Spatial morphing - quantile mapping extensions - #2448
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gavinevans wants to merge 21 commits into
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gavinevans wants to merge 21 commits into
gavinevans wants to merge 21 commits into
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…ransition point between forecast sources.
…ight precipitation.
…ons sources is absent.
…s not a weighted average of the input sources, as this can introduce artefacts. Instead, a weighted average of the quantiles is used, and a spatial union mask is created. This avoids the quantile mapping introducing a fade-in/fade-out behaviour, which is undesirable and the spatial union helps avoid scattered showers becoming broader areas of low precipitation.
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Addresses #2441
Description
This PR builds on #2441. Only this commit 56023bd is actually relevant for this PR.
After looking at some more case studies, it was realised that although the initial use of quantile mapping, immediately after the spatial morphing could have benefits, in some situations, it could also introduce artefacts in other situations. Problematic situations are similar to as shown below where the use of a weighted average between source 1 and source 2 as the quantile mapping reference has introduced two adjacent fronts in the interpolated image, over the North Sea, relative to the inputs. This is purely an artefact of the Quantile Mapping reference forecast.
To address this issue, this PR doesn't use a simple weighted average as the quantile mapping reference but instead uses a weighted average of the quantiles, which avoids this kind of spatial artefacts. This, however, can lead to an apparent excess of precipitation in some situations e.g. as shown below, where there is a broader area of low precipitation over Orkney and to the east of Orkney than may be expected, given the smaller scattered showers present in the input.
To reduce this artefact, I've restricted the precipitation to the spatial union of the inputs, which helps avoid broader areas of precipitation occurring.
Testing: