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Concept
Cloud forcing
Résumé
In meteorology, cloud forcing, cloud radiative forcing (CRF) or cloud radiative effect (CRE) is the difference between the radiation budget components for average cloud conditions and cloud-free conditions. Much of the interest in cloud forcing relates to its role as a feedback process in the present period of global warming.
The following equation calculates this change in the radiation budget at the top of the atmosphere
The net cloud radiative effect can be decomposed into its longwave and shortwave components. This is because net radiation is absorbed solar minus the outgoing longwave radiation shown by the following equations
The first term on the right is the shortwave cloud effect (Qabs ) and the second is the longwave effect (OLR).
The shortwave cloud effect is calculated by the following equation
Where So is the solar constant, ∝cloudy is the albedo with clouds and ∝clear is the albedo on a clear day.
The longwave effect is calculated by the next following equation
Where σ is the Stefan–Boltzmann constant, T is the temperature at the given height, and F is the upward flux in clear conditions.
Putting all of these pieces together, the final equation becomes
All global climate models used for climate change projections include the effects of water vapor and cloud forcing. The models include the effects of clouds on both incoming (solar) and emitted (terrestrial) radiation.
Clouds increase the global reflection of solar radiation from 15% to 30%, reducing the amount of solar radiation absorbed by the Earth by about 44 W/m2. This cooling is offset somewhat by the greenhouse effect of clouds which reduces the outgoing longwave radiation by about 31 W/m2. Thus the net cloud forcing of the radiation budget is a loss of about 13 W/m2. If the clouds were removed with all else remaining the same, the Earth would gain this last amount in net radiation and begin to warm up.
These numbers should not be confused with the usual radiative forcing concept, which is for the change in forcing related to climate change.
Source officielle
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