Estimates of the Seasonal Mean Vertical Velocity Fields of the Extratropical Northern Hemisphere

Abstract

Wintertime and summertime mean 500 mb vertical velocity fields for the extratropical Northern Hemisphere are calculated by several methods from general circulation statistics compiled from National Meteorological Center (NMC) operational analyses for 11 winters and 12 summers and are compared to each other and to seasonal mean patterns of cloudiness and precipitation. The methods are: 1) seasonal averages of NMC 6 h forecasts; 2)solution of the quasi-geostrophic omega equation in the form introduced by Hoskins et al. (1978) involving the Q vector; and 3) integration of the continuity equation employing the divergence calculated as a residual from the seasonal mean vorticity budget. In addition, the adiabatic vertical velocity is calculated from the thermodynamic equation. Plots of the seasonal mean Q vector indicate the pattern of ageostrophic flow in the upper troposphere. The time-mean flow is shown to be more important than the transient eddies in forcing seasonal mean vertical motion. While little confidence can be placed in the magnitude of the seasonal mean vertical velocity determined from NMC data, the omega equation and the vorticity balance yield very similar vertical motion patterns that, over much of the extratropical Northern Hemisphere, are in agreement with the vertical velocity pattern suggested by cloudiness and precipitation. The other two methods yield somewhat similar patterns that are less successful in matching the pattern suggested by cloudiness and precipitation. The similarity of several features in the adiabatic vertical velocity field to corresponding features in the vertical velocity field calculated from the vorticity balance implies that over many areas of the extratropical Northern Hemisphere diabatic heating is secondary term in the time-averaged mid-tropospheric heat balance.