The role of water vapour in the atmosphere. A short overview from a climate modeller's point of view

Abstract Water vapour plays a dominant role in the radiative balance and the hydrological cycle. It is a principal element in the thermodynamics of the atmosphere, it transports latent heat, it contributes to absorption and emission in a n umber of bands and it condenses into clouds that reflect and adsorb solar radiation, thus directly affecting the energy balance. In the lower atmosphere, the water vapour concentrations can vary by orders of magnitude from place to place. This variability creates a fundamental problem in climate modelling due to the very high temporal and spatial resolution needed to resolve all processes creating the sharp gradients which are related to the variability. The contribution of water vapour to atmospheric phenomena on different time and space scales for today's and future climates are discussed as well as the importance of water vapour monitoring. The latter is a prerequisite for model validation and an important contribution to the understanding of the behaviour of the atmosphere. It is shown that only the validation of more than one component of the hydrological cycle leads to a better understanding and an improvement of the simulations.