In this order of ideas, applying the principles of thermodynamics, we present the formulation of two simple semi-empirical parameterizations that determine the decrease of the total cloud cover (ε which includes low cloud, middle cloud and height cloud) in terms of the tropospheric temperature increase (the terms decrease or increase denoted by ∆ should be understood as changes of thermodynamic state). These models include the parameterization of turbulence, convective dynamical systems, cloud microphysics and radiative transfer processes, as well as evaporation, condensation and precipitation, among other parameterizations on which cloud formation depends however, some of these complicated processes are unrealistic and therefore the response of clouds to climate change remains uncertain 14, 15, 16. The cloud formation processes in modern climate models (such as CMIP5), implies the development of several parameterizations that interact with each other. Since early 1980s it has been recognized, as a good approximation, that the tropospheric relative humidity remains constant in a climate change, mainly over oceanic regions near the surface 11, 12, 13, 14. To numerically simulate the response to warming, should also be considered the phenomenon of the warming-trend slowdown, a hiatus in global warming probably due to natural variability 10. Models that simulate a higher-than-average warming also simulate a larger than average shortwave net increase, along with a higher-than-average reduction of the cloud screening (albedo) effect 9. The cloud-aerosol interactions and total cloud cover calculation are also a large source of uncertainty, which may cause large bias in radiation simulation. The importance of cloud cover parameterization can be found in several works 6, 7, 8, addressing that the aerosols produced by desert dust acting as cloud condensation nuclei indirectly influence the cloud cover impacting the Earth’s energy budget by scattering and absorbing solar radiation, absorbing, and emitting thermal outgoing radiation. Besides characterizing the weather, clouds also have and important effect on the dynamics and structure of weather systems, such as precipitation distribution and tropical and extratropical circulation 4. The cloud role in climate consists in modifying the radiation budget regionally and at planetary scales, as clouds impact in both shortwave and longwave radiation 2. Depending on the CM, and even in the simplest model configuration, the patterns of clouds and precipitation response to warming might have substantial variations 5. ![]() More recently, it has been mentioned that one of the shortcomings in CMs are the representation of clouds, precipitation, and circulation 4. ![]() In this way, physical processes are ideally represented by these conservation laws, however, when the processes cannot be represented on certain scales of space and time, semi-empirical parameterizations are necessary to have as many equations as unknowns.Įarly works addressed the importance of a proper cloud parameterization 2, 3. Laws of conservation of energy, momentum, and mass describe the thermodynamics and dynamics of the different processes simulated by the CMs 1. The negative sign of the factor is independent on the conservation or non-conservation of relative humidity in the troposphere under climate change.Ītmospheric climate models (CMs), along with observations, are some of the main tools for weather prediction and climate research. The contribution of the present work consists in finding that the negative sign of the proportionality factor is due to the Clausius–Clapeyron equation that is, to the magnitude of the derivative of the saturation vapor pressure at the typical standard surface temperature of 288 K. ![]() If the shortwave reflection effect of the cloud cover is dominant on a global scale, this parameterization leads to a predominant positive feedback: if the temperature increases like in the current climate change, the cloud cover decreases and more solar radiation reaches the surface increasing the temperature even more. ![]() But if the relative humidity is not conserved, then the cloud cover decreases (increases) by 7.6 pp. If the relative humidity is conserved throughout the troposphere, a 1 ☌ heating (cooling) of the mid troposphere, decreases (increases) the cloud cover by 1.5 percentage points (pp). On a global and annual average, we find a parameterization in which the cloud cover increase is proportional to the mid tropospheric temperature increase, with a negative proportionality factor.
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