Online Briefing report 5.11 Weather Generator
The Weather Generator has been run for a control period corresponding to the UKCP09 baseline climate (1961–1990) and future time periods to estimate the changes in key climate indices at the daily level. Two types of analyses are presented here, for the differences between the baseline and future projection. Firstly, tables are presented of detailed measures for 4 sites across the UK. Secondly, maps of 25 km grids are presented showing patterns of changes.
There are different ways of sampling from a probabilistic distribution to develop Weather Generator output (see the User Guidance). The results shown below are Weather Generator outputs corresponding to change factors randomly sampled from the appropriate PDF for future climate under the Medium emissions scenario for the 2080s. Amongst the most notable changes related to temperature are increases in number of days with high temperatures nationwide and particularly in the south-east, along with reductions in frost days. Amongst changes related to rainfall are increases in dry spell frequency related to summer drying. Table 3 shows future and control percentiles of various temperature indices for 4 representative sites. The changes at 10, 50 and 90th percentile levels of a number of derived indices have been calculated. Major increases in numbers of hot days (above both 25 and 28ºC) are found for the future projection.
Table 4 shows future and baseline percentiles of dry spell frequency for 4 representative sites. There are significant increases in the 10-day dry spell frequency associated with summer drying.
Ensembles of weather generator outputs were produced on a 25 km grid across the UK in a similar manner as for the sites. A sample of the outputs is shown here. Note that maps such as these are not available from the User Interface.
The largest increase in numbers of hot days is found in the south east of England (see Figure 23), where at the 50% probability level () an increase from around 20 to more than 50 days per year is expected. The maps should be interpreted as showing the differences (increases) in frequency of hot days between the same percentile level. For example, the maps of change at 90th percentile level show the number of days per year which is exceeded on average in the top 10 percent of years. Finally, changes in the pattern of dry spells are shown in Figure 24, where modest increases are found across the country and substantial increases in the south and east associated with lower summer rainfall.