In prep

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Manual sections in preparation

This page contains incomplete and/or unreviewed information. It should not be used for reference

Precipitation downscaling from 1h to 5min

For use in UMEP these data must be interpolated to 5-min resolution. Generally variables are linearly interpolated (accounting for the different time stamp conventions); the downscaling for incoming shortwave radiation includes an additional stage using local sunrise and sunset times; for precipitation, the situation is more complex. In the absence of more detailed information the simplest approach is to equally distribute rainfall over all 5-min intervals within the 3-hour period, but this significantly overestimates the duration of rainfall (Ward et al., in review). A more advanced downscaling procedure for precipitation can be applied. The input parameters for the disaggregation model are derived from fits to empirical data and should therefore be updated to represent the location studied. The values provided are for London, UK, based on three years of radar data (2012-2014 inclusive).

  • When useRainModel=0 rainfall accumulations are spread evenly across the model time steps (Ward et al., in review).
  • When useRainModel=1 a time-varying rainfall sequence is generated at the model time resolution using a stochastic process (Ward et al., in review, Gabey et al., in review).

In both cases the accumulated rainfall occurring within each bin of the input rainfall data is conserved.

Name Definition Units Value Site
rainIntensityLogMean Geometric mean of a log-normal distribution describing the rain intensity specified in log-space, i.e. log(geometric mean). mm h-1 -1.104 London, UK (2012)
rainIntensityLogSd Log-standard deviation of a log-normal distribution describing the rain intensity specified in log-space, i.e. log(geometric std dev) log(mm h-1) 0.366 London, UK (2012)
rainExtentScale Scale of two-parameter Weibull distribution fitted to the CDF of temporal extent (start of first rain pulse to end of final rain pulse) of rainfall clusters.
  • A cluster is defined as a series of rain pulses separated by no more than X minutes, where X is 180 min in the example dataset.
min 218.9 London, UK (2012)
rainExtentShape Shape of two-parameter Weibull distribution fitted to the CDF of temporal extent of rainfall clusters 0.650 London, UK (2012)
rainExponent Exponent of a power law function fitted to the empirical power spectrum of the rainfall time series in the frequency interval (0.001, 0.1] -1.013 London, UK (2012)
rainWetProb Mean proportion of cluster time occupied by actual rainfall, i.e. total time spent raining ÷ total duration of rain clusters. A cluster is a sequence of wet and dry spells in which dry spells are shorter than 3 hours. 0.524 London, UK (2012)