How do the forecasts from the UM 4 km and 1.5 km models and the WRF differ?

The most obvious difference between the models, at first glance, is the grid step (this parameter is mentioned when deciding which model to choose: UM 1.5 km, UM 4 km, WRF 3.4 km). For this reason, the 1.5 km UM can include many fine details that are not mapped on the sparser WRF model grid.

The difference in the number of calculation levels, UM 70 versus WRF 48, is important for the diagnosis and prediction of the vertical structure of atmospheric temperature and humidity. A larger number of computational levels results in better forecasts of cloud types, cloud thickness, vertical extent and a better starting point for precipitation forecasts.

Each model slightly differently parameterises (approximates) various physical phenomena such as radiation transfer, cloud and precipitation formation, etc. The models also have different substrate characteristics (different grid and different substrate classes) and are fed with data from different global models, which sometimes affects the outcome of the final forecast.