HOW ARE MODEL VERTICAL VELOCITIES COMPUTED?
All of NCEP’s models use their ‘vertical momentum’ equation for ultimately getting vertical velocity. There is no kinematic approach used so there is no need to apply an O’Brien correction. The third equation of motion involves the material derivative of the vertical coordinate, i.e. d(sigma or eta)/dt – what is written quite often as sigma-dot or eta-dot. Through the hydrostatic assumption, this material derivative equation is reduced to the model’s continuity equation involving integrated mass-flux divergence and the surface pressure tendency. The values of sigma-dot or eta-dot get converted to a conventional vertical velocity in the models’ post-processor.
