The Damping of Potential Enstrophy in the Large-Scale Transient Eddies in the Wintertime Troposphere

Eero Holopainen Department of Meteorology, University of Helsinki, 00100 Helsinki, Finland

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Carl Fortelius Department of Meteorology, University of Helsinki, 00100 Helsinki, Finland

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Abstract

On the average, there has to be a balance in the atmosphere between the rate at which the potential enstrophy of the transient eddies (TE) is generated by conversion from the time-mean flow and the rate at which it is damped by diabatic and frictional processes. In the quasi-geostrophic framework this conversion rate (and, hence, also the damping rate) can be estimated from conventional circulation statistics for temperature and horizontal wind. This estimation does not require data on vertical velocity and horizontal wind divergence, which are poorly known and play a crucial role in the TE energetics. Thus, from the data, one can presumably get more reliable information about the damping of the TE potential enstrophy than about the diabatic and frictional damping of the TE energy.

An observational study is made of the amount and maintenance of the (quasi-geostrophic) potential enstrophy of the large-scale transient eddies in the troposphere over the Northern Hemisphere in February 1979 by using two sets of FGGE (level III-b) analyses. Both datasets give approximately the same results. The rate of the damping of the TE potential enstrophy appears to have a maximum around 300 and 400 hPa, as does also the TE potential enstrophy itself. The associated time scale is of the order of ten days. Synoptic and physical interpretation of the results is given.

It is suggested that potential enstrophy might provide more useful a framework than energetics for verification and intercomparison of atmospheric large-scale models.

Abstract

On the average, there has to be a balance in the atmosphere between the rate at which the potential enstrophy of the transient eddies (TE) is generated by conversion from the time-mean flow and the rate at which it is damped by diabatic and frictional processes. In the quasi-geostrophic framework this conversion rate (and, hence, also the damping rate) can be estimated from conventional circulation statistics for temperature and horizontal wind. This estimation does not require data on vertical velocity and horizontal wind divergence, which are poorly known and play a crucial role in the TE energetics. Thus, from the data, one can presumably get more reliable information about the damping of the TE potential enstrophy than about the diabatic and frictional damping of the TE energy.

An observational study is made of the amount and maintenance of the (quasi-geostrophic) potential enstrophy of the large-scale transient eddies in the troposphere over the Northern Hemisphere in February 1979 by using two sets of FGGE (level III-b) analyses. Both datasets give approximately the same results. The rate of the damping of the TE potential enstrophy appears to have a maximum around 300 and 400 hPa, as does also the TE potential enstrophy itself. The associated time scale is of the order of ten days. Synoptic and physical interpretation of the results is given.

It is suggested that potential enstrophy might provide more useful a framework than energetics for verification and intercomparison of atmospheric large-scale models.

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