Synoptic comparison of cold events in winter and summerin Melbourne and Perth     

I. Simmonds  T. Richter 《Theoretical and Applied Climatology》2000,67(1-2):19-32

Summary  Unseasonably cold weather episodes have the potential to cause dislocation to many aspects of society, regardless of the season in which they occur. In this work we devise a method for quantitatively identifying extreme cold events in such a way that it is not biased to the winter season (as is usual in most other studies). We have applied this method to the daily maximum temperatures (over the period January 1972 to June 1991) in the southern Australian cities of Melbourne and Perth. We identify 10 cold events in winter and summer for the cities. Analyses were performed to determine the synoptic environment in which these events occurred. The most common synoptic type in these samples was the ‘classic’, which is characterised by, amongst other factors, the passage of a cold front over the city on the day of the outbreak, and the transport of air from subantarctic latitudes. Melbourne recorded five such events in summer and six in winter, while seven and eight occurred in the two seasons for Perth. The circulation features and characteristics of other synoptic types identified with these episodes is also examined. The mean synoptic anomalies which are coincident with these cold events are analysed. For both cities and seasons there is a ‘high-low’ anomalous dipole in the regional MSLP pattern, with the high located in the ‘upstream’ quadrant from the anomalous cyclone. Having said this, the relative importance of the two features of the dipole in being associated with the cold event strongly depended on the city and season under consideration. The research shows that the regional structures associated with cold events in Melbourne and Perth bear some similarity, but also display a number of significant differences. These differences are associated partly with the different climatological and synoptic settings in which these cities find themselves, and the nature of their seasonality. Received October 10, 1999/Revised April 7, 2000  相似文献   

On the vertical structure of Mediterranean explosive cyclones   总被引：1，自引：0，他引：1  

John Kouroutzoglou  Helena A. Flocas  Kevin Keay  Ian Simmonds  Maria Hatzaki 《Theoretical and Applied Climatology》2012,110(1-2):155-176

An attempt is made to explore the vertical structure of the surface explosive cyclones in the Mediterranean on a climatological basis during the cold period of the year in order to get a better insight in the interaction between the upper and lower levels responsible for the genesis and evolvement of the phenomenon. The vertical profile of the explosive cyclones was examined with the aid of the vertical tracing software of the University of Melbourne Cyclone Tracking Algorithm, using the 1?×?1° spatial resolution of ERA-40 reanalysis data. It was found that about 57?% of the track steps of surface explosive cyclones extend up to 500?hPa. The north-westward tilting of the surface cyclones with height during the stage of explosive cyclogenesis, with a mean distance of 350?km between mean sea and 500?hPa levels, confirms the importance of baroclinicity. About 45?% of the surface explosive cyclones reached their maximum depth before their 500?hPa counterparts, implying the role of surface processes.  相似文献   

Short‐term forecasting with a multi‐level spectral primitive equation model part II ‐ Hemispheric prognoses and verification     

Roger Daley  Claude Girard  John Henderson  Ian Simmonds 《大气与海洋》2013,51(2):117-134

The spectral global baroclinic primitive equation model described in Part I of this paper has been extensively tested. The model has been run daily from operational analyses for over a year. From this large sample of forecasts, verification statistics have been collected and compared with similar statistics collected from three competitive grid‐point models. The spectral model is also compared with the grid‐point models in a synoptic case study.

A second case study demonstrates the effect of horizontal resolution and physical effects on spectral model forecasts. The results of these experiments demonstrate that the spectral model is highly competitive with other models, in terms of both accuracy and computational efficiency. On 18 February 1976 the spectral model became the operational Canadian large‐scale forecast model.  相似文献   

A climatology of Southern Hemisphere anticyclones   总被引：1，自引：0，他引：1  

David A Jones  Ian Simmonds 《Climate Dynamics》1994,10(6-7):333-348

A climatology of anticyclones generated by an objective automatic scheme applied to 15 years of once-daily Australian Bureau of Meteorology hemispheric analyses is presented. Contour maps of the anticyclone system density, positions of formation and dissipation together with other statistics are shown. The distribution of anticyclones through the hemisphere was found to be dominated by a mid latitude belt of high density, located in the band 25–42°S, typically 24° south of the time-mean subtropical ridge. Within this band the anticyclone density displays considerable structure with greater system numbers over the eastern parts of the three subtropical ocean basins in the vicinity of the three subtropical ocean time-mean anticyclones. During winter the system density displays a bifurcation in the New Zealand sector, with the highest density along the 30 and 45°S latitude bands. The movement of systems in the subtropical ocean basins was found to be in a general easterly direction with a weak equatorwards component, the transport of systems closely following the orientation of the belt of highest system density. In the vicinity of the African and South American continents, movement was more complex with east-south-east motion upstream, and east-north-east movement downstream, the net transport being such as to encourage a general steering of systems around the continental land masses more particularly during the warmer seasons. To highlight the dynamic role played by these systems and their cyclonic counterparts, we present a limited investigation of the response of Southern Hemisphere synoptic systems to variations of the broader atmospheric system and compare these findings to those obtained by more traditional analysis techniques.  相似文献   

Comment on ‘Marine GIS: Identification of mesoscale oceanic thermal fronts’     

F. J. Simmonds  B. G. Lees 《International journal of geographical information science》2013,27(3):369-373

In this paper, we aim to clear up a significant conceptual error in the use of the ‘sink method’ as presented by Valavanis et al. (2005) for oceanic thermal front detection. We argue that the features identified by the authors in their paper are mostly cyclonic or cold ring eddies in the Aegean Sea.  相似文献   

