Regional model

This experiment runs simulations of perturbed physics versions of a regional climate model running inside a global climate model.

• This experiment is now in the alpha test stage.

Dynamical models of the global climate system, such as the model used in the main climateprediction.net experiment, divide the atmosphere and ocean into blocks and simulate the transfer of energy, mass, moisture, and other properties between those blocks. Earth is very big so these models need to use a large number of blocks. The blocks used in the atmospheric component of the main climateprediction.net experiment model are about 300km across (more precisely, 3.75 degrees longitude by 2.5 degrees latitude) with 19 blocks stacked on top of each other, while those in the highest resolution climate model currently being run on a supercomputer are about 100km across. Getting them smaller is difficult: halving their size in each dimension means slowing down the model by a factor of 16 (because the time step also has to be shorter for smaller blocks). But this is much larger than the resolution needed to figure out what has happened or may happen in your area. Also, keep in mind that useful information from these models is only produced at much larger scales (i.e. when they stop looking blocky). Is there any way around this?

Maybe there is. One idea is to run a much higher resolution climate model over a smaller area than the globe, covering a few million square kilometres. At each time step this "regional climate model" takes the weather at its edges (including the surface) from what a global model says it is. The regional model then simulates all the weather going on inside this domain using its much higher resolution.

This sounds like a neat idea, but there are still many things we do not know about regional modelling. This experiment addresses the question of how important uncertainty in parameters in the regional model are for simulating the regional climate. Essentially, it is the main climateprediction.net experiment but using a regional climate model.

This experiment will run both the global and regional models together. The global model will be HadCM3L, identical to what is run in the main climateprediction.net experiment. Like the main experiment, values of uncertain parameters in the model will differ across simulations. The regional model will be HadRM3, used by the UK Met Office's PRECIS programme. Values of uncertain parameters in the regional model will also be altered across simulations.

This experiment will focus on two regions: one set of simulations will have the regional model placed over western North America, while the other set will have the regional model placed over southern Africa.

For the first time, climateprediction.net will perform regional climate modeling for western North America. Regional modeling provides better spatial detail, which is critically important in mountainous regions. By producing thousands of simulated model futures, this regional experiment will for the first time provide detailed probabilistic answers to key questions about aspects of climate change of great societal relevance that go beyond changes in annual mean temperature and precipitation: frost days, measures of heat waves, number of consecutive dry days, extreme daily precipitation, wind speed, extreme wind events, snowpack, and coastal upwelling, to name a few. Changes in these quantities could affect agriculture, energy demand, human health, coastal ecosystems, flood risk, water supply, and many more aspects of economic and environmental values.

The domain covered by the regional model for the Western North American experiment is shown in this image (showing topography in metres):

The resolution is about 24km. Note how individual mountain ranges are visible here; in standard global climate models these mountain ranges are merged into a blob. Thus, the higher resolution of the regional model should lead to a more accurate representation of how the mountains affect the weather here.

The domain covered by the regional model for the Southern African experiment is shown in this image:

Because this region is bigger than the Western North American region, resolution here will be lower, about 49km. Southern Africa contains a number of mountainous regions. Also, weather in the tropical areas is dominated by small thunderstorms, rather than the large weather systems typical in mid latitudes. So we expect the increased resolution of the regional model to be important here too.

This project addresses the two key limitations on climate change science in southern Africa:

• a constrained capacity to undertake robust exploration of the range of possible future climate at a regional scale;
• the limited understanding of the complex interaction of key physical processes governing the regional climate response to anthropogenic forcing.

Richard Jones, Simon Wilson (UK Met Office)
Myles Allen, Tolu Aina, Milo Thurston, Hiro Yamazaki (University of Oxford)
Philip Mote, Eric Salathé, Valérie Dulière (University of Washington)
Bruce Hewitson, Dáithí Stone, Chris Jack (University of Cape Town)

This project is funded by Microsoft Research.