Feature - Counting the Clouds

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When meteorologists model atmospheric conditions, the forecast they create is a kind of photograh made of ‘pixels’ as well. For climate models, such as ECHAM5, used widely in Europe, these ‘pixels’ are columns of air approximately 1 kilometer thick (vertically) and 250 by 250 kilometers wide (horizontally). Anything smaller than the pixel size, cumulus clouds a few kilometers wide for example, are not in resolution.  

Researchers Petri Räisänen and Heikki Järvinen at the Finnish Meteorological Institute are working to change this.

“There are many things we do not know about clouds,” says Räisänen. “What we want to do is improve the methods of how to calculate their impact of radiation in the atmosphere.”

Clouds affect the earth’s energy balance in two ways. Some incoming solar radiation reflects off clouds back to space, which lends a cooling effect. Clouds also act like a blanket, capturing thermal radiation emitted from the earth, warming the climate. Qualitatively, meteorologists know the cooling effect is stronger. Quantitatively, however, the model is limited.

Calculating the transfer of radiation through the atmosphere requires assumptions about the structure of clouds. So far, these assumptions have been written within models describing the transfer of radiation. To avoid over-cumbersome  calculations, simplified and even unrealistic assumptions have been widely used. A prime example is the assumption that the density of cloud water is horizontally uniform within each column of air several hundred kilometers wide.

Räisänen is using a different approach. “We are taking the cloud structure outside the radiative transfer model. In practice, this means it can be made much more flexible and hopefully more accurate.”

Räisänen's work effectively increases the resolution detail of these models, to include cloud details, without altering the radiative transfer codes themselves—codes that are complicated and difficult to modify.

“That’s the beauty of this thing really,” he says. “If I was using the old approach I’d be making it much more complicated and more expensive.”

Räisänen makes use of an approach called Monte Carlo Independent Column Approximation.  A small additional component is included in the climate model that generates the structure of clouds within each column of air (i.e. "pixel"), using statistical assumptions about the variations of cloud properties within that column.  In this way, small-scale variations in cloud water density, size of cloud droplets for example, can be included in the calculations, which was not feasible
before. The results from this approximation can be put back into the model and, Räisänen says, “it is essentially as simple as it was before.”  This enhanced, higher-resolution model offers a clearer, more accurate representation of the atmosphere.

The work of Räisänen and Järvinen has been implemented into the ECHAM5 model. They plan to continue working towards increasingly comprehensive climate models, particularly one that would include sea surface temperatures. For their calculation intense work, they use grid computing resources from DEISA—Distributed European Infrastructure for Supercomputing Applications.

- Danielle Venton
iSGTW Editor