Hello Everyone
I have been working on ERA5 Monthly drought indices and the dataset i get from it contains some unrealistic values. while the realistic range of the dataste lies between -3 and +3 (though infinity on both sides, but obtaining values across these range is severely unrealistic) but the values i got formony months is -8.22 for different categories of spi. Like May 1984, the spi1 values for this month is -8.22, similarly i have many months from 1978 till 2015 that contains similiar unrealistic values. Is there a possibilty that i have committed a mistake or is there something really wrong in the dataset. Kindly guide me for this case. Thank you
HI Mohammad. I think the value of -8.22 is a placeholder for NoData. I may be wrong, but that is my understanding.
In general, values larger than +8 and smaller than -8 are on the edge of the distribution; most likely the event is wetter/drier than any event during the reference period used to fit the distribution (1991–2020, as per WMO recommendations). Therefore, we suggest interpreting both SPI and SPEI only in ranges, such as the ones outlined in our accompanying paper ( ERA5–Drought: Global drought indices based on ECMWF reanalysis | Scientific Data ). For example, any value smaller than -3 should simply be interpreted as an extreme drought, and the magnitude of the index should not be analysed.
Still, in some cases, the fit does not work that well — these cases can be filtered out with the quality criteria (probability of zero precipitation and test for normality). Have you checked if these parameters help to mask out this odd behaviour / grid cell? Kindly let me know; I’ll take a closer look at this specific case if the issue persists.
Thank you again for your feedback!