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John P. Weiss and Jeffrey B. Weiss

Abstract

The seasonal dependence of predictability in ENSO manifests itself in the so-called spring barrier found in the cyclostationary lag autocorrelations, or persistence. This work examines the statistics of persistence, with particular focus on the phase-of-year-dependent pattern found in ENSO data, the barrier. Simple time series of one sine wave produce a barrier if the frequency is a biennial cycle or one of its harmonics. Time series of two sine waves produce a barrier if one frequency is a biennial cycle or a harmonic thereof. They additionally produce a barrier if their frequencies sum to unity. Time series with continuous but narrow spectral peaks at barrier-producing frequencies produce barriers only if the phase angles vary slowly or coherently across the peaks. The shape of the barrier seen in these simple time series is used to construct a model persistence map, which is a combination of an idealized barrier and the persistence of a red-noise process. A nonlinear least squares fit of the persistence of a time series to the model persistence provides a quantitative measure of the properties of the persistence barrier in any time series. Application of the measure to the Southern Oscillation index and sea surface temperature in the NINO3 region of the equatorial Pacific indicates that the ENSO persistence barrier is a feature that is statistically distinguishable from the theoretical persistence of a red-noise process. The ENSO barrier results from phase coherency of the continuum of interannual modes near the biennial frequency. Measuring the barrier on windowed data shows that there was a weak persistence barrier from approximately 1915 to 1945, a strong barrier during the 1960s and early 1970s, and a weakening barrier in the late 1970s.

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Bradley G. Illston, Jeffrey B. Basara, Christopher Weiss, and Mike Voss

The WxChallenge, a project developed at the University of Oklahoma, brings a state-of-the-art, fun, and exciting forecast contest to participants at colleges and universities across North America. The challenge is to forecast the maximum and minimum temperatures, precipitation, and maximum wind speeds for select locations across the United States over a 24-h prediction period. The WxChallenge is open to all undergraduate and graduate students, as well as higher-education faculty, staff, and alumni. Through the use of World Wide Web interfaces accessible by personal computers, tablet computer, and smartphones, the WxChallenge provides a state-of-the-art portal to aid participants in submitting forecasts and alleviate many of the administrative issues (e.g., tracking and scoring) faced by local managers and professors.

Since its inception in 2006, 110 universities have participated in the contest and it has been utilized as part of the curricula for 140 classroom courses at various institutions. The inherently challenging nature of the WxChallenge has encouraged its adoption as an educational tool. As its popularity has grown, professors have seen the utility of the Wx-Challenge as a teaching aid and it has become an instructional resource of many meteorological classes at institutions for higher learning. In addition to evidence of educational impacts, the competition has already begun to leave a cultural and social mark on the meteorological learning experience.

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Gregory S. Duane, Peter J. Webster, and Jeffrey B. Weiss

Abstract

Teleconnections between the midlatitudes of the Northern and Southern Hemispheres are diagnosed in National Centers for Environmental Prediction–National Center for Atmospheric Research reanalysis data and separately in European Centre for Medium-Range Weather Forecasts reanalysis data. The teleconnections are manifested as a small but significant tendency for blocking to occur simultaneously in the two hemispheres, though at different longitudes and different relative latitudes, during boreal winters over the period 1979–94 in both datasets.

One way to explain the correlations between blocking events is as an instance of synchronized chaos, the tendency of some coupled chaotic systems to synchronize, permanently or intermittently, regardless of initial conditions. As the coupling is weakened, the systems no longer synchronize completely, but small correlations between the states of the coupled systems are observed instead. In previous work, such behavior was observed in an idealized coupled-hemisphere model constructed from a midlatitude model due to de Swart, which extended the earlier Charney–DeVore spectral truncation of the barotropic vorticity equation by including a few extra modes. The direct coupling of the two midlatitude systems in the coupled-hemisphere model represented the exchange of Rossby waves through the upper-tropospheric “westerly ducts” in the Tropics.

