Extracting statistical parameters of extreme precipitation from a NWP model

Precipitation simulations in an 8×8 km grid by the PSU/NCAR Mesoscale Model MM5 are used to find the M5 and Ci statistical parameters in order to support the M5 map used for flood estimates by Icelandic engineers. It is known a priori that especially wind anomalies do occur on a considerably smaller scale than 8 km. The simulation period used is 1962–2005 and 73 meteorological stations do have records long enough in this period to provide an observation value set. Of these only 1 is in the central highlands, so the highland values of the existing M5 map are estimations. The comparison between the simulated values and the observational value set show a M5 average difference (observed-simulated) of 5 mm/24 h with standard deviation 17 mm, 3 outliers excluded. This is within expected limits, computational and observational errors considered. Suggested correction procedure brings these values down to 4 mm and 11 mm.