A precipitation phase discriminator for IMPROVER

Knowledge of the expected precipitation phase is crucial for mitigating the impacts snow and ice on national infrastructure. This is sensitive to the altitude of the modelled forecast grid point which varies between models.

The IMPROVER project aims to blend probabilistic model variables from different models. This presentation describes the approach used to standardise the phase change levels of falling precipitation from the Met Office UK and Global models over the high-resolution UK domain.

The method uses wet-bulb temperature profiles to identify the surface where snow changes to sleet and sleet changes to rain, interpolates these surfaces through model orography and below sea level, then extracts the predicted phase at the altitude of the standard high-resolution UK orography. This is performed for each model realization to maintain the multivariate connection between precipitation and precipitation phase.

The precipitation phase discriminators are used to categorise precipitation rate and accumulation probability data into rain, sleet and snow phases which in turn inform a categorical most-likely weather code.

We present results from a one-month trial using data from February 2020 comparing the weather code forecasts with site observations across the UK.