Significant correlations are found between blocking events, which are chaotically timed in each hemisphere considered singly, even without several of the idealizations used in the previous study. In a model modified to include an extended tropical region, the correlations are little affected by attenuation and phase shift of the Rossby waves that couple the two midlatitude systems. Variations in the relative longitudes of topographic features in the two hemispheres leave significant correlations or anticorrelations. The annual cycle, which imposes directionality on the coupling, since the Northern Hemisphere is more strongly forced than the Southern Hemisphere at the times when the hemispheres are coupled, increases the correlations slightly. A two-hemisphere model constructed from a higher-order (wavenumber 3) truncation of the barotropic vorticity equation exhibits regime transitions between blocked and zonal flow at a more realistic rate in each hemisphere but still exhibits interhemispheric correlations.

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Brad E. Beechler, Jeffrey B. Weiss, Gregory S. Duane, and Joseph Tribbia

Abstract

Because of position errors traditional methods of data assimilation can broaden and weaken jets or other flow structures leading to reduced forecast skill. Here a technique to assimilate properties of coherent structures is developed and tested. Focusing on jets, the technique identifies jets in both the modeled and observed fields and warps the model grid so that the jet positions are better aligned prior to further assimilation of observations. The technique is tested using optimal interpolation on the flow in a two-layer quasigeostrophic channel. The results show that a simple and fast jet position correction algorithm can significantly improve the skill of a 12-h forecast. Furthermore, the results indicate that this method of position correction maintains its utility when observations become sparse.

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Vineel Yettella, Jeffrey B. Weiss, Jennifer E. Kay, and Angeline G. Pendergrass

Abstract

Climate variability and its response to increasing greenhouse gases are important considerations for impacts and adaptation. Modeling studies commonly assess projected changes in variability in terms of changes in the variance of climate variables. Despite the distant and impactful covariations that climate variables can exhibit, the covariance response has received much less attention. Here, a novel ensemble framework is developed that facilitates a unified assessment of the response of the regional variances and covariances of a climate variable to imposed external forcings and their time of emergence from an unforced climate state.

Illustrating the framework, the response of variability and covariability of land and ocean temperatures is assessed in the Community Earth System Model Large Ensemble under historical and RCP8.5 forcing. The results reveal that land temperature variance emerges from its preindustrial state in the 1950s and, by the end of the twenty-first century, grows to 1.5 times its preindustrial level. Demonstrating the importance of covariances for variability projections, the covariance between land and ocean temperature is considerably enhanced by 2100, reaching 1.4 times its preindustrial estimate. The framework is also applied to assess changes in monthly temperature variability associated with the Arctic region and the Northern Hemisphere midlatitudes. Consistent with previous studies and coinciding with sea ice loss, Arctic temperature variance decreases in most months, emerging from its preindustrial state in the late twentieth century. Overall, these results demonstrate the utility of the framework in enabling a comprehensive assessment of variability and its response to external climate forcings.

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Casey B. Griffin, Christopher C. Weiss, Anthony E. Reinhart, Jeffrey C. Snyder, Howard B. Bluestein, Joshua Wurman, and Karen A. Kosiba

Abstract

During the second Verification of the Origins of Rotation in Tornadoes Experiment (VORTEX2) field campaign, mobile radars observed a previously undocumented feature: the low-reflectivity ribbon (LRR). The LRR was characterized by reduced reflectivity Z H and differential reflectivity Z DR through a narrow region extending from the intersection of the hook and forward-flank regions of supercells. This study synthesizes kinematic and polarimetric radar observations with in situ measurements taken by the “StickNet” observing network. StickNet data have been used to establish that the LRR is associated with a localized minimum in pseudoequivalent potential temperature Pronounced drops in are observed by nine separate probes in three different supercell thunderstorms. Both single- and dual-Doppler analyses are used to examine the two- and three-dimensional structures of the winds within the LRR, revealing that the LRR is associated with cyclonic vertical vorticity aloft. Polarimetric radar observations are used to study the hydrometeor characteristics and the processes that cause those hydrometers to be present. Special consideration is given to the analysis of the vertical distribution of traditional and polarimetric variables, as well as the evolution of the kinematic fields retrieved by dual-Doppler analysis. The combination of thermodynamic, kinematic, and inferred microphysical observations supports a hypothesis that the LRR comprises sparse, large hail.

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