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Feature - Clearing the air: solving an atmospheric controversy with DEISA

Feature - Clearing the air: solving an atmospheric controversy with DEISA

The PINNACLE project tests climate models. Image courtesy UCAR

Scientists seeking to develop models for predicting weather, climate and air quality have long been confronted with the fundamental problem of how to accurately forecast the height of the atmospheric boundary layer (ABL) as it develops during daytime heating.

In an attempt to solve this controversy, a team of scientists from the Delft University of Technology in the Netherlands, together with Imperial College London and the National Center for Atmospheric Research in Colorado, initiated the PINNACLE project, using the resources of the DEISA grid of supercomputers.

The ABL is the lower layer of the atmosphere, the part which we live in. Its height grows throughout the day, from a few hundred meters in the morning to one kilometer or more in the afternoon. The ABL has a large Reynolds number (a measure of the turbulence of the system), which means that the motion of the rising hot air within is highly turbulent.

"Turbulence in the atmospheric boundary layer mixes heat, momentum, and bio(chemical) species originating on the surface throughout the entire boundary layer; any inaccurate calculation of the boundary layer height will result in flawed predictions of - for example - temperature and pollutant concentrations," notes Harm Jonker of the Delft University of Technology in the Netherlands, and PINNACLE's lead researcher. "Therefore, for weather, climate, and air quality models, it is of vital importance to correctly forecast the height of the boundary layer as it develops under daytime heating. To put it bluntly: if a model cannot get the boundary layer height correct, it cannot get anything correct."

Ascertaining how to predict the evolution of the daytime ABL, and determine a "growth-rate law," is problematic. There are a number of different incompatible models proposed, and the most widely-used law is controversial because it rests on mutually-inconsistent results from simulations, laboratory experiments and atmospheric observations.

Daytime heating causes the ABL height to rise. Image courtesy UCAR


The PINNACLE project sought to solve the problem by using computer simulations to recreate the different classical laboratory experiments which existing ABL growth-rate laws are based on. By varying the Reynolds number up or down by as much as thousand, and analyzing the impact it made, researchers were able to resolve some of the controversies surrounding the competing theories.

The findings suggest that fluid properties play a much greater part in determining boundary growth than commonly assumed and, more importantly, reveal which of the growth-rate laws is correct. "This law can now be used with full confidence in weather, climate and air quality models," concludes Jonker.

PINNACLE also sheds light on why different laboratory experiments, conducted in the past by various groups using different methods, gave different growth-rate laws.

"One of the most interesting outcomes of the project is our finding that the experiment that historically was most influential in the field was actually right - but for the wrong reasons," remarks Jonker. "In that experiment, the fluid used was heated water in a tank. Compared to the atmosphere, the Reynolds number was too low; however, compared to the fluid in the atmosphere (air), the water's conductivity was also too low. We found that these two elements effectively cancelled each other out, so that the correct 'atmospheric' growth law emerged - somewhat fortuitously - from the experiment."

Because the total computational cost of the simulations exceeded by far anything that can normally be requested at the national level, the team relied on the DEISA framework. The resource allocation was the equivalent of 1.9 million CPU-hours, divided equally among the different supercomputing centers that constitute the DEISA consortium. Some of the simulations were so demanding that the resource allocation on each individual platform was insufficient, and a "super-run" on the Jülich BlueGene supercomputer, at the Jülich Research Centre, was required, which made use of 32,768 processors - half of the machine's full capacity.

-Excerpted from DEISA newsletter 4/09, "Determining the Growth-Rate Law of the Atmospheric Boundary Layer: One Step Closer to Ensuring the Accuracy of Weather Models," by Euan MacDonald. Adapted by Seth Bell, iSGTW

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