Tuning the precipitation phase discriminator in IMPROVER

IMPROVER (Integrated Model Post-Processing and Verification) has been developed by the Met Office as an open-source probability-based post-processing system to fully exploit our convection permitting, hourly cycling ensemble forecasts. Post-processed MOGREPS-UK model forecasts are blended with deterministic UKV model forecasts and data from the coarser resolution global ensemble, MOGREPS-G, to produce seamless probabilistic forecasts from now out to 7 days ahead. For precipitation, an extrapolation nowcast is also blended in at the start. Forecasts are converted to probabilities at the start, and all initial stages of post-processing are performed on gridded data, with site-specific forecasts extracted as a final step, helping to ensure consistency. Data are processed on a 10km global grid and on a 2km UK-centred grid. An overview of IMPROVER will be given in a separate talk.

 

The IMPROVER post-processing system includes a precipitation phase discriminator to determine the phase of precipitation after accounting for the change in orography from the modelled grid to the standardised, unsmoothed grid so that forecasts from different resolution models can be blended together seamlessly. The method calculates the melting experienced by precipitation as a product of positive wet-bulb temperature and atmospheric depth. This discriminator includes two empirical thresholds for where snow starts to melt and where melting is complete which are used to separate falling snow, mixed-phase precipitation, and falling rain. Where liquid precipitation falls onto a frozen surface, freezing rain is diagnosed.

This talk describes how the phase discrimination is performed, and how the empirical thresholds have been tuned by comparing spot forecasts with current weather SYNOP observations.