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JungKyu Rhys Lim, Brooke Fisher Liu, and Michael Egnoto

Abstract

On average, 75% of tornado warnings in the United States are false alarms. Although forecasters have been concerned that false alarms may generate a complacent public, only a few research studies have examined how the public responds to tornado false alarms. Through four surveys (N = 4162), this study examines how residents in the southeastern United States understand, process, and respond to tornado false alarms. The study then compares social science research findings on perceptions of false alarms to actual county false alarm ratios and the number of tornado warnings issued by counties. Contrary to prior research, findings indicate that concerns about false alarm ratios generating a complacent public may be overblown. Results show that southeastern U.S. residents estimate tornado warnings to be more accurate than they are. Participants’ perceived false alarm ratios are not correlated with actual county false alarm ratios. Counterintuitively, the higher individuals perceive false alarm ratios and tornado alert accuracy to be, the more likely they are to take protective behavior such as sheltering in place in response to tornado warnings. Actual country false alarm ratios and the number of tornado warnings issued did not predict taking protective action.

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Brooke Fisher Liu, Michael Egnoto, and JungKyu Rhys Lim

Abstract

Mobile home residents experience higher fatality rates from tornadoes than “fixed home” residents. Yet, research on how mobile home residents understand and respond to tornado warnings is lacking. Such research can help meteorologists and their partners better communicate tornado risk. We conducted four surveys with residents of the southeastern United States. This region has the highest concentration of tornado fatalities and killer tornadoes, in part because of the high density of mobile homes. Findings reveal that today’s tornado warning system inadequately prepares mobile home residents to respond safely to tornadoes. The study offers recommendations for how to improve tornado communication for mobile and fixed home residents.

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John D. Farrara, Michael Fisher, Carlos R. Mechoso, and Alan O'Neill

Abstract

The early winter (mid-April to mid-July) circulation in the Southern Hemisphere stratosphere is studied. Emphasis is placed on the evolution of strong disturbances with structures dominated by the zonal wavenumber-1 component of the flow (wave 1). The approach to this investigation is based on analysis of 12 years (1979–90) of observational data and comparative analyses of control and hypothesis-testing simulations with a three-dimensional primitive equation model of the stratosphere and mesosphere.Considerable interannual variability is found in both the intensity and timing of wave-1 amplification during early winter. Though usually quasi-stationary, there are six extended periods in the dataset when wave 1 travels steadily eastward and is of large amplitude. Two of these periods (June 1980 and June 1985) are examined in detail. The evolution of the circulation in these two cases resembles that during Canadian warmings in the Northern Hemisphere in several ways. First, there is a large, eastward-moving disturbance with a nearly equivalent barotropic structure, with the largest amplitude in the lower and middle stratosphere. Second, temperature increases are smaller than those observed during final warmings in the Southern Hemisphere. Third, irreversible buckling of contours of Ertel's potential vorticity takes place in a region well away from the zero-wind line. Owing to their geographical preference for development over the South Pacific, wave-1 events in the southern stratosphere during early winter are referred to as South Pacific warmings.The hypothesis-testing simulations suggest that the development of South Pacific warmings is connected with the amplification of wave 1 at 100 mb and that the eastward propagation of the disturbances requires eastward propagation of wave 1 at 100 mb. In addition, the results suggest that development of stratospheric disturbances in the southern stratosphere during early winter depends more on the intensity of wave 1 at 100 mb than on the structure of the zonal-mean flow in the stratosphere.

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Kettyah C. Chhak, Andrew M. Moore, Ralph F. Milliff, Grant Branstator, William R. Holland, and Michael Fisher

Abstract

At midlatitudes, the magnitude of stochastic wind stress forcing due to atmospheric weather is comparable to that associated with the seasonal cycle. Stochastic forcing is therefore likely to have a significant influence on the ocean circulation. In this work, the influence of the stochastic component of the wind stress forcing on the large-scale, wind-driven circulation of the North Atlantic Ocean is examined. To this end, a quasigeostrophic model of the North Atlantic was forced with estimates of the stochastic component of wind stress curl obtained from the NCAR Community Climate Model. Analysis reveals that much of the stochastically induced variability in the ocean circulation occurs in the vicinity of the western boundary and some major bathymetric features. Thus, the response is localized even though the stochastic forcing occurs over most of the ocean basin. Using the ideas of generalized stability theory, the stochastically induced response in the ocean circulation can be interpreted as a linear interference of the nonorthogonal eigenmodes of the system. This linear interference process yields transient growth of stochastically induced perturbations. By examining the model pseudospectra, it is seen that the nonnormal nature of the system enhances the transient growth of perturbation enstrophy and therefore elevates and maintains the variance of the stochastically induced circulations in the aforementioned regions. The primary causes of nonnormality in the enstrophy norm are bathymetry and the western boundary current circulation.

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Kettyah C. Chhak, Andrew M. Moore, Ralph F. Milliff, Grant Branstator, William R. Holland, and Michael Fisher

Abstract

As discussed in Part I of this study, the magnitude of the stochastic component of wind stress forcing is comparable to that of the seasonal cycle and thus will likely have a significant influence on the ocean circulation. By forcing a quasigeostrophic model of the North Atlantic Ocean circulation with stochastic wind stress curl data from the NCAR CCM3, it was found in Part I that much of the stochastically induced variability in the ocean circulation is confined to the western boundary region and some major topographic features even though the stochastic forcing is basinwide. This can be attributed to effects of bathymetry and vorticity gradients in the basic state on the system eigenmodes. Using generalized stability theory (GST), it was found in Part I that transient growth due to the linear interference of nonnormal eigenmodes enhances the stochastically induced variance. In the present study, the GST analysis of Part I is extended and it is found that the patterns of wind stress curl that are most effective for inducing variability in the model have their largest projection on the most nonnormal eigenmodes of the system. These eigenmodes are confined primarily to the western boundary region and are composed of long Rossby wave packets that are Doppler shifted by the Gulf Stream to have eastward group velocity. Linear interference of these eigenmodes yields transient growth of stochastically induced perturbations, and it is this process that maintains the variance of the stochastically induced circulations. Analysis of the large-scale circulation also reveals that the system possesses a large number of degrees of freedom, which has significant implications for ocean prediction. Sensitivity studies show that the results and conclusions of this study are insensitive and robust to variations in model parameters and model configuration.

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Andrew M. Moore, Javier Zavala-Garay, Youmin Tang, Richard Kleeman, Anthony T. Weaver, Jérôme Vialard, Kamran Sahami, David L. T. Anderson, and Michael Fisher

Abstract

The optimal forcing patterns for El Niño–Southern Oscillation (ENSO) are examined for a hierarchy of hybrid coupled models using generalized stability theory. Specifically two cases are considered: one where the forcing is stochastic in time, and one where the forcing is time independent. The optimal forcing patterns in these two cases are described by the stochastic optimals and forcing singular vectors, respectively. The spectrum of stochastic optimals for each model was found to be dominated by a single pattern. In addition, the dominant stochastic optimal structure is remarkably similar to the forcing singular vector, and to the dominant singular vectors computed in a previous related study using a subset of the same models. This suggests that irrespective of whether the forcing is in the form of an impulse, is time invariant, or is stochastic in nature, the optimal excitation for the eigenmode that describes ENSO in each model is the same. The optimal forcing pattern, however, does vary from model to model, and depends on air–sea interaction processes.

Estimates of the stochastic component of forcing were obtained from atmospheric analyses and the projection of the dominant optimal forcing pattern from each model onto this component of the forcing was computed. It was found that each of the optimal forcing patterns identified may be present in nature and all are equally likely. The existence of a dominant optimal forcing pattern is explored in terms of the effective dimension of the coupled system using the method of balanced truncation, and was found to be O(1) for the models used here. The implications of this important result for ENSO prediction and predictability are discussed.

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Paul Poli, Hans Hersbach, Dick P. Dee, Paul Berrisford, Adrian J. Simmons, Frédéric Vitart, Patrick Laloyaux, David G. H. Tan, Carole Peubey, Jean-Noël Thépaut, Yannick Trémolet, Elías V. Hólm, Massimo Bonavita, Lars Isaksen, and Michael Fisher

Abstract

The ECMWF twentieth century reanalysis (ERA-20C; 1900–2010) assimilates surface pressure and marine wind observations. The reanalysis is single-member, and the background errors are spatiotemporally varying, derived from an ensemble. The atmospheric general circulation model uses the same configuration as the control member of the ERA-20CM ensemble, forced by observationally based analyses of sea surface temperature, sea ice cover, atmospheric composition changes, and solar forcing. The resulting climate trend estimations resemble ERA-20CM for temperature and the water cycle. The ERA-20C water cycle features stable precipitation minus evaporation global averages and no spurious jumps or trends. The assimilation of observations adds realism on synoptic time scales as compared to ERA-20CM in regions that are sufficiently well observed. Comparing to nighttime ship observations, ERA-20C air temperatures are 1 K colder. Generally, the synoptic quality of the product and the agreement in terms of climate indices with other products improve with the availability of observations. The MJO mean amplitude in ERA-20C is larger than in 20CR version 2c throughout the century, and in agreement with other reanalyses such as JRA-55. A novelty in ERA-20C is the availability of observation feedback information. As shown, this information can help assess the product’s quality on selected time scales and regions.

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