Positive Feedback

Positive climate feedback is a process that some initial change in the climate causes some secondary change that in turn increases the effects of the initial change, essentially magnifying the initial effect.

Positive feedback in cloud, clouds likely responded to carbon dioxide induced global warming by amplifying hat warming. The feedback effect of the cloud according to the radiation-convection model and GCM shows a positive feedback pattern.

Radiative-Convective Model

The Radiative-Convective Model is a one-dimensional model for calculating the temperature and vertical distribution of radiative components in the Earth-atmospheric machine. Because it does not include a kinetic equation that can determine vertical or horizontal motion, it only calculates the temperature at which the solar radiation and Earth radiation reach equilibrium. This model reproduces the average vertical temperature distribution of the Earth through a convective control process that simplifies the detailed radiation process and convection process. In this model, the energy sources that cause convection are latent heat and sensible heat generated on the surface of the earth. However, the Radiative-Convective Model has limitations such as that the mechanical process and the circulation of water vapor are not included.

As a result of analyzing the surface temperature according to the cloud distribution through this model, the higher the height of the cloud, the greater the increase in the surface temperature due to long-wave radiation.

General Circulation Model (GCM)

Solar radiation and earth radiation energy absorbed by the Earth-atmospheric climate system are important factors in determining atmospheric temperature and surface temperature, so they vary greatly by the spatial distribution of cloud and water vapor amounts. Meanwhile, the spatial distribution of clouds and water vapor amounts is determined by the form of atmospheric circulation.

Studies inferring the role of the radiation process in the atmospheric circulation model in particular, showed a similar distribution when the ambient environmental conditions were vertical data in the mid-latitude summer region and there is no cloud, with the difference in heating rates between 1 degree/day. On the other hand, in the case of clouds, the difference was very large.

The Impact of Clouds on the Surface Longwave Radiation Budget

The results of analyzing the effect on the surface longwave radiation budget according to the cloud type are as follows.

① The maximum amount of cloud of Cirrus and Cirrostratus appears in areas where convective activity is active, and the low temperature of the upper clouds maximizes the greenhouse effect of clouds. However, the longwave radiation emitted from the cloud bottom is absorbed into the atmosphere between the cloud bottom and the surface, and the effect on the surface is minimal. However, in the case of thin upper clouds, it is known to have a positive feedback effect on the surface temperature because it plays a large role in increasing the surface temperature by releasing longwave radiation from the lower part to the surface while transmitting solar radiation.

② Convective clouds with large optical thickness have cloud layers lying from the middle to upper layers of the atmosphere. In convective clouds, the highest region of cloud is located at a low latitude, but the maximum region of radiation effect appears as a median latitude. In general, convective clouds are generated when the average surface temperature is 25 degrees Celsius or higher, or when the surface is unevenly heated by strong sunlight, resulting in convection. Therefore, cloud feedback is concentrated in the hot environment in the equatorial area or the mid-latitude summer, which is the development of the cyclone route.

③ In the case of mid-level clouds (AC, NS), clouds are widely distributed in the high-latitude region. Similar to the distribution of the amount of cloud, the radiation effect of mid and high-latitude clouds is also large.

④ In the case of lower clouds (stratus, stratocumulus), they are mainly distributed in a wide range of marine areas. They are abundant in mid-latitude oceans where sea level temperatures are low. The radiation effect of clouds on the surface indicates that they are greatly dominated by lower clouds with low altitudes and large amounts. In particular, lower clouds are well developed in the reversion layer area of the ocean, as this reversion of temperature enables higher temperatures than lower temperatures of seawater. Therefore, a lot of radiant energy can be supplied to the sea level by the lower cloud.

However, where the reversion layer does not exist and is not an ocean area, the lower clouds are less affected by long-wave radiation emitted from Earth due to their relatively warm cloud temperature compared to the upper clouds. Instead, it has a much greater effect on reflecting sunlight, which is visible light. Therefore, when these lower clouds increase when the surface temperature increases, it causes a surface cooling effect.

Seasonal cloud-long wave radiation feedback

During the summer, upper clouds (Ci, Cs) appear at their maximum in the Indian Ocean and the Western Pacific Ocean. In particular, almost all regions show a radiation effect of up to 3Wm^-2.

During the winter

① The upper clouds (Ci, Cs) move to the Indian Ocean and SPCZ regions in the southern hemisphere, releasing the cloud's long-wave radiation to its maximum extent. However, the long-wave radiation effect on the surface was smaller than in summer.

② In the case of thick convective clouds, the amount of clouds increases due to the development of active cyclones in the mid-latitude region of the northern hemisphere in winter, and the radiation effect also increases compared to summer.

③ Among middle-level clouds, the seasonal changes in the amount of Cirrostratus and the radiation effect are weak. On the other hand, as the amount of Nimbostratus increases in the northern hemisphere's high altitude region, the cloud radiation effect of more than 10Wm^-2 appears in the northern hemisphere's cyclone path.

④ In the case of lower clouds, the amount of cloud has decreased, but the radiation effect on the ocean has reached 40Wm^-2, contributing greatly to the surface energy budget regardless of seasonal changes.

A mix of clouds and aerosols

The radiative value of the aerosol cloud location according to the radiation can be calculated using the radiative transfer model, and the radiative transfer model used here is SBDART. As for the cloud input conditions, low cloud conditions were assumed and the aerosol layer was located below the cloud, the cloud and the aerosol layer were mixed, and the aerosol was above the cloud.

The figure above shows the radiative transfer process according to the positions of aerosols and clouds. When clouds and aerosols coexist, reflection by clouds is reduced due to light absorption by aerosols, and at the same time, indirect effects are induced. When the aerosol is above the cloud layer, the amount of radiation transmitted through the aerosol layer is mostly reflected by the bright cloud, but is reduced according to the transmittance of the aerosol layer. In this case, a positive radiative forcing effect is expected.

Clouds and aerosols are the main factors affecting the global radiation budget, and when clouds and aerosols coexist, they appear more complex than the radiative forcing effects each exerts alone. Due to the geographical conditions of the continent and the ocean, the frequency of clouds is frequent in the Korean Peninsula, and the radiative forcing effect is highly likely to appear due to the influence of yellow dust. It can be seen that the effect of the aerosol altitude on the radiative forcing is relatively large when the cloud exists at a certain altitude (2~3 km) and the altitude of the aerosol layer increases from 1 km to 5 km. Aerosols with light-absorbing properties absorb radiant energy by themselves in the atmosphere and cause the effect of raising the temperature. will act as a factor.