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David J. Karoly

Abstract

Composite seasonal mean and anomaly fields prepared from operational numerical analyses have been used to describe the Southern Hemisphere (SH) circulation features associated with El Niño-Southern Oscillation (ENSO) events. The period of analyses available (1972–83) has limited the composites to include only three ENSO events. The reliability and stability of the composites has been tested using multiple permutation methods and by comparison with the results obtained using a longer period (1950–79) of SH rawinsonde station data.

In the SH winter, a weak equivalent-barotropic wavetrain pattern of anomalies extends over Australia and the South Pacific Ocean to South America. This wavetrain pattern is quite variable in amplitude and location between ENSO events, although it is more stable over the subtropical Pacific. In the SH summer, the circulation anomalies are more zonally symmetric, with increased height at low and high latitudes and decreased height in middle latitudes. The circulation anomalies in the SH summer are more stable than in winter, with similar patterns of anomalies in the subtropics and middle latitudes in all events.

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Melissa S. Bukovsky
and
David J. Karoly

Abstract

Several aspects of the precipitation climatology from the North American Regional Reanalysis (NARR) are analyzed and compared with two other reanalyses and one set of gridded observations over a domain encompassing the United States. The spatial distribution, diurnal cycle, and annual cycle of precipitation are explored to establish the reliability of the reanalyses and to judge their usefulness. While the NARR provides a much improved representation of precipitation over that of the other reanalyses examined, some inaccuracies are found and have been highlighted as a warning to potential users of the data.

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David J. Karoly
and
Abraham H. Oort

Abstract

Two sets of observed atmospheric circulation statistics for the Southern Hemisphere (SH) are compared. The first set was compiled at the Geophysical Fluid Dynamics laboratory (GFDL) and consists of global objective analyses of circulation statistics accumulated at individual rawinsonde stations for the period May 1963–April 1973. The second set was obtained from daily hemispheric numerical analyses prepared operationally at the World Meteorological Centre, Melbourne, Australia for the period September 1972–August 1982. This study extends the earlier comparison of circulation statistics from station-based and from numerical analysis-based methods by Lau and Oort for the Northern Hemisphere to the Southern Hemisphere.

The domain used for the comparison is a 5° × 5° latitude–longitude grid from 10° to 90°S and seven pressure levels from 1000 to 100 mb. The circulation statistics examined include (i) ten-year averages of the monthly mean fields (measures of the mean circulation), (ii) ten-year averages of the standard deviations and covariances of daily values (measures of the daily transient eddy variability) and (iii) year-to-year standard deviations of the monthly mean fields (measures of the interannual variability). The statistics are presented using horizontal maps on pressure surfaces and latitude–pressure sections of zonal averages.

The two sets of circulation statistics were derived using very different analysis methods and they apply for different time periods. The similarities and differences between the statistics from the two datasets indicate the reliability of the statistics and can be used to define a better composite set of circulation statistics for the SH.

The relatively large differences in the statistics can generally be attributed to the sparse conventional observation network in the SH, particularly over the large ocean regions, and deficiencies in the analysis methods. The two sets agree reasonably well from 850 to 500 mb over the land masses, where the observation network is less sparse. In the upper troposphere, the magnitudes of the daily transient eddy statistics from the Australian dataset are smaller due to the analysis method and the inclusion of satellite data. Over the data-sparse regions, the use of the zonal average as the first guess for the GFDL dataset has led to reduced spatial variability, smoother fields and underestimation of extreme values.

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Brian J. Hoskins
and
David J. Karoly

Abstract

Motivated by some results from barotropic models, a linearized steady-state five-layer baroclinic model is used to study the response of a spherical atmosphere to thermal and orographic forcing. At low levels the significant perturbations are confined to the neighborhood of the source and for midlatitude thermal forcing these perturbations are crucially dependent on the vertical distribution of the source. In the upper troposphere the sources generate wavetrains which are very similar to those given by barotropic models. For a low-latitude source, long wavelengths propagate strongly polewards as well as eastwards. Shorter wavelengths are trapped equatorward of the poleward flank of the jet, resulting in a split of the wave-trains at this latitude. Using reasonable dissipation magnitudes, the easiest way to produce an appreciable response in middle and high latitudes is by subtropical forcing. These results suggest an explanation for the shapes of patterns described in observational studies.

The theory for waves propagating in a slowly varying medium is applied to Rossby waves propagating in a barotropic atmosphere. The slow variation of the medium is associated with the sphericity of the domain and the latitudinal structure of the zonal wind. Rays along which wave activity propagates, the speeds of propagation, and the amplitudes and phases along these rays are determined for a constant angular velocity basic flow as well as a more realistic jet flow. They agree well with the observational and numerical model results and give a simple interpretation of them.

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David J. Karoly
and
Karl Braganza

Abstract

Variations of Australian-average mean temperature and diurnal temperature range over the twentieth century are investigated. The observed interannual variability of both is simulated reasonably well by a number of climate models, but they do not simulate the observed relationship between the two. Comparison of the observed warming and reduction in diurnal temperature range with climate model simulations shows that Australian temperature changes over the twentieth century were very unlikely to be due to natural climate variations alone. It is likely that there has been a significant contribution to the observed warming during the second half of the century from increasing atmospheric greenhouse gases and sulfate aerosols.

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Melissa S. Bukovsky
and
David J. Karoly

Abstract

In this study, the Weather Research and Forecasting (WRF) model is employed as a nested regional climate model to dynamically downscale output from the National Center for Atmospheric Research’s (NCAR’s) Community Climate System Model (CCSM) version 3 and the National Centers for Environmental Prediction (NCEP)–NCAR global reanalysis (NNRP). The latter is used for verification of late-twentieth-century climate simulations from the WRF.

This analysis finds that the WRF is able to produce precipitation that is more realistic than that from its driving systems (the CCSM and NNRP). It also diagnoses potential issues with and differences between all of the simulations completed. Specifically, the magnitude of heavy 6-h average precipitation events, the frequency distribution, and the diurnal cycle of precipitation over the central United States are greatly improved. Projections from the WRF for late-twenty-first-century precipitation show decreases in average May–August (MJJA) precipitation, but increases in the intensity of both heavy precipitation events and rain in general when it does fall. A decrease in the number of 6-h periods with rainfall accounts for the overall decrease in average precipitation. The WRF also shows an increase in the frequency of very heavy to extreme 6-h average events, but a decrease in the frequency of all events lighter than those over the central United States. Overall, projections from this study suggest an increase in the frequency of both floods and droughts during the warm season in the central United States.

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Sophie C. Lewis
and
David J. Karoly

Abstract

Diurnal temperature range (DTR) is a useful index of climatic change in addition to mean temperature changes. Observational records indicate that DTR has decreased over the last 50 yr because of differential changes in minimum and maximum temperatures. However, modeled changes in DTR in previous climate model simulations of this period are smaller than those observed, primarily because of an overestimate of changes in maximum temperatures. This present study examines DTR trends using the latest generation of global climate models participating in phase 5 of the Coupled Model Intercomparison Project (CMIP5) and utilizes the novel CMIP5 detection and attribution experimental design of variously forced historical simulations (natural-only, greenhouse gas–only, and all anthropogenic and natural forcings). Comparison of observed and modeled changes in DTR over the period of 1951–2005 again reveals that global DTR trends are lower in model simulations than observed across the 27-member multimodel ensemble analyzed here. Modeled DTR trends are similar for both experiments incorporating all forcings and for the historical experiment with greenhouse gases only, while no DTR trend is discernible in the naturally forced historical experiment. The persistent underestimate of DTR changes in this latest multimodel evaluation appears to be related to ubiquitous model deficiencies in cloud cover and land surface processes that impact the accurate simulation of regional minimum or maximum temperatures changes observed during this period. Different model processes are likely responsible for subdued simulated DTR trends over the various analyzed regions.

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Diandong Ren
,
Lance M. Leslie
, and
David Karoly

Abstract

In this study, landslide potential is investigated, using a new constitutive relationship for granular flow in a numerical model. Unique to this study is an original relationship between soil moisture and the inertial number for soil particles. This numerical model can be applied to arbitrary soil slab profile configurations and to the analysis of natural disasters, such as mudslides, glacier creeping, avalanches, landslips, and other pyroclastic flows. Here the focus is on mudslides.

The authors examine the effects of bed slope and soil slab thickness, soil layered profile configuration, soil moisture content, basal sliding, and the growth of vegetation, and show that increased soil moisture enhances instability primarily by decreasing soil strength, together with increasing loading. Moreover, clay soils generally require a smaller relative saturation than sandy soils for sliding to commence. For a stable configuration, such as a small slope and/or dry soil, the basal sliding is absorbed if the perturbation magnitude is small. However, large perturbations can trigger significant-scale mudslides by liquefying the soil slab.

The role of vegetation depends on the wet soil thickness and the spacing between vegetation roots. The thinner the saturated soil layer, the slower the flow, giving the vegetation additional time to extract soil moisture and slow down the flow. By analyzing the effect of the root system on the stress distribution, it is shown that closer tree spacing increases the drag effects on the velocity field, provided that the root system is deeper than the shearing zone.

Finally, the authors investigated a two-layer soil profile, namely, sand above clay. A significant stress jump occurs at the interface of the two media.

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Ailie J. E. Gallant
and
David J. Karoly

Abstract

Changes in the area of Australia experiencing concurrent temperature and rainfall extremes are investigated through the use of two combined indices. The indices describe variations between the fraction of land area experiencing extreme cold and dry or hot and wet conditions. There is a high level of agreement between the variations and trends of the indices from 1957 to 2008 when computed using (i) a spatially complete gridded dataset without rigorous quality control checks and (ii) spatially incomplete high-quality station datasets with rigorous quality control checks. Australian extremes are examined starting from 1911, which is the first time a broad-scale assessment of Australian temperature extremes has been performed prior to 1957. Over the whole country, the results show an increase in the extent of hot and wet extremes and a decrease in the extent of cold and dry extremes annually and during all seasons from 1911 to 2008 at a rate of between 1% and 2% decade−1. These trends mostly stem from changes in tropical regions during summer and spring. There are relationships between the extent of extreme maximum temperatures, precipitation, and soil moisture on interannual and decadal time scales that are similar to the relationships exhibited by variations of the means. However, the trends from 1911 to 2008 and from 1957 to 2008 are not consistent with these relationships, providing evidence that the processes causing the interannual variations and those causing the longer-term trends are different.

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Jonty D. Hall
,
Adrian J. Matthews
, and
David J. Karoly

Abstract

The observed relationship between tropical cyclone activity in the Australian region and the Madden–Julian oscillation (MJO) has been examined using 20 yr of outgoing longwave radiation, NCEP–NCAR reanalysis, and best track tropical cyclone data. The MJO strongly modulates the climatological pattern of cyclogenesis in the Australian region, where significantly more (fewer) cyclones form in the active (inactive) phase of the MJO. This modulation is more pronounced to the northwest of Australia. The relationship between tropical cyclone activity and the MJO was strengthened during El Niño periods. Variations of the large-scale dynamical conditions necessary for cyclogenesis were explored, and it was found that MJO-induced perturbations of these parameters correspond with the observed variation in cyclone activity. In particular, 850-hPa relative vorticity anomalies attributable to the MJO were found to be an excellent diagnostic of the changes in the large-scale cyclogenesis patterns.

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