcing, resulting in a net cooling of the earth.

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39 Geo-engineering of clouds

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41 10. Deep convective clouds (thunderstorms) emit little longwave radiation at the top, but a lot at the bottom. They also reflect much of the incoming shortwave radiation. Their cloud-greenhouse and albedoforcings are both large, but nearly in balance, resulting in neither warming nor cooling.

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44 11. The total cloud radiative forcing on an annual mean basis is approximately 17 W/m 2. This implies that clouds have a net cooling effect on the earth s climate. Removing all the clouds in the atmosphere would result in a warming of about o C.

45 Units = W/m2

46 Without clouds the Earth would be warmer about 20 degrees Celcius

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48 Clouds in GCMs

49 High Clouds in GCMs

50 Low clouds in GCMs

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55 Homework: Koren et al., 2007: On the twilight zone between clouds and aerosols, Geophys. Res. Lett., 34, L08805, doi: /2007gl029253