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Ian Simmonds

Abstract

Analytic fields, with several spectral variance power laws, are prescribed and evaluated at a finite number of equally-spaced points. For a given accuracy of interpolation, an unaliased truncated Fourier series is found to require less degrees of freedom than both cubic spline and two-point interpolation. With the input truncation chosen here, cubic spline is superior to linear interpolation, except for the roughest field. Very similar results hold for the accuracy of the first derivatives implied by these interpolation schemes.

When the errors in the first derivatives are examined only at the data points, however, the derivative of the aliased series is more accurate than that of the cubic spline. An even more accurate series of the same length can be obtained by analyzing the cubic spline passed through the points. The two finite-difference schemes tested have the largest errors.

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Ian Simmonds

Abstract

Use of the primitive equations in spectral models of the atmosphere raises certain questions about the representation of the horizontal components of velocity therein. A barotropic model is constructed and integrated using two types of velocity representation. In the first, (u,v) are expanded directly in spherical harmonics while the second represents (u cosθ, v cosθ) in like manner, where θ is latitude. These integrations are compared and it is concluded that the direct representation of (u,v) is an allowable procedure.

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Ian Simmonds

Abstract

Some experiments are performed which are designed to compare the representation efficiency of dewpoint depression, relative humidity and mixing ratio, with a view to incorporating moisture into a spectral model. A score is defined based on the ability of a finite spherical harmonic series for a given variable to reconstitute the grid-point fields of dewpoint depression, relative humidity and mixing ratio. Based on this measure, the conclusion is reached that a finite series of dewpoint depression is slightly superior to a similar series of relative humidity in being able to represent the grid-point structure of the three fields, and a great deal better than mixing ratio.

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Ian Simmonds

Abstract

The techniques developed and tested by Simmonds (1976) for inserting grid-point data into a barotropic spectral model have been extended to the multi-level case. Theoretical and numerical analyses suggest that such a model can provide a good data assimilation vehicle if inertia gravity waves are adequately suppressed. Efficient mechanisms in the present model involve the application of divergence diffusion, a time filter and a modest amount of vorticity diffusion. Two high-resolution versions of this model were constructed, the first containing all the physical processes usually included in general circulation models, the second being the same but for the omission of moist and radiative processes. Data assimilations were undertaken with these two models to determine the necessity of including the physical processes. In the upper levels and at the surface the latter model was found to produce better analyses in the one 6-day assimilation carried out. Forecasts initialized from these analyses supported the view that the assimilation without physics produced a more accurate representation of the atmosphere. The experiments show that, at the very least, the tested spectral model is an adequate data assimilator.

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Ian Simmonds

Abstract

Techniques for inserting data into spectral models have been developed and are shown to be capable of assimilating data. Real 500 mb winds and heights are inserted into a free-surface spectral model at locations which simulate the global observing network. A series of 6-day assimilation experiments are undertaken which are designed to test the effect of a semi-implicit time-stepping algorithm, different frequencies of data insertion, and various adjustment mechanisms on the quality of the resulting analyses. Scrutiny of these experiments, in both real and spectral space, gives insight into the model's response to inserted data and suggests methods by which the assimilation may he improved. Some methods, which are particularly suited to spectral models, are subjected to testing and found to be effective.

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Ian Simmonds

Abstract

This paper addresses the extent to which sea level pressure cyclones change size as they develop. A state-of-the-art cyclone tracking scheme has been applied to the global “reanalyses” produced by the National Centers for Environmental Prediction for the four-decade period 1958–97. The analysis is based on all the cyclones found in the analyses, and on those which halfway through their lifetimes are located in the 30°–50° and 50°–70° latitude bands. Systems in both the Northern Hemisphere (NH) and Southern Hemisphere are considered, as are those in the December–February and June–August periods.

The results show that the radius of surface cyclonic systems increases as they evolve to maturity. This finding holds for the two baroclinic domains considered in both hemispheres and in both winter and summer. In the NH winter in the 30°–50°N and 50°–70°N belts the average increase in size of systems that last longer than 3 days is about 33% over 4 days. In the northern summer the rate of increase in radius is less marked, particularly in the midlatitude belt. In the Southern Hemisphere winter the mean rate of size increase is somewhat more modest than in the northern winter. The increase in size in the southern summer is greater than in the north, particularly in the 50°–70° band.

The small number of studies on this topic have indicated that over specific domains and limited samples the size of cyclones increase as they evolve from their point of first identification. The present results show that these increases occur in the extratropics of both hemispheres and in both winter and summer.

