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STORMPAC 4.1 User Guide
C4
DISAGGREGATION PROCEDURE
C4.1
Summary of the Algorithm
The disaggregation procedure can be summarised as follows. A 1-hour rainfall depth is read in
from a file, which contains the hourly series to be disaggregated. A 5-minute series is simulated
using the fitted model. The 5-minute simulated series is summed and the 1-hour total of the
simulated series compared to the 1-hour total that was read in. The simulated series is discarded
if the absolute difference between the simulated 1-hour total and the total read in exceeds
0.05 mm. The process is repeated until the totals are in agreement to within 0.05 mm. The
simulated series then represents a possible realisation of 5-minute data representative of the 1hour value that was read in. Following this procedure, a record of 1-hour rainfall depths can be
disaggregated.
In the above procedure, some additional rules are applied. These include using overlapping 5minute values from the previous disaggregated hour provided the total of these overlapping
values do not exceed the 1-hour total to be disaggregated. This allows for some influence, due to
overlapping cells, from the previous hour. In addition, the disaggregated series are scaled to
achieve an exact match to the hourly series (although clearly this is only a very minor adjustment,
given they fall to within 0.05 mm of the hourly total).
It should be noted that the disaggregation procedure is not accounted for when fitting the model –
it is a separate procedure that could be applied to any model capable of simulating 5-minute
series. Therefore it is likely that some (probably small) bias would be introduced into the
distributional properties of the resultant disaggregated series.
C4.2
Tests
The Farnborough data were aggregated to hourly values and then disaggregated using the
above procedure. The distribution of the resultant disaggregated series was compared to the
historical 5-minute distribution using quantile plots; the results are shown in Figures C3-5.
Figure C3 gives the overall result, where the data for all months have been pooled. From this
figure it can be seen that the distribution of the disaggregated series compares favourably to the
distribution of the historical 5-minute series. The exception is a slight curvature in the mid to lower
part of the plot, which would translate into a slight over-estimation in part of the distribution tail.
The plots for January and July show a similar pattern, i.e. a slight over-estimation for some of the
distribution tail (Figures C4 and C5).
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