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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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David J. Karoly

Abstract

The vacillatory behavior of the general circulation model described by Hunt (1978a,b) is analyzed using transformed Eulerian-mean diagnostics. This model was shown by Hunt to have large time variations in the troposphere and stratosphere with a period of ∼20 days. These diagnostics are used to show the coupling between the troposphere and the stratosphere and the forcing of mean state changes during the vacillation cycle.

The time variations of the wave-induced form on the mean flow and the Coriolis torque are in approximate balance throughout the vacillation cycle. Thus mean flow changes are small and the effect of the mean state on wave propagation is approximately constant. The vacillation cycle in the model is apparently due to variations in baroclinic wave activity in the troposphere and not to wave, mean flow interaction in the upper troposphere and stratosphere.

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David J. Karoly

Abstract

Eliassen-Palm cross sections and residual meridional circulations are presented for the Northern and Southern Hemispheres for summer and winter based on the data of Newell et al. (1972, 1974). The cross sections are similar to those presented by Edmon et al. (1980) for the Northern Hemisphere except that, in the Southern Hemisphere, there is much less variation between summer and winter than in the Northern Hemisphere.

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David J. Karoly and Qigang Wu

Abstract

Trends in surface temperature over the last 100, 50, and 30 yr at individual grid boxes in a 5° latitude–longitude grid are compared with model estimates of the natural internal variability of these trends and with the model response to increasing greenhouse gases and sulfate aerosols. Three different climate models are used to provide estimates of the internal variability of trends, one of which appears to overestimate the observed variability of surface temperature at interannual and 5-yr time scales. Significant warming trends are found at a large fraction of the individual grid boxes over the globe, a much larger fraction than can be explained by internal climate variations. The observed warming trends over the last 50 and 30 yr are consistent with the modeled response to increasing greenhouse gases and sulfate aerosols in most of the models. However, in some regions, the observed century-scale trends are significantly larger than the modeled response to increasing greenhouse gases and sulfate aerosols in the atmosphere. Warming trends consistent with the response to anthropogenic forcing are detected at scales on the order of 500 km in many regions of the globe.

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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

This note examines the sensitivity of simulated U.S. warm-season precipitation in the Weather Research and Forecasting model (WRF), used as a nested regional climate model, to variations in model setup. Numerous options have been tested and a few of the more interesting and unexpected sensitivities are documented here. Specifically, the impacts of changes in convective and land surface parameterizations, nest feedbacks, sea surface temperature, and WRF version on mean precipitation are evaluated in 4-month-long simulations. Running the model over an entire season has brought to light some issues that are not otherwise apparent in shorter, weather forecast–type simulations, emphasizing the need for careful scrutiny of output from any model simulation. After substantial testing, a reasonable model setup was found that produced a definite improvement in the climatological characteristics of precipitation over that from the National Centers for Environmental Prediction–National Center for Atmospheric Research global reanalysis, the dataset used for WRF initial and boundary conditions in this analysis.

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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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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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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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