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Damien Irving and Ian Simmonds

Abstract

Southern Hemisphere mid- to upper-tropospheric planetary wave activity is characterized by the superposition of two zonally oriented, quasi-stationary waveforms: zonal wavenumber 1 (ZW1) and zonal wavenumber 3 (ZW3). Previous studies have tended to consider these waveforms in isolation and with the exception of those studies relating to sea ice, little is known about their impact on regional climate variability. A novel approach is taken to quantifying the combined influence of ZW1 and ZW3, using the strength of the hemispheric meridional flow as a proxy for zonal wave activity. The methodology adapts the wave envelope construct routinely used in the identification of synoptic-scale Rossby wave packets and improves on existing approaches by allowing for variations in both wave phase and amplitude. While ZW1 and ZW3 are both prominent features of the climatological circulation, the defining feature of highly meridional hemispheric states is an enhancement of the ZW3 component. Composites of the mean surface conditions during these highly meridional, ZW3-like anomalous states (i.e., months of strong planetary wave activity) reveal large sea ice anomalies over the Amundsen and Bellingshausen Seas during autumn and along much of the East Antarctic coastline throughout the year. Large precipitation anomalies in regions of significant topography (e.g., New Zealand, Patagonia, and coastal Antarctica) and anomalously warm temperatures over much of the Antarctic continent were also associated with strong planetary wave activity. The latter has potentially important implications for the interpretation of recent warming over West Antarctica and the Antarctic Peninsula.

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Ian Simmonds and Kevin Keay

Abstract

An analysis of the variability and trends exhibited by many aspects of Southern Hemisphere (SH) mean sea level extratropical cyclones during the period 1958–97 is presented. The investigation is undertaken by applying a state-of-the-art cyclone finding and tracking scheme to the 6-hourly reanalyses produced by the National Centers for Environmental Prediction. The outcome of this is arguably the most reliable analysis of SH cyclone variability undertaken to date.

Across the 40-yr period the annual and seasonal mean cyclone densities have undergone reductions at most locations south of about 40°S (with the greatest reductions near 60°S), and increases to the north. This pattern of change resembles the “high-latitude mode” identified in many studies of SH circulation features. It is shown that the mean radius of SH extratropical cyclones displays almost everywhere a significant positive trend, and there are also increases in annual mean cyclone “depth” (i.e., the pressure difference between the center and the “edge” of a cyclone).

The annual average number of cyclones per SH analysis rose from the start of the period to a maximum of about 39 in 1972. Since then, the numbers have shown an overall decline, the counts in the 1990s being particularly low. Similar behavior was evident when the count was confined to the 30°–50°S and 50°–70°S latitude bands. Least squares best fit to the three time series exhibit significant slopes of −0.58, −0.26, and −0.58 cyclones per analysis per decade, respectively. Between 30° and 70°S the annual mean number of cyclones found per analysis assumed a maximum about 1970, but that number has dramatically decreased by about 10% since then. (This analysis suggests that the downward trends in cyclone numbers are associated with a warming Southern Hemisphere.) The overall structure of the time series of annual cyclone per analysis over 30°–50°S and 50°–70°S are similar, but their year-to-year changes are shown to be negatively correlated; hence, there tends to be an interannual compensation of cyclone density between the middle and higher latitudes.

The extent to which changes in the semiannual oscillation over the last few decades could be said to have influenced how cyclones are distributed across seasons is briefly examined. The results show, in particular, that the interannual relationship between spring and winter cyclone density cannot be explained in terms of a response to a change in the amplitude of the semiannual oscillation.

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Ian Simmonds and Martin Dix

Abstract

There are a number of atlases that display the distribution of ocean-atmosphere sensible and latent heat fluxes over various regions. Many are based on the “classical” method, in which time mean quantities are used in the bulk aerodynamic formulas, rather than the more accurate “sampling” method, which computes the mean of the instantaneous fluxes. Much of the justification for the use of this approximation comes from some studies conducted in the North Pacific and Atlantic oceans. How valid is it when applied globally?

In this study we use large datasets, generated by January and July simulations of a general circulation model of the atmosphere, to examine comprehensively the differences between the two methods. We find that the ocean zonal averages of the two fluxes differ by less than 10 W m−2 at most latitudes in both months. However, at high southern latitudes in winter the sensible heat fluxes north of the Antarctic ice pack are up to 17 W m−2 (25%) greater when calculated with the sampling method. We show that the two methods differ due to four temporal covariances of various atmospheric quantities Some of the covariances contribute up to 12 W m−2 to the zonal mean fluxes. The fact that the classical and sampling methods give similar results over most of the globe is due, in large part, to the near cancellation of the covariances. However, this is not always true around the periphery of Antarctica.

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Ian Simmonds and A. Rocha

Abstract

Two general circulation model experiments have been conducted with a view toward determining the sensitivity of Australian winter circulation and precipitation to the imposition of idealized sea surface temperature anomalies around the western half of Australia. In the first, a positive anomaly was imposed to the northwest of the continent and a negative anomaly to the southwest, while in the second this pattern was reversed. In the former, major changes were simulated in the circulation over the Australian sector, and significant precipitation increases were induced over more than half of the continent. By contrast, the responses to the forcing in the latter experiment were considerably less, particularly for precipitation over the continent, which showed virtually no change of significance. The results, taken in concert with those of Simmonds, suggest the important role played by warm ocean temperatures to the northwest of Australia in influencing winter rainfall.

The results suggest that the addition of negative ocean temperature anomalies in the extratropics just to the west of Australia serves to strengthen the response over Australia over and above that induced by the warming of the ocean in the northwest. This is consistent with the results of analyses undertaken with the COADS data, which suggested that an even greater strengthening would be induced by a westward shift of the negative anomaly.

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