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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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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
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
Kevin Keay

Abstract

This paper presents a new climatology of Southern Hemisphere (SH) extratropical cyclones. This has been compiled by applying a state-of-the-art cyclone tracking scheme to the 6-hourly National Centers for Environmental Prediction–National Center for Atmospheric Research (NCEP–NCAR) global reanalyses spanning the period 1958–97. The results show there to be, on average, between 35 and 38 cyclonic systems per analysis (depending on season), with the greatest density [exceeding 6 × 10−3 cyclones (deg lat)−2] found south of 60°S in all seasons and in the Indian and west Pacific Oceans in autumn and winter. For the most part, there is a net creation of cyclones (i.e., cyclogenesis exceeds cyclolysis) north of about 50°S, and a net destruction to the south of this latitude. Having said this, the most active cyclogenesis takes place south of 45°S. The NCEP–NCAR reanalyses indicate that most SH cyclogenesis occurs at very high latitudes, and the axis of the maximum lies on, or to the south of, 60°S. This is in agreement with the deductions of many modern studies of SH cyclone behavior. The region is also host to even greater levels of cyclolytic activity.

The authors consider measures of the importance and influence (e.g., for eddy fluxes) of cyclonic systems. It is suggested that the “depth” of a system (the pressure difference between the center and the “edge” of a cyclone) is a relatively bias-free and useful measure of a cyclone’s status and effect on the circulation. The greatest climatological depths are seen to lie at about 60°S, well to the north of the circumpolar trough and of the region of greatest cyclone density. The mean lifetime of cyclones that last at least 1 day is just over 3 days. Those that are located between 50° and 70°S (at their half-lifetime) endure, on average, almost one day longer than all other systems. The mean track length of winter systems is 2315 km, which reduces to 1946 km in summer.

The significance of the work presented here lies in a number of factors. First, the climatology has been derived from 40 yr of analysis, a period longer than any considered heretofore. Further, the (re)analyses used can be regarded as one of the best representations of the global atmosphere. The availability of these analyses at 6-hourly intervals means that the uncertainties with tracking of cyclones are greatly diminished. Finally, it has been compiled using one of the most sophisticated and reliable automatic cyclone finding and tracking schemes. This climatology of SH extratropical cyclones is arguably the most accurate and representative set yet assembled.

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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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Irina Rudeva
and
Ian Simmonds

Abstract

For the last few decades the Northern Hemisphere midlatitudes have seen an increasing number of temperature extreme events. It has been suggested that some of these extremes are related to planetary wave activity. In this study we identify wave propagation regions at 300 hPa using the ERA-Interim dataset from 1980 to 2017 and link them to temperature extremes in densely populated regions of the Northern Hemisphere. Most studies have used background flow fields at monthly or seasonal scale to investigate wave propagation. For a phenomenon that is influenced by threshold incidents and nonlinear processes, this can distort the net Rossby wave signal. A novel aspect of our investigation lies in the use of daily data to study wave propagation allowing it to be diagnosed for limited but important periods across a wider range of latitudes, including the polar region. We show that winter temperature extremes in the midlatitudes can be associated with circulation anomalies in both the Arctic and the tropics, while the relative importance of these areas differs according to the specific midlatitude region. In particular, wave trains connecting the tropical Pacific and Atlantic may be associated with temperature anomalies in North America and Siberia. Arctic seas are markedly important for Eurasian regions. Analysis of synoptic temperature extremes suggests that pre-existing local temperature anomalies play a key role in the development of those extremes, as well as amplification of large-scale wave trains. We also demonstrate that warm Arctic regions can create cold outbreaks in both Siberia and North America.

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