The role of clouds in the climate system

Abstract

Abstract. Clouds are important for global climate since they have a strong impact on solar and terrestrial radiation as well as on the formation of precipitation. The different types of clouds in the atmosphere are linked to the climate system by a multitude of dynamical and thermodynamical processes including numerous feedback mechanisms. In present-day climate, on average, clouds cool our planet, the net cloud radiative forcing at the top of the atmosphere is about -20 Wm -2 . One of the most interesting questions concerning clouds is: how will they respond to a change in climate? A slight change in cloud amount or a shift in the vertical distribution of clouds might have a considerable impact on the energy budget of the Earth. The Third Assessment Report of the Intergovernmental Panel on Climate Change (IPCC) states clearly that cloud processes and related feedbacks are among the physical processes leading to large uncertainties in the prediction of future climate. The main reason for this is that many microphysical and dynamical processes controlling the life cycle and radiative properties of clouds are not adequately implemented in global climate models. The interaction of aerosols and clouds and the resulting radiative forcing (indirect and semidirect aerosol effect) is one of the major fields of active cloud research at present. This chapter introduces various aspects of the cloud-climate relation and summarizes the discussions of topics currently under scientific debate.

HOLIMO II: a digital holographic instrument for ground-based in situ observations of microphysical properties of mixed-phase clouds

“…The number concentrations of the water droplets (d < 34 µm) showed a mean concentration of 116 cm −3 , which is in agreement of the typical value of 100 cm −3 observed for altostratus and altocumulus clouds (Quante, 2004).…”

Use of a Lidar Forward Model for Global Comparisons of Cloud Fraction between the ICESat Lidar and the ECMWF Model

“…Clouds play a major role in the earth's radiation budget and predictions of future climate (Stephens et al 1990;Solomon et al 2007;Quante 2004), yet both ice and liquid clouds are often poorly represented in general circulation models (Arking 1991), which is one of the major factors limiting the accuracy of future climate predictions. In numerical weather prediction, clouds are important for a number of reasons, including the model radiation scheme, surface temperature forecasts, visibility, aircraft icing forecasts, and their role in the formation of precipitation.…”

Water in the Earth's atmosphere

“…Beside this vital role, clouds contribute to the vertical and horizontal redistribution of water vapour in the atmosphere. As a result of their significance in the radiation and energy budget of the Earth [6,7,79], in many regions of the globe clouds determine the rates of evaporation and influence regional and local circulation systems through the release of latent heat or heating-and cooling rates associated with radiative processes. Substantial requirement for the effective formation of clouds are the water vapour saturation of the environment and the existence of suited cloud condensation nuclei (CCN) and ice nuclei (IN), respectively [80][81][82][83].…”

“…It is the most important greenhouse gas in the atmosphere [2]. Clouds, which backscatter radiation in the visible spectral range and absorb in the infrared, cover large areas of the sky and thereby modify, in a dominant way, the radiation balance and thus climate [3][4][5][6][7]. Water is also important because it occurs in its three phases in the atmosphere, ice, liquid and gas.…”