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R. A. Plumb, R. A. Vincent, and R. L. Craig

Abstract

Studies of the quasi-two-day wave show that it is a summertime phenomenon. In the summer of 1983–84 at Adelaide (35°S, 138°E), the main phase of the wave appeared as a pulse in mid-January which lasted about seven cycles (14 days). Coincident with the onset of the pulse a temporary but substantial change occured in the prevailing circulation throughout a deep layer of the upper mesosphere; a perturbation of more than 10 m s&−1 occurred in the northward flow, whereas the change in the zonal flow (about 30 m s−1 westward) actually caused a reversal of the prevailing eastward simulation above 84 km.

It is suggested that these changes in the prevailing circulation were a response to the wave pulse. A simple calculation is performed to estimate the anticipated response to the observed wave event; under plausible assumptions about the magnitude of mean and eddy dissipation processes, predicted circulation changes agree reasonably well with those observed.

It is concluded that such events have a substantial, if temporary, impact on the prevailing circulation in the upper mesospere and may be important in the transport of atmospheric constituents at these heights during summer.

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Robert L. Walko, Craig J. Tremback, Roger A. Pielke, and William R. Cotton

Abstract

An existing two-way interactive grid-nesting technique is generalized to accommodate stretched grids and a spatially variable grid-nesting ratio. The new scheme applies the same reversibility constraint and possesses the same scalar and momentum conservation properties as its predecessor. The scheme can be applied in any coordinate direction but was motivated primarily by the common use of vertical grid stretching. The scheme is successfully toted in a simulation of a collapsing cold pool.

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T. H. Vonder Haar, A. C. Meade, R. J. Craig, and D. L. Reinke

Abstract

Advanced software routines have been developed for digital imaging systems to obtain three- and four- dimensional computer-generated images from meteorological satellite, radar, and conventional data. Time sequences of these digital images provide a truly four-dimensional view of evolving atmospheric conditions. Applications of this technique for convective storms research and teaching, for forecaster information, and for pilot briefing are presented.

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Craig S. Schwartz, Glen S. Romine, Kathryn R. Fossell, Ryan A. Sobash, and Morris L. Weisman

Abstract

Precipitation forecasts from convection-allowing ensembles with 3- and 1-km horizontal grid spacing were evaluated between 15 May and 15 June 2013 over central and eastern portions of the United States. Probabilistic forecasts produced from 10- and 30-member, 3-km ensembles were consistently better than forecasts from individual 1-km ensemble members. However, 10-member, 1-km probabilistic forecasts usually were best, especially over the first 12 h and at rainfall rates ≥ 5.0 mm h−1 at later times. Further object-based investigation revealed that better 1-km forecasts at heavier rainfall rates were associated with more accurate placement of mesoscale convective systems compared to 3-km forecasts. The collective results indicate promise for 1-km ensembles once computational resources can support their operational implementation.

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William R. Cotton, Ming-Sen Lin, Ray L. McAnelly, and Craig J. Tremback

Abstract

A composite analysis technique is used to investigate the evolution of mesoscale features of mesoscale convective complexes (MCCs). The early stage of the MCC lifecycle is characterized by convergence, vertical motion and heating being centered in the lower troposphere. As the MCC matures the level of peak upward motion and heating shifts to the upper troposphere. The system achieves and maintains its maximum divergence, upward motion, and anticyclonic vorticity in the upper troposphere during the latter half of the life cycle. This is in contrast to GATE tropical clusters where the maximum divergence, upward motion, and anticyclonic vorticity occurred at the mature stage of the cluster and then weakened. This difference might be explained by an MCC being an inertially stable form of mesoscale convective system whose radius exceeds the Rossby radius of deformation.

The MCC is shown to be an efficient rain producer, exhibiting a precipitation efficiency exceeding 100% at the mature stage due to the accumulation of water substance in the stratiform anvil cloud during the earlier, predominantly convective stages of the system.

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Craig S. Schwartz, Glen S. Romine, Kathryn R. Smith, and Morris L. Weisman

Abstract

Convection-permitting Weather Research and Forecasting (WRF) Model forecasts with 3-km horizontal grid spacing were produced for a 50-member ensemble over a domain spanning three-quarters of the contiguous United States between 25 May and 25 June 2012. Initial conditions for the 3-km forecasts were provided by a continuously cycling ensemble Kalman filter (EnKF) analysis–forecast system with 15-km horizontal grid length. The 3-km forecasts were evaluated using both probabilistic and deterministic techniques with a focus on hourly precipitation. All 3-km ensemble members overpredicted rainfall and there was insufficient forecast precipitation spread. However, the ensemble demonstrated skill at discriminating between both light and heavy rainfall events, as measured by the area under the relative operating characteristic curve. Subensembles composed of 20–30 members usually demonstrated comparable resolution, reliability, and skill as the full 50-member ensemble. On average, deterministic forecasts initialized from mean EnKF analyses were at least as or more skillful than forecasts initialized from individual ensemble members “closest” to the mean EnKF analyses, and “patched together” forecasts composed of members closest to the ensemble mean during each forecast interval were skillful but came with caveats. The collective results underscore the need to improve convection-permitting ensemble spread and have important implications for optimizing EnKF-initialized forecasts.

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Craig S. Schwartz, Glen S. Romine, Ryan A. Sobash, Kathryn R. Fossell, and Morris L. Weisman

Abstract

This expository paper documents an experimental, real-time, 10-member, 3-km, convection-allowing ensemble prediction system (EPS) developed at the National Center for Atmospheric Research (NCAR) in spring 2015. The EPS is particularly unique in that continuously cycling, limited-area, mesoscale ensemble Kalman filter analyses provide diverse initial conditions. In addition to describing the EPS configurations, initial forecast assessments are presented that suggest the EPS can provide valuable severe weather guidance and skillful predictions of precipitation. The EPS output is available to operational forecasters, many of whom have incorporated the products into their toolboxes. Given such rapid embrace of an experimental system by the operational community, acceleration of convection-allowing EPS development is encouraged.

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Craig L. Stevens, Edward R. Abraham, C. Mark Moore, Philip W. Boyd, and Jonathan Sharples

Abstract

Current-meter, temperature, and microstructure observations of the large-amplitude internal tide shoaling on the continental shelf of the east coast of northern New Zealand show the complexity of the internal kinematics and mixing. The propagation speed of the main internal wave was around 0.3 m s−1, and nonstationary time series analysis was used to locate the trailing short-wavelength internal waves in frequency (periods of around 40 min) and tidal-phase space. The average energy dissipation rate (5 × 10−8 m2 s−3) was an order of magnitude smaller than that observed on the open shelf in other studies, but peaks in dissipation rate were measured to be much greater. The vertical diffusivity of heat was around 10−4 m2 s−1, comparable to, or greater than, other studies. Examples of the scale and sporadic nature of larger mixing events were observed. The behavior was complicated by the nearby steeply shoaling coast of the Poor Knight Islands. Consistent reflected wave energy was not apparent.

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Craig S. Schwartz, Glen S. Romine, Ryan A. Sobash, Kathryn R. Fossell, and Morris L. Weisman

Abstract

Beginning 7 April 2015, scientists at the U.S. National Center for Atmospheric Research (NCAR) began producing daily, real-time, experimental, 10-member ensemble forecasts with 3-km horizontal grid spacing across the entire conterminous United States. Graphical forecast products were posted in real time to the Internet, where they attracted a large following from both forecasters and researchers across government, academia, and the private sector. Although these forecasts were initially planned to terminate after one year, the project was extended through 30 December 2017 because of the enthusiastic community response. This article details the motivation for the NCAR ensemble project and describes the project’s impacts throughout the meteorological community. Classroom and operational use of the NCAR ensemble are discussed in addition to the diverse application of NCAR ensemble output for research purposes. Furthermore, some performance statistics are provided, and the NCAR ensemble website and data visualization approach are described. We hope the NCAR ensemble’s success will motivate additional experimental forecast demonstrations that transcend current operational capabilities, as forward-looking forecast systems are needed to accelerate operational development and provide students, young scientists, and forecasters with glimpses of what future modeling systems may look like. Additionally, the NCAR ensemble dataset is publicly available and can be used for meaningful research endeavors concerning many meteorological topics.

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Ryan A. Sobash, Craig S. Schwartz, Glen S. Romine, Kathryn R. Fossell, and Morris L. Weisman

Abstract

Probabilistic severe weather forecasts for days 1 and 2 were produced using 30-member convection-allowing ensemble forecasts initialized by an ensemble Kalman filter data assimilation system during a 32-day period coinciding with the Mesoscale Predictability Experiment. The forecasts were generated by smoothing the locations where model output indicated extreme values of updraft helicity, a surrogate for rotating thunderstorms in model output. The day 1 surrogate severe probability forecasts (SSPFs) produced skillful and reliable predictions of severe weather during this period, after an appropriate calibration of the smoothing kernel. The ensemble SSPFs exceeded the skill of SSPFs derived from two benchmark deterministic forecasts, with the largest differences occurring on the mesoscale, while all SSPFs produced similar forecasts on synoptic scales. While the deterministic SSPFs often overforecasted high probabilities, the ensemble improved the reliability of these probabilities, at the expense of producing fewer high-probability values. For the day 2 period, the SSPFs provided competitive guidance compared to the day 1 forecasts, although additional smoothing was needed to produce the same level of skill, reducing the forecast sharpness. Results were similar using 10 ensemble members, suggesting value exists when running a smaller ensemble if computational resources are limited. Finally, the SSPFs were compared to severe weather risk areas identified in Storm Prediction Center (SPC) convective outlooks. The SSPF skill was comparable to the SPC outlook skill in identifying regions where severe weather would occur, although performance varied on a day-to-day basis.

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