a Comparison of the Observed Extratropical Northern Hemisphere Summertime and Wintertime Circulations.
Abstract
The three-dimensional structure of the extratropical Northern Hemisphere (north of 20(DEGREES)N) summertime general circulation is documented, making use of twice -daily National Meteorological Center (NMC) operational analyses of nine pressure levels (1000-100 mb) for 12 summers (1966-1977). Two distinct regimes of standing waves quite unlike the wintertime standing waves appear: a subtropical regime which is clearly a monsoonal response to thermal forcing and a higher latitude regime in which the standing waves are smaller scale and much weaker than in winter and are equivalent barotropic in structure with almost no vertical tilt. While the transient eddies shift northward from their wintertime location and are weaker in summer, their properties and relation to the mean flow are much the same as in winter. In both seasons, the regions of strongest transient activity generally coincide with the regions where the height scale of the most unstable baroclinic wave is large compared to the atmospheric scale height, except where strongly sloping terrain inhibits baroclinic instability. Wintertime and summertime mean 500 mb vertical velocity fields for the extratropical Northern Hemisphere are calculated by several methods from general circulation statistics for 11 winters and 12 summers and are compared to each other and to the seasonal mean patterns of cloudiness and precipitation. The methods are: (1) seasonal averages of NMC six-hour 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. The seasonal mean thermally direct and indirect ageostrophic circulations are depicted by plots of the Q vector, which indicates the direction of low-level ageostrophic flow. 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 implied by cloudiness and precipitation. The other two methods yield somewhat similar patterns that are less successful in matching the pattern implied 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 a secondary term in the time-averaged mid-tropospheric heat balance.
- Publication:
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Ph.D. Thesis
- Pub Date:
- 1981
- Bibcode:
- 1981PhDT........65W
- Keywords:
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- Physics: Atmospheric Science