User:Hyogase/Greenhouse effect

Clouds
Clouds play an important part in greenhouse effect. They can absorb and emit infrared radiation and thus affect the radiative properties of the atmosphere. Clouds include liquid clouds, mixed-phase clouds and ice clouds. Liquid clouds are low clouds and have negative radiative forcing. Mixed-phases are clouds coexisted with both liquid water and solid ice at subfreezing temperatures and their radiative properties are substantially influenced by the liquid content. Ice clouds are high clouds and their radiative forcing depends on the ice crystal number concentration, cloud thickness and ice water content.

The radiative properties of liquid clouds depend strongly on cloud microphysical properties, such as cloud liquid water content and cloud drop size distribution. The liquid clouds with higher liquid water content and smaller water droplets will have a stronger negative radiative forcing. The cloud liquid contents are usually related to the surface and atmospheric circulations. Over the warm ocean, the atmosphere is usually rich with water vapor and thus the liquid clouds contain higher liquid water content. When the moist air flows converge in the clouds and generate strong updrafts, the water content can be much higher. Aerosols will influence the cloud drop size distribution. For example, in the polluted industrial regions with lots of aerosols, the water droplets in liquid clouds are often small.

The mixed phase clouds have negative radiative forcing. The radiative forcing of mix-phase clouds has a larger uncertainty than liquid clouds. One reason is that the microphysics are much more complicated because the coexistence of both liquid and solid water. For example, Wegener–Bergeron–Findeisen process can deplete large amounts of water droplets and enlarge small ice crystals to large ones in a short period of time. Hallett-Mossop process will shatter the liquid droplets in the collision with large ice crystals and freeze into a lot of small ice splinters. The cloud radiative properties will change dramatically during these processes because small ice crystals can reflect much more sun lights and generate larger negative radiative forcing, compared with large water droplets. Precipitation is also an important factor. Since the mixed phase clouds are usually related to the precipitation, such as storms, large droplets and ice crystals will deplete quickly and cloud water content will drop significantly after the precipitation.

Cirrus clouds can either enhance or reduce the greenhouse effects, depending on the cloud depth. Thin cirrus is usually considered to have positive radiative forcing and thick cirrus has negative radiative forcing. Some scientific research suggests doing some cirrus seeding to thin cirrus clouds and reduce its greenhouse effects, but some other studies doubt its efficiency and think it would be useless to fight with global warming. Ice water content and ice size distribution determines cirrus radiative properties. The larger ice water content is, the more cooling effects cirrus have. When cloud ice water contents are the same, cirrus with more smaller ice crystals have larger cooling effects, compared with cirrus with fewer larger ice crystals.