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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

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

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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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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Damien Irving and Ian Simmonds

Abstract

The Pacific–South American (PSA) pattern is an important mode of climate variability in the mid-to-high southern latitudes. It is widely recognized as the primary mechanism by which El Niño–Southern Oscillation (ENSO) influences the southeast Pacific and southwest Atlantic and in recent years has also been suggested as a mechanism by which longer-term tropical sea surface temperature trends can influence the Antarctic climate. This study presents a novel methodology for objectively identifying the PSA pattern. By rotating the global coordinate system such that the equator (a great circle) traces the approximate path of the pattern, the identification algorithm utilizes Fourier analysis as opposed to a traditional empirical orthogonal function approach. The climatology arising from the application of this method to ERA-Interim reanalysis data reveals that the PSA pattern has a strong influence on temperature and precipitation variability over West Antarctica and the Antarctic Peninsula and on sea ice variability in the adjacent Amundsen, Bellingshausen, and Weddell Seas. Identified seasonal trends toward the negative phase of the PSA pattern are consistent with warming observed over the Antarctic Peninsula during autumn, but are inconsistent with observed winter warming over West Antarctica. Only a weak relationship is identified between the PSA pattern and ENSO, which suggests that the pattern might be better conceptualized as a preferred regional atmospheric response to various external (and internal) forcings.

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Ian Simmonds and Cher Chidzey

Abstract

Many climate models of the energy balance type parameterize the zonally-averaged infrared flux at the top of the atmosphere in terms of the surface (or sea level) temperature T and cloud cover n in the form
IBTDTn
Most recent studies have used the annual average data of Ellis and Vonder Haar (1976) to determine the coefficients of this regression relation, and it leads to tolerable parameterization errors. However, we see here that when such formulas are used to simulate the seasonal cycle, very large errors are incurred. These errors are not greatly reduced if the regression coefficients are deduced by fitting the seasonal data.

The more recent and comprehensive data set Winston et al. (1979) has been used to define and evaluate regression equations for the longwave emission at the top of the atmosphere. Whatever the relative accuracy of these two sets, it is found that regressions developed on the latter are about 2–5 W m−2 (or 20–50%) more accurate for the annual mean and 6–7 W m−2 (∼50%) more accurate over the seasonal cycle. A better fit over polar regions is most evident.

It is found that the inclusion of the nonlinear term in the parameterization makes little change in the accuracy to which the data are fitted. However, the explicit inclusion of the effects of clouds is found to be important. The analysis also reveals that clouds exert a significant feedback mechanism on climate. When the infrared flux formula is tuned to fit the later satellite data set, the implied extent of this feedback lies within the range determined by model calculations. It is also found that the sensitivity of climate implied by the formula, as expressed through the sensitivity of the longwave to surface temperature, is somewhat greater than that presented in recent determinations.

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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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Irina Rudeva and Ian Simmonds

Abstract

Presented here is a global analysis of frontal activity variability derived from ERA-Interim data over the 34-yr period of January 1979–March 2013 using a state-of-the-art frontal tracking scheme. In December–February over that epoch, there is a northward shift of frontal activity in the Pacific in the Northern Hemisphere (NH). In the Southern Hemisphere (SH), the largest trends are identified in the austral summer and are manifested by a southward shift of frontal activity over the Southern Ocean.

Variability of frontal behavior is found to be closely related to the main modes of atmospheric circulation, such as the North Atlantic Oscillation (NAO) for the Atlantic–European sector in the NH and the southern annular mode (SAM) in the middle and high latitudes of the SH. A signal associated with El Niño and hence emanating from the tropics is also apparent in the behavior of frontal systems over the Pacific by a reduction in the number of fronts in the middle South Pacific and intensification of frontal activity in high and low latitudes throughout the year. It is shown in general that the associations of the large-scale modes with frontal variability are much stronger than with cyclones. This indicates that the quantification of the behavior of fronts is an important component of understanding the climate system. At the very high latitudes, it is also shown here that, in the recent years of rapid sea ice reduction in the Arctic, there have been fewer summer fronts observed over the Canadian Arctic.

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