Experimental Setup

The Western US Drought 2015 experiment is an ensemble of thousands of weather@home simulations run from December 2013 to May 2015, over the Western USA region.

There are three types of simulations within our ensemble:

  1. World with climate change – actual simulations: these simulate observed, current climate conditions using observed sea-surface temperatures and green-house gas concentrations.
  2. World with climate change but no “blob” – climatological simulations: these simulate the current climate conditions, but without the observed “blob” of unusually warm water in the Pacific. Instead of observed sea-surface temperatures, we use the average sea-surface temperatures from 2000-2010. This gives us an idea of the behaviour of the atmosphere with the ocean in an ‘average state’.
  3. World that might have been without climate change – natural simulations: these use pre-industrial green-house gas concentrations, and sea-surface temperatures which are like the observed sea-surface temperatures but with an anthropogenic (human-caused) warming component subtracted. See weather@home 2014 experimental design for more details.

For each of these “worlds” we will submit thousands of simulations to our volunteers, each starting from slightly different initial states of the atmosphere. We will initially submit 5,000 actual simulations, 5,000 climatological simulations and 12,000 natural simulations. Each model is run on the home computer of one of our volunteers and is then uploaded back to our servers.

By comparing the actual simulations and the natural simulations, we can assess the extent to which humans have contributed to the likelihood of the current drought.

And by comparing actual simulations and the climatological simulations, we can assess the extent to which the observed ocean currents and pattern of sea-surface temperatures with the “blob” changed the likelihood of the current drought compared to an average year without a “blob” but with human-induced climate change. 

If the result is too close to call after we receive these results, we may add extra simulations which will increase the statistical confidence in our results.

Sample results modelling precipitation (mm/day) over the Pacific Northwest Region.

– Back to weather@home 2015: Western US Drought