Introduction

Clouds are one of the main causes of uncertainty in predictions of future climate, and are an important weather forecasting problem principally via their role in precipitation formation. We use advanced remote sensing techniques, particularly radar and lidar, to improve understanding of cloud processes and to provide the essential information on clouds necessary to improve their representation in numerical weather forecast and climate models.

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• Associated research groups: The Radar Group | The Radiation Group

Code for download

• Stochastic cirrus model. A simulator to generate realistic 3D cirrus fields using Fourier techniques.
• Multiple scattering algorithms. A fast forward model for calculating the effect of multiple scattering on radar and lidar.
• Variational rain-hail retrieval. A retrieval algorithm for polarimetric radar.
• Maxwell2D. Animations of the propagation of electromagnetic waves to understand a wide range of atmospheric phenomena and optical instruments.

Clouds and Climate

The problem of clouds for climate prediction is illustrated in the diagrams below. The figure on the left indicates that we are confident that the planet will warm over the coming century (predominantly due to the increase in greenhouse gases), but that the magnitude of the change is uncertain. One of the main reasons for the uncertainty is that the global distribution of clouds could alter in response to a surface warming, which would then have a "feedback" effect on surface temperature. For example, reduced coverage of low cloud would result in less of the suns rays being reflected back to space and the surface would warm still further. The figure on the right demonstrates the current uncertainty in the magnitude and even the sign of this feedback in a selection of climate models.

Predicted change in average global surface temperature over the next 100 years, from the 2001 report of the Intergovernmental Panel on Climate Change. The various lines correspond to different emission scenarios and the bars on the right indicate the uncertainty, as indicated by the range of predictions from the various climate models.

Effect of clouds to amplify or counteract the warming due to increased greenhouse gases, in 18 climate models. Panel a shows that this can range from a 30% reduction to a doubling of the warming. Panel b shows that this is due to very different changes in the way clouds interact with solar (shortwave) and thermal infrared (longwave) radiation.

Before the problem of cloud feedback can be addressed we need to better understand the distribution and behaviour of clouds in the current climate. Most current operational measurements of clouds lack either the necessary vertical resolution or the ability to retrieve the parameters important for climate, such as the average size of the particles or the amount of condensed water they contain. This motivates the development of new observational techniques based around "active" sensors, such as radar and lidar. Learn more...