How are storms calculated?

Storms are calculated by the UM model. First, model nodes are found where the GWP, or graupel water path exceeds a threshold value. The electrical charge is then parameterised at these nodes. If this charge exceeds a certain value or a multiple thereof, a discharge (or several discharges) is considered to have occurred. The remainder is passed on to the next calculation step. The discharge density presented is the quotient of the number of discharges per time period divided by the area belonging to the model node (16 km2).

For example: a value of 1 on the storm map corresponds to 16 discharges for the last 3 hours in the model node.
The representation of storms on the meteogram is explained in a separate question.