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What is it?

The UKCP09 Weather Generator produces a set of a minimum of 100 daily (and hourly) time series from 30 to 1000 years in length (length specified by user with different options available for daily and hourly outputs).

The UKCP09 Weather Generator does not generate predicted weather for any actual time in the future. To remind users of this fact, the first year in the generated outputs is always 3001 with subsequent years labelled as 3002, 3003, etc. As such, users should not be using the provided set of time series to look for a specific date or time period. Instead they should be using the entire set of time series to explore the nature of the future climate (extremes, thresholds, etc.) or as input to vulnerability, impacts and adaptation assessments.

There are no graphical products available through the UKCP09 User Interface based on the Weather Generator outputs. Due to the nature of the outputs, graphical products are not appropriate or practical. Some maps based on specially run ensembles of UKCP09 Weather Generator outputs are available in the Weather Generator report, but these are not available from the UKCP09 User Interface.

Variables

The Weather Generator produces time series with each daily and hourly output within the series consisting of daily mean temperature, daily temperature range, vapour pressure and sunshine duration. From these variables are calculated potential evapotranspiration, maximum and minimum temperature, relative humidity and direct and diffuse radiation.

Hourly time series (if requested) are comprised of variables estimated from the daily values using simple disaggregation rules (see section 5 in Weather Generator report) derived from the baseline observed climate with the rules remaining fixed for future time periods.

Users will always be provided with a full set of variables (i.e. there is not an option to deselect variables when building a request within the Weather Generator).

Options available when building a Weather Generator request

When configuring a request for the UKCP09 Weather Generator within the User Interface, user can select to use 100 to 1000 model variants from the selected probabilistic projections (sampled data for a particular location, emission scenario and 30-year time period). The minimum number of variants from with the sampled data that can be selected is 100 with the default being that these are randomly selected from the 10,000 available within the specified sample data.

User can also further refine their choice by specifying specific model variant ID. Users may wish to exploit this functionality where there is a need to examine the internal variability, such as when conducting a flood risk assessment. In this case, users would choose to request 100 to 1000 runs using a single model variant (selected by ID). Users are strongly discouraged from using this option unless they fully understand the implications. The results from an impacts assessment based on  considering this limited selection of outputs should not be used for decision making and are highly susceptible to misinterpretation.

Nature of the output

Users will receive the following set of outputs:

1. Control run – 100 time series of 30 years for the baseline period

2. Future climate runs – minimum of 100 time series of user-defined runs perturbed using a given future climate. The number and length of the time series can be set within specified limits by the user when configuring the Weather Generator request within the UKCP09 User Interface (the default is 30 years and 100 time series).

3. Weather Generator driving statistics file – presenting the absolute values of variable statistics use to drive the Weather Generator. This is similar to the change factor file, but provides absolute values rather than transformations of climate change values.

4. Metadata – includes information on the request parameters selected, internal Weather Generator settings, seed values, model variant IDs used, etc.

For each of the Control run and the future climate runs, each time series consists of a single row for each time step (day or hour) with the columns in each row providing, in order:

Daily time series:

• Year, month, day, day count within year, transition, mean total daily precipitation rate (mm/day), minimum daily temperature (ºC), maximum daily temperature (ºC), vapour pressure (hPA), relative humidity (%), sunshine hours (hours), downward diffuse radiation (Wh/m2), direct radiation (Wh/m2), and potential evapotranspiration (mm/day) 

Hourly time series:

• Year, month, day, hour, total hourly precipitation (mm), mean hourly temperature (ºC), vapour pressure (hPA), relative humidity (%), sunshine (fraction of an hour), downward diffuse radiation (Wh/m2), and direct radiation (Wh/m2).

These are the default outputs. User choices cannot reduce the amount of output provided. Depending on choices made when configuring the Weather Generator run, the resulting output files could potentially be up to 1GB in size (when zipped or in binary format). This would typically take about 6 hours to download with a 1 Mbit/sec broadband connection.