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Online Briefing report 3.1.3 Emissions uncertainty 4.1 Changes in daily climate

Changes in daily climate, such as the frequency of hot or very wet days, are likely to be more significant for many climate impacts than changes in or seasonal averages. Whilst, as we saw in the previous section, we are not able to project changes in storm tracks and anticyclones with confidence, we can project how the characteristics of daily time series (weather) could be affected by changes in the more basic aspects of future climate, such as monthly mean temperature and precipitation and other aspects of their distributions, which we have more confidence in projecting.

In order to provide consistency between impact studies, we have incorporated a weather generator in UKCP09 to supply plausible realisations of how future daily time series could look, consistent with changes in the characteristics of monthly-average climate. The UKCP09 Weather Generator provides such synthetic of temperature (mean, maximum and minimum), precipitation, relative humidity, vapour pressure, (PET) and sunshine (from which we also estimate diffuse and direct downward solar radiation) at a resolution of , for each of the three emission scenarios and each of the future 30-yr time periods — 2020s, 2030s, etc.

As pointed out in Box 2, there is a cascade of confidence in climate projections at different scales, and the UKCP09 Weather Generator does not add any additional climate change information over that which is present in the probabilistic projections. The 5 km scale is used to add local topographic information (e.g. hills, valleys) and it is based on observed data which is representative of that scale. Users can average changes from a small number (up to 40) of 5 km squares to get changes over a bigger area.

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