ESRL Quarterly Newsletter - Fall 2010

Director's Corner

"I've looked at clouds from both sides now..."

The words of a popular song written decades ago have relevance to the science going on today in ESRL.

On the front page of this issue is a discussion of the August Nature paper by Chemical Science Division’s (CSD) Graham Feingold and a team that looked at a type of cloud field seen over large areas of the globe. In the paper, an interesting temporal sequence of the cellular structure of cloud fields is discussed, as is the role of aerosols within them. The role of aerosols in cloud, including how they affect planetary albedo (reflection of sunlight) has been a fundamental research topic for climate scientists. The large areas of stratus cloud, particularly prevalent over oceans off the west coast of continents, play a major role in the Earth’s radiation balance. If added greenhouse gases reduce the area covered by this type of cloud, that would enhance global warming (a positive feedback). Conversely, if these cloud fields increase, it would slow the rise in global temperature. Clouds are important at all length and time scales—they are one of the best examples of fractals in nature. Thus, understanding the emergent behavior of the clouds seen in these studies should be helpful in improving atmospheric prediction from the shortest time scales (weather) to the longest (climate).

There is another interesting body of cloud-related work in ESRL’s Physical Sciences Division (PSD), led by George Kiladis. He has used satellite imagery and other techniques to look at tropical waves. As a graduate student, I recall having to repeat the derivation of a very elegant analysis of atmospheric waves in the “equatorial waveguide,” originally done by Taroh Matsuno (and later by Adrian Gill). Kiladis and collaborators have used Outgoing Longwave Radiation (essentially cloud structures observed from satellites) to determine the structure of tropical waves, and find a whole bestiary of such cloud clusters, including Kelvin Waves, eastward and westward propagating Rossby waves, and inertia-gravity waves. These equatorial cloud structures—most notably the Madden-Julien Oscsillation (a type of cloud cluster that propagates westward along the tropical band, taking many weeks to cross the Pacific)—offer the exciting potential of extending predictions beyond the chaotic limits of a few weeks to allow significant predictability in the range of a couple of months. I regard the over-emphasis of the meteorological community on mid-latitude meteorology (where most meteorologists live) as having limited the progress that we could be making in the tropics. For example, spectral models are great for mid-latitude cyclones, but are poor in the tropical belt for several reasons, most notably because their resolution is less there than in mid-latitudes. This was part of the rationale behind ESRL’s development of global models on icosahedral grids, and the results in the FIM model have shown improved tropical skill.

Within ESRL’s Global Systems Division (GSD), there are important new advances in computing and global modeling. In computing, Mark Govett and collaborators are leading in the next big revolution in geophysical supercomputing: using Graphical Processing Units (GPUs) to run atmospheric models. Govett’s group has demonstrated speed-ups greater than 30 times using GPUs on the new Nonhydrostatic Icosahedral Model (led by Jin Luen Lee of GSD). A calculation by Jaques Middelecoff estimates that given the growth of GPU speeds, it may be possible to run twice-daily 10-day global cloud-resolving models (horizontal resolution of four kilometers or less) as early as 2012.

Advances in geophysical prediction typically follow a sequence: they start with observations, followed by scientific understanding of the relevant processes, and ultimately are incorporated into predictive models. Here in ESRL, we can see the entire process in action. A dream of mine would be that GSD’s new cloud-resolving global models would use the advances from CSD and PSD to capture clouds realistically in weather and climate prediction in a way that is unequaled elsewhere in the world.

At ESRL, we look at clouds from all sides now.

--Alexander MacDonald