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Point Rainfall (from "ecPoint")

In April 2019 ECMWF introduced a new type of experimental product - "Point Rainfall" - into ecCharts, following several years of development work. The issue it aims to address is illustrated on Figure 8.1.14.1.

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Figure 8.1.14.2. Two CDF examples comparing raw gridscale (red), post-processed bias-corrected gridscale (green), and post-processed point rainfall (blue). The first plot is for a site in Spain in summer at day 2. The second plot is for a southern England site in autumn, at day 4. Note that for each case the areas to the left of the green and blue curves, that represent the mean gridbox rainfall over the ENS, should be the same.

Output formats

Output from ecPoint can in principle be for pre-defined periods of time, or instances in time. For point rainfall we currently make available overlapping 12h periods up to day 10, namely T+0-T+12, T+6-T+18,... T+234-T+246. In future we plan also to make available equivalent products for 6 hour and 24 hour intervals.

Calibration

As with any post-processing system ecPoint has to be calibrated. For this it uses short-range Control run forecasts of 12h rainfall covering one year (the "training period"), which are individually compared with rainfall observations, for the same times, within the respective gridboxes. The full procedure is not described here, but involves segregation according to gridbox-weather-types, which each have different sub-grid variability structures and/or different bias corrections associated. The 12h point rainfall system introduced into operations in April 2019 incorporated 214 such types. The type definitions are currently based on the following parameters: convective rainfall fraction, total 12h precipitation forecast, 700hPa wind speed, CAPE, 24h clear-sky solar radiation.

Verification

1 year of global verification of 12h point rainfall products indicates that when compared to point observations, and relative to raw ENS forecasts, the point rainfall forecasts are much more reliable, and have a much better discrimination ability. The net frequency of point observations of no (measurable) rain is much higher in reality than it is in raw ENS gridbox forecasts, whereas the net frequency in point rainfall forecasts is almost perfect, as shown by the verification reliability metric. Meanwhile large totals, such as 50mm/12h, are much better delineated by the point rainfall; using the ROC area metric day 10 forecasts in the point rainfall are as good as day 1-2 forecasts from the raw ENS for this threshold.

Some Uses of Point Rainfall

By giving, for example, non-zero probabilities for very large totals that had a zero probability in the raw ENS output, the Point Rainfall can provide a useful new pointer to when flash floods are possible locally. Likewise if one wants a better idea of how likely it is that a given period remains dry, the point rainfall can usually provide better guidance; indeed in convective situations the point rainfall probabilities for dry should be much better. And where a certain criteria has to be met as the basis for a warning or alert, which might not always be a large total, again the point rainfall should overall provide customers with better guidance than the raw ENS.

The ecCharts Products

Users can type "point" into the "Add Layers" filter box to find point rainfall layers. The first release of point rainfall products into ecCharts in April 2019 came with two new layer options for map display: 

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Figure 8.1.14.5: Probabilities of >10mm from the raw ENS (first plot) and point rainfall (second plot)

Limitations

Users should be aware of some known limitations of point rainfall output, which are listed below:

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Future products may address items 3 and 4, and include a cdf comparison option.

Recent upgrades to ecPoint

From the 12UTC ensemble runs on Monday 23rd May 2022 onwards the post-processing algorithm differs in the following three ways:

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See https://confluence.ecmwf.int/display/FCST/ecPoint+output+improved for further information.

Additional Sources of Information

(Note: Some aspect of older material may now be out of date)

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