This page describes the way the anomaly and uncertainty of the ensemble forecasts in the sub-seasonal and seasonal products is are determined using the climatology as reference. This includes also how the expected forecast anomaly category (the 'dominant' one of the 7 predefined ones) and the uncertainty category (of the 3 predefined ones of low/medium/high) of the ensemble forecasts are determined. This is a generic procedure, which is the same for both EFAS and GloFAS, as it is executed the same way for each river pixel, regardless of the resolution, and also the same for the sub-seasonal and seasonal products, as it works in the exact same way regardless of whether it is weekly mean values, as in the sub-seasonal, or monthly mean values, as in the seasonal.
The characterisation of the forecast signal in both the sub-seasonal and seasonal is based on the ensemble member's extremity in the context of the model climatological distribution, which is explained further below.
Climatological percentiles and forecast anomaly categories
From the climate sample, 99 climate percentiles are determined, which represent equally likely (1% chance) segments of the river discharge value range that occurred in the 20-year climatological sample (both sub-seasonal and seasonal is currently based on 20 years). Figure 1 shows an example generic climate distribution, either based on weekly means or monthly means, with the percentiles represented along the y-axis. Only the deciles (every 10%), the quartiles (25%, 50% and 75%), of which the middle (50%) is also called median, and few of the extreme percentiles are indicated near the minimum and maximum of the climatological range indicated shown by black crosses. Each of these percentiles have an equivalent river discharge value along the x-axis. From one percentile to the next, the river discharge value range is divided into 100 equally likely bins (separated by the percentiles), some of which is indicated in Figure 1, such as bin1 of values below the 1st percentile, bin2 of values between the 1st and 2nd percentiles or bin 100 of river discharge values above the 99th percentiles, etc.
Figure 1. Schematic of the forecast anomaly categories, defined by the climatological distribution.
Based on the percentiles and the related 100 bins, there are seven categories defined, which will be used as anomaly categories (Table 1). These are also indicated by vertical lines separating them in Figure 1 by shading. The two most extreme categories are the bottom and top 10% of the climatological distribution (<10% as red 'Extreme low' and 90%< as blue'Extreme high'). Then the moderately low and high river discharge categories from 10-25% (orange'Low') and 75-90% (middle-dark blue'High'). The smallest negative and positive anomalies are defined by 25-40% ('Bit low') and 60-75% and displayed by yellow and light blue colours in Figure 1('Bit high'). Finally, the normal condition category is defined as 40-60%, so the middle 1/5th of the distribution, coloured grey the area called 'Normal' in Figure 1.
In addition, for some web products the middle three categories of 'Bit low', 'Normal' and 'Bit high' are merged into one larger 'Near normal' category, covering the percentiles from 25th to 75th. This 'Near normal' category is also indicated in Figure 1 and Table 1.
| Anomaly categories | Name | Ranks | Description | |
|---|---|---|---|---|
| Cat-1 | Extreme low | 1-10 | bottom 10% of the climatological distribution | |
| Cat-2 | Low | 10-25 | 15% from the 1st decile to the 1st quartile | |
| Cat-3 | Bit low |
(25-75) | 25-40 | 15% from the 1st quartile to the 2nd quintile |
| Cat-4 | Near normalNormal | 40-60 | 20% from the 2nd to the 3rd quintile | |
| Cat-5 | Bit high | 60-75 | 15% from the 3rd quintile to the 3rd quartile | |
| Cat-6 | High | 74-90 | 15% from the 3rd quartile to the 9th decile | |
| Cat-7 | Extreme high | 90-100 | top 10% of the climatological distribution | |
Table 1: Definition and description of the 7 anomaly categories. The possible value ranges in the 'Ranks column' are inclusive at the start and exclusive at the end, so for example for Cat-1 the possible ranks are 1, 2, 3, ... and 10. Depending on the For some web products, sometimes the middle three categories (Cat3, Cat-4 and Cat-5) are combined into one extended 'Near normal' category.
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