ecPoint post-processed ensemble forecasts, for 12h rainfall (available in interactive form in ecCharts and static form in Open Charts) have changed.
From the 12UTC ensemble runs on Monday 23rd May 2022 onwards the post-processing algorithm differs in the following three ways:
- The mathematical representation of the mapping functions, that convert ensemble member gridbox rainfall into point rainfall, essentially changes from a discretised to a continuous format, whilst remaining non-parametric. The distribution tails, which are all-important for capturing localised extremes, improve as a result. Map plots attain a smoother appearance, particularly at short leads.
- The mapping functions and decision tree structure have been completely re-built for model cycle 47r3, partly to account for the fact that 47r3 included substantial changes to model physics.
- Whilst the five governing variables in the decision tree are basically the same, two are now more representative of reality:
- The "maximum CAPE" variable now uses the maximum of 12 CAPE values during the valid period whilst previously it used just 3.
- Our use of the "clear sky solar radiation" variable now takes better account of aerosol.
Together these changes should result in post-processed forecasts that are more reliable and more skillful.
In tests the changes unsurprisingly led to differences in ecPoint rainfall distributions. Mostly these are quite small, but locally the upper (wet) percentile values can differ by 50% or more. The higher percentiles that are the most extreme on a global plot tend to be even larger in the new version. The bias-corrected forecast gridbox rainfall totals (=mean of point rainfall distribution) also exhibit local differences but are mostly similar. Globally-averaged forecast rainfall hardly changes.
The table below demonstrates, for different areas, the impact of applying the changes listed above on some pre-selected percentiles on a particular day. For all plots the data time is 00UTC 13th May 2022 and the valid period 06-18UTC 15th. Scales are mm/12h.
| 50th percentile (median) | 90th percentile | 99th percentile | |
|---|---|---|---|
| Global |
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| Europe |
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|  |
| Tropics / Sub-tropics |
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|  |