Day-of-the week variation of rainfall and maximum temperature in Melbourne,Australia     

I. Simmonds  J. Kaval 《Theoretical and Applied Climatology》1986,36(3-4):317-330

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Association between modes of variability of January Northern Hemisphere snow cover and circulation     

D. J. Walland  I. Simmonds 《Theoretical and Applied Climatology》1997,58(3-4):197-210

Summary An Empirical Orthogonal Function (EOF) analysis has been applied to NOAA/NESDIS snow concentration data. The major modes of variability in January Northern Hemisphere snow concentration have been extracted and analysed. The analysis was completed separately over Eurasia and North America. Strong, coherent patterns were found for each of the first three EOFs that were analysed over both continents. Over Eurasia the first EOF showed much of Europe as well as western and central Asia in phase but eastern Asia of the opposite phase although the signal was somewhat weaker. North America had a very similar first EOF with a large positive anomaly centered over Montana reaching loadings of over 0.8. East of the Great Lakes, the anomaly changes sign, although again, its magnitude is much smaller.An EOF examination was also made of the anomalous 700 hPa geopotential height fields. These modes of variability were correlated with those of snow cover with the aim of investigating the mechanisms by which the surface boundary snow and the overlying circulation can interact. The stronger correlations were discussed and logical physical scenarios were presented for each. The results indicate that there was no common pattern whereby one medium was always forcing the other but rather a complex array of interactions where each medium could influence the other. To support the physical basis of the relationships being depicted by the EOF study, a case study of January 1981 was made.The presence of intercontinental relationships was also investigated and such relations were strongly suggested. It was proposed that the large scale organisation of the atmosphere between the two continents could go some way to explaining these links in snow variability.With 6 Figures  相似文献   

A critique of the UK’s JNCC seismic survey guidelines for minimising acoustic disturbance to marine mammals: Best practise?     

E.C.M. Parsons  Sarah J. Dolman  Michael Jasny  Mark P. Simmonds 《Marine pollution bulletin》2009,58(5):643-651

The United Kingdom’s statutory conservation agency, the Joint Nature Conservation Committee (JNCC), developed guidelines in 1995 to minimise acoustic disturbance of marine mammals by oil and gas industry seismic surveys. These were the first national guidelines to be developed and have subsequently become the standard, or basis, of international mitigation measures for noise pollution during seismic surveys. However, relatively few aspects of these measures have a firm scientific basis or proven efficacy. Existing guidelines do not offer adequate protection to marine mammals, given the complex propagation of airgun pulses; the difficulty of monitoring in particular the smaller, cryptic, and/or deep-diving species, such as beaked whales and porpoises; limitations in monitoring requirements; lack of baseline data; and other biological and acoustical complications or unknowns. Current guidelines offer a ‘common sense’ approach to noise mitigation, but in light of recent research and ongoing concerns, they should be updated, with broader measures needed to ensure adequate species protection and to address data gaps.  相似文献   

Precipitation changes due to the introduction of eddy-resolved sea surface temperatures into simulations of the “Pasha Bulker” Australian east coast low of June 2007     

Christopher R. S. Chambers  Gary B. Brassington  Ian Simmonds  Kevin Walsh 《Meteorology and Atmospheric Physics》2014,123(1-2):1-15

Stratocumulus is often present offshore of Peru and northern Chile and exists at the top of a cool, moist and well-mixed marine boundary layer (MBL) under a marked temperature inversion maintained by large-scale subsidence. The subtropical MBL and stratocumulus has been the focus of many recent studies, but mid-latitude systems can exert a strong influence. However, this connection is not well established due to debatable model results and few in situ measurements south of 20°S. During a 2-week field campaign in August 2011 at Robinson Crusoe Island (~700 km offshore at 33.6°S), radiosondes were launched to observe the response of the MBL to mid-latitude synoptic forcing. During the observation period a broad, slow-moving cutoff low (COL) passed over the region. Other observations include COSMIC GPS, infrared satellite imagery, TRMM radar reflectivity, and operational radiosondes from the Chilean weather service. A numerical simulation is included to diagnose the synoptic features. The inversion prior to the COL was maintained and lifted above 5 km as the COL passed over the island. Soon after the COL center passed the island, the MBL top did not descend or reform near the surface and then deepen, but rather an inversion reformed at ~2.7 km. Using a variety of datasets, the height of the reformation of the inversion is related to the cloud top height of the scattered shallow cumulus convection under the COL, which coincides with the level of maximum convergence of the vertical velocity.  相似文献   

Quantifying uncertainty in extreme values of design parameters with resampling techniques   总被引：1，自引：0，他引：1  

Ying Li  Dave Simmonds  Dominic Reeve   《Ocean Engineering》2008,35(10):1029-1038

In this paper, a methodology for the selection of statistical models for describing the extreme wave heights on the basis of resampling techniques is presented. Two such techniques are evaluated: the jackknife and the bootstrap. The methods are applied to two high-quality datasets of wave measurements in the Mediterranean and one from the East Coast of the USA. The robustness of the estimates of the extreme values of wave heights at return periods important for coastal engineering design is explored further. In particular, we demonstrate how an ensemble error norm can be used to select the most appropriate extreme probability model from a choice of cumulative distribution functions (CDFs). This error norm is based on the mean error norm of the optimised CDF for each resampled (replicate) data series. The resampling approach is also used to present confidence intervals of the CDF parameters. We provide a brief discussion of the sensitivity of these parameters and the suitability of each model in terms of uncertainty with resampling techniques. The advantages of resampling are outlined, and the superiority of the bootstrap over the jackknife in quantifying the uncertainty of extreme quantiles is demonstrated for these records.  相似文献