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Eero Holopainen

Abstract

Time-filtered grid-point data for the northern extratropics in winter have been used to study the local effect on the time-mean flow, arising from the “synoptic-scale” transient eddies (TE), which have periods between 2.5 and 6 days. The local TE effect is defined as the net work done on the time-mean flow by the TE induced force. In the interior of an air column, this force is determined by the irrotational TE flux of quasi-geostrophic potential vorticity; at the upper and lower boundaries, it is determined by the irrotational TE heat flux. The net work in the entire air column consists of the sum of a “baroclinic” part, which affects only the vertical shear of the time-mean flow, and of a “barotropic” part, which affects mainly the vertically-averaged component of the time-mean flow.

The results show that the baroclinic part of the net TE work dominates and, in the storm track regions, gives an energy conversion from the time-mean flow to the synoptic-scale eddies. The barotropic part of the TE work shows in these same regions a partially compensating conversion of the TE energy back to the mean flow. These eddies thus tend to dissipate the baroclinic component and to strengthen the barotropic component of the time-mean flow with the concomitant tendency of strengthening the surface westerlies in the storm track regions and of shifting the time-mean jet stream polewards.

The “energy method” introduced in the present paper for the description of the interplay between the time-mean flow and the transient eddies is compared with other methods suggested in the literature for this purpose. The effect of synoptic-scale eddies on the time-mean flow is further compared briefly with that exerted by low-frequency fluctuations with periods between 10 and 90 days.

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Eero Holopainen
and
Carl Fortelius

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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Eero Holopainen
and
Carl Fortelius

Abstract

In this paper we investigate the role of high-frequency transient eddies in the maintenance of the blocking high which occurred over Europe on 16-25 February 1979. Time-filtering is used to decompose the atmospheric large-scale flow into contributions from high-frequency and low-frequency fluctuations. the separation being roughly at the period of six days. The net effect on the ten-day mean flow of the high-frequency cyclone-scale eddies is studied using several methods.

The vorticity flux convergence associated with the high-frequency eddies, although small in comparison with the dominating terms in the vertically averaged vorticity equation, appears to be important for counterbalancing the effects of the time-mean surface stress, especially in the vicinity of the storm tracks. The eddies are thus important for the distribution of the time-mean surface pressure. Also the “tendency method” of illustrating the eddy effects supports this view.

The hemispheric average of the barotropic energy conversion from the high-frequency eddies to the time-mean flow is much stronger during the blocking episode than during the remaining part of the month.

The response of a linear, hemispheric stationary wave model to the forcing caused by the high-frequency eddies is calculated by taking the eddy forcing and the zonally averaged mean flow as observed during 16–25 February 1979. Comparison of this calculated response with the observed time-mean waves further supports the view that during this blocking episode the high-frequency eddies had an important effect on the structure and maintenance of the time-mean flow in the lower troposphere.

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Eero Holopainen
and
Jussi Kaurola

Abstract

The invertibility principle of potential vorticity (PV) is used, within a quasigeostrophic framework, to formally partition the observed atmospheric flow in various ways. The first method separates the contributions of the interior PV and temperature at the lower boundary. In the literature, this approach has mainly been applied to some idealized flows. In real situations, however, this partitioning appears questionable because any nonzero temperature anomaly at the boundary automatically implies a nonzero stability part of the interior PV and, hence, these two contributions never occur independent of each other in nature. At large horizontal scales, the two contributions have a large amplitude at every level and almost cancel each other. The second method decomposes the 3D flow into contributions arising from the vorticity and the stability parts of the PV. This formal partitioning appears essentially to decompose the flow into its barotropic and baroclinic part. The third method explores the role of PV of various layers in inducing the flow. This approach shows that the upper-level flow is very little influenced by PV at the lower levels. In the lower troposphere, however, the large-scale, flow is dictated by PV in the upper troposphere and lower stratosphere, whereas the smaller-scale features are induced by the local PV, the latter including the effect of the boundary temperature. The third method may be useful as means of verification of atmospheric general circulation models.

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Carl Fortelius
and
Eero Holopainen

Abstract

The distributions of the net sources of atmospheric dry and latent energy are evaluated by the residual technique using the reanalyzed ECMWF FGGE level IIIb data for February and July 1979. Their sum (i.e., the residual estimate of the source of total energy) is compared to simultaneous Nimbus-7 ERB estimates of the net radiation at the top of the atmosphere. Over land the estimated total energy source should be nearly equal to the net radiation.

The best agreement is found in July 1979 over the Northern Hemisphere middle latitudes, where monthly averages over large land areas agree to within 10 W m−2. Elsewhere, differences of up to 60 W m−2 occur for corresponding averages. Over low latitude continents, the residuals are generally too small over convectively active regions and too large over subsidence regions. A reason for this may be known deficiencies in the radiation parameterizations of the forecast model used in the data assimilation.

The distribution of latent heat sources, obtained from the circulation data, is compared with climatological estimates based on the one hand, on surface observations and, on the other hand, on rawinsonde data only. It appears that recent developments in data assimilation methods have improved the usefulness of The “aerological method.”

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Eero Holopainen
and
Carl Fortelius

Abstract

A short review of atmospheric energy transport studies is given, and the importance of the Global Weather Experiment for such studies is emphasized. The accuracy of energy flux (divergence) estimates is then discussed, comparing results obtained for February 1979 from two independent sets of analyses using the same procedures. It is concluded that at present the uncertainty in estimates of the zonally averaged monthly mean poleward transport of (dry static) energy is of the order of 1 × 1015 W, being less in the extratropics than in the tropics.

The results indicate that the accuracy in the estimation of the global distribution of diabatic heating has been much improved by the Global Weather Experiment. This accuracy is, however, not yet good enough for the satellite/atmospheric measurement method (“Oort-Vonder Haar method”) to be useful for oceanographic purposes. Future progress in our ability to diagnose atmospheric heat sources and sinks is intimately tied to progress in the development of data assimilation methods.

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Eero Holopainen
,
Carl Foptelius
, and
Kimmo Ruosteenoja

Abstract

The effect of transient eddies (TE) on stationary eddies (SE) is studied in terms of how the TEs force the SE isobaric height variance in the northern extratropics. The method used correlates the observed SE height field with the TE geopotential tendencies, which describe the net forcing effect that arises from the TE fluxes of beat and momentum and from the associated secondary circulations. Diagnostic estimates are made based on atmospheric circulation statistics for February 1979 and for an ensemble of several winters. The results for the TE forcing of the SE isobaric height variance are compared with those of the SE potential enstrophy.

Although the net forcing effect of all TEs on the SE potential enstrophy is a damping one at all levels, the TEs tend to maintain the SE isobaric height variance in the lower troposphere. This effect is caused by the high-frequency synoptic-scale TEs. The low-frequency TEs, by contrast, tend to damp the SE isobaric height variance at all levels. The sensitivity of the method to uncertainties in the input data is estimated by making calculations from different datasets valid for the same period.

The methods applied in this paper might be used as a diagnostic tool in the validation of general circulation models.

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Ngar-Cheung Lau
and
Eero O. Holopainen

Abstract

The forcing of the time-mean flow by transient eddies is examined within the framework of a quasi-geostrophic equation relating the geopotential tendency to the convergence of transient eddy transports of heat and vorticity. The forcing functions of this equation are computed using observed circulation statistics for the wintertime Northern Hemisphere, and solutions are sought for the three-dimensional structure of geopotential and temperature tendencies associated with eddies of different time scales.

In general, the geopotential tendencies associated with vorticity fluxes are of the same sign within a given atmospheric column; whereas the polarity of the geopotential tendencies associated with heat fluxes in the lower troposphere is opposite to that in the upper troposphere. The geostrophic wind tendencies associated with synoptic-scale eddies with periods between 2.5 and 6 days are strongest in the vicinity of the oceanic storm tracks. The enhanced poleward heat transports by active disturbances in these regions lead to eastward accelerations of the geostrophic flow in the lower troposphere, westward accelerations in the upper troposphere, and hence a reduction in the vertical shear of the eastward flow along the storm tracks. The vorticity transports by eddies with synoptic time scales are associated with eastward accelerations throughout the troposphere over the storm tracks. The geostrophic wind tendencies associated with the vorticity fluxes tend to dominate in the upper troposphere, so that the combined effect of the eddy transports of heat and vorticity by synoptic-scale eddies is to accelerate the eastward current at all vertical levels in middle latitudes. The geopotential tendencies associated with eddies with periods between 10 days and a season are generally stronger than those associated with synoptic-scale disturbances. In the upper troposphere, the transports of both heat and vorticity by the low-frequency eddies are accompanied by tendencies which act to destroy the departure from zonal symmetry of the time-averaged geopotential height field. The forcing of the geopotential height field due to vorticity transports by low-frequency eddies is stronger than the corresponding forcing due to heat transports.

The temperature tendencies associated with eddy heat transports are much stronger than those associated with eddy vorticity transports. The thermal forcing due to synoptic-scale disturbances is characterized by dipole-like structures over the western oceans, with positive temperature tendencies (warming) north of the cyclone tracks and negative tendencies (cooling) further south. In the lower troposphere, the tendencies associated with low-frequency eddies act to destroy the zonally asymmetric component of the stationary temperature field. The typical magnitude of temperature tendencies as computed using the present method, which implicitly takes into account the combined effects of eddy flux convergences and the associated secondary circulations, is about 60–70% of the corresponding values obtained by considering the convergence of eddy heat fluxes alone.

The effects of transient disturbances as depicted by tendencies associated with eddy fluxes are contrasted with earlier results based on eddy transports of quasi-geostrophic potential vorticity. The distinction between these two approaches is discussed.

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Eero O. Holopainen
,
L. Rontu
, and
Ngar-Cheung Lau

Abstract

The effect of horizontal transports of momentum and heat by transient eddies (TE) on the time-mean flow is studied by examining the relevant terms in a local budget of quasi-geostrophic potential vorticity. Two long-term observational data sets are used, and results for the northern Hemisphere winter are presented.

The results indicate that eddy beat fluxes in the free atmosphere exert a dissipative influence on both the zonally averaged flow and the stationary waves. On the oilier hand, eddy momentum transports tend to force cyclonic circulations over the semi-permanent Icelandic and Aleutian surface lows, and anticyclonic circulations over the oceanic high pressure cells in the subtropics. The forcing of the time-mean flow arising from horizontal TE heat transports is generally stronger than the forcing associated with eddy momentum transports. The net eddy of eddy transports of heat and momentum is to dissipate the potential enstrophy of the stationary waves. The characteristic time scale associated with this dissipative effect is of the order of 4–5 days.

The relative contribution to the eddy forcing by low-frequency fluctuations (with periods between 10 days and a season) and by synoptic-scale fluctuations (with periods between 2.5 and 6 days) are examined. The forcing associated with low-frequency eddies generally dominates. The forcing associated with synoptic-scale eddies is concentrated in the cyclone tracks near the cast coats of Asia and North America, where a certain degree of counterbalancing between the heat flux forcing and the momentum flux forcing takes place.

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