Explained: the differences between the predicted precipitation and the measured precipitation

Why has it rained much more at my farm than at the neighbour’s? And why does the expected precipitation differ so much from what actually fell?

Due to the heavy rainfall of the past few days, we get a lot of questions at SmartFarm about the differences between the predicted rainfall amounts and those actually measured with the FieldMate(s). These differences may seem unreliable and strange, but they can be explained.

A shower occurs in summer over land as warm moist air rises. With a constant supply of heat and moisture, clouds (water droplets) form locally and, under the right conditions, can grow to a height of more than 10 km. The water droplets grow in size in the cloud and will also freeze at a certain height. The stronger the upward movement, the stronger the ice and droplet formation and the greater the resulting rainfall. The rainfall (or hail) also causes a downward movement in the cloud of cold air that sinks to the bottom as the precipitation falls. This powerful downward flow caused the major damage in Leersum (NL) last Friday (18 June 2021) and also ensures that the energy supply of a shower is cut off and extinguished. The duration of a single shower is therefore relatively short (max. 0.5 to 1 hour).

The horizontal extent of a shower is also limited, especially compared to a weather model, which only calculates the weather for a limited number of points on the earth. For global weather models, the grid distance is more than 10 km and the local character of showers makes them difficult to calculate with global models. More accurate models are better able to do this, but they also have difficulties with the exact location and intensity of showers. The figure shows the radar image of Friday 18 June 17:25 (Source: buienradar.nl) and the model calculation of the Harmoniemodel of the KNMI (Source: WeatherOnline.co.uk – 15 UTC grid ~6 x 6 km). It is clear that a number of shower complexes are well modelled, but smaller showers are missing and the shower intensity is often less.

Not surprisingly, there can be large differences between what is reported in the weather forecast and what we actually measure with our Fieldmate(s)!

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