While the QPFs are related to the orographic model precipitation, they are not produced from direct running of the model programs on a daily basis. They are derived based on the repeatability of 700mb wind direction-dependent orographic precipitation patterns over a basin and with precipitation magnitudes scaled by wind speed, moisture depth, and temperature. The steps in the procedure are described below, following a brief description of the orographic model. The remaining fraction , 1-E, becomes the imported condensate for the next grid interval downstream. Precipitation efficiency is a parameter which can be varied to better match the climatological areal distribution of precipitation when setting up the model over a new area.
Calculations proceed along one grid line at a time.
Total precipitation from all layers is calculated at each grid point after accounting for possible sub-cloud evaporation. A key feature of the model-produced precipitation patterns is their strong dependence on 700mb wind direction. The model needs an observed, predicted, or contrived sounding as input. The Basic QPF Objective Aid First. These runs use model topographic grids with a 5 km horizontal interval.