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Patricia M. Dare and Philip J. Smith

Abstract

Eddy kinetic energy budgets are presented for both moist and dry 48 h forecasts and corresponding observations of a developing, winter extratropical cyclone. Forecasts are from the Drexel-NCAR Limited Area Mesoscale Prediction System initialized at 1200 GMT 9 January 1975. Observations consist of North American rawinsonde data objectively analyzed to the model grid. The energetics quantities are calculated for both model and observed data on a domain including most of the contiguous United States and southern Canada for times corresponding to the model initialization and the 12, 24, 36 and 48 h forecasts.

Results show that the eddy kinetic energy content is dominated by the jet stream development for both model and observed data, with maxima flanking the trough at approximately 300 mb. The area-averaged, vertically-integrated eddy kinetic energy content is approximately 25% greater for the model initialization than the observations. The excess is localized in the jet maximum west of the trough, where geostrophic first-guess values are retained during initialization in place of missing winds. The increased kinetic energy west of the trough may be a factor in the excessive upper-air wave development predicted in the 24 h forecast and in the southeastward displacement of the cyclone system in the latter half of the forecast. Despite the larger initial eddy kinetic energy content, both models predict only a doubling of the integrated value, whereas the observed value tripled.

The observed increase in eddy kinetic energy content primarily results from an upper-tropospheric surplus of the source due to horizontal flux convergence over the