The role of the atmosphere in the water cycle
The hydrological cycle is a consequence of the conservation of water substance in the climatic system in its three phases and must be regarded in toto with the aerial and terrestrial branches.
The atmosphere due to its mobility and its capacity to promote phase transitions which lead to precipitation, forces, in a way, the terrestrial branch through the two common boundary hydrological elements: the evaporation and precipitation.
The atmosphere is responsible for the transport of water from the oceans into the continents. The atmosphere plays an important role in modulating the precipitation and also in the radiative forcing at the land surface, through the radiative effects of clouds, trace gases including carbon dioxide, water vapour and aerosols. They profoundly alter the radiative proprieties of the atmosphere. Clouds are the prime regulator of the energy exchanges in the atmosphere. Their feedbacks are the greatest uncertainty in modelling the general circulation of the atmosphere.
Actually Hydrology is based upon the so called classical equation of hydrology, which is a balance equation for the terrestrial branch. It is only a condition equation that does not involve any dynamic principle. However, the balance equation for the moisture in the atmosphere allowing the use of meteorological quantities to define the hydrological elements is based on the dynamics of the general circulation of the atmosphere. Thus, due to the forcing of the terrestrial branch by the aerological branch the dynamics of the terrestrial hydrology can be linked to the dynamics of the aerial branch.
Using aerological data, the basic fields of precipitable water and water vapour transport were analyzed on a global scale. In order to study its intrinsic structure the transport water field was partitioned into a nonrotational part (the potential field) and a nondivergent part (the streamfunction field).
The connection between the atmospheric branch and the hydrology of the Earth’s surface can be studied through the divergence field of water vapour transport, through the two dimensional mean paths of water vapour in the atmosphere and the vertical mean streamlines.
Due to the release of latent heat associated with cloud formation there is a strong generation of entropy within the atmosphere, which is equivalent to the entropy generated by the absorption of solar radiation.
This work points to the importance of the atmosphere for the study of the hydrological cycle showing that the dynamics of Hydrology is, in part, ruled by the dynamics of the general circulation of the atmosphere.
KeywordsWater Vapour Southern Hemisphere Climate System Entropy Generation Hydrological Cycle
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