Online Weather Generator report 4 Projections of change
The Weather Generator projects changes in temperature and rainfall variables and various derived indices which are not directly available from the climate model output. These changes are driven by Change Factors from the climate model analysis, and the Weather Generator outputs are therefore consistent with the climate model .
Amongst the most notable changes related to temperature are increases in heat wave frequency in the south and east, and major increases in maximum temperatures nation wide, along with reductions in frost days. Amongst changes related to rainfall are increases in dry spell frequency related to summer drying and increases in the annual wettest day amounts.
The Weather Generator has been run for a control period corresponding to the baseline (1961–1990) and future projections to estimate the changes in key climate indices and to check that the changes are consistent with those from the climate model PDFs where possible. The method involves first running the WG for the control (i.e. fitted to observed statistics) 100 times with a , thus generating an ensemble of 100 different time series of length 30 years. Then the WG is run once for each of 1000 climate model output variants for a future projection. The variants are sampled randomly across the PDF. Differences between climate indices calculated from this future ensemble and the baseline ensemble are then derived. This strategy is followed because the variability in the WG baseline is small relative to the variation in the future climate PDF obtained from the climate model outputs, more so for more distant futures.
Two types of analyses are presented here for the differences between the control and future projections. First, tables are presented of detailed measures for eight sites across the UK shown in Figure 6. Second, maps of 25 km grids are presented showing patterns of changes.