Downscaling is the term given to the process by which climate projections on a large (coarse) spatial scale are processed to provided projections for smaller (finer) scales.
For instance model runs that give climate projections at the 250km scale might be downscaled by a factor of 10 to provide projections at the 25km scale.
There are two types of downscaling that have been used in UKCP09 in order to produce finer resolution projections:
1. Dynamical downscaling: Regional Climate Model (RCMs) are a type of dynamical downscaling. Dynamical downscaling uses atmospheric interactions in the coarser resolution Global Climate Model (GCM) to develop a finer resolution model. In UKCP09 the dynamically downscaled RCM, HadRM3 has been used to produce 11 runs of regional climate projections at the medium Emissions scenarios (A1B) on a daily time scale. These 11 runs were used to downscale the larger Ensemble of GCM runs.
2. Stochastic and statistical downscaling: The UKCP09 Weather Generator is an example of stochastic and statistical downscaling. It is based around a stochastic model that simulates future rainfall sequences. Other weather variables are derived from the rainfall states. Statistical measures within the Weather Generator are then modified according to the UKCP09 probabilistic projections.
- An overview of how probabilistic projections are created and presented in UKCP09 is described in Box 3 of the UKCP09 Briefing report.
- More details are provided in Chapter 3 of the UKCP09 Climate change projections report, including an overview of the methodology. The dynamical downscaling approach used is explained in more detail in section 3.2.11.
- An overview of how the Weather Generator works is provided in the UKCP09 Weather Generator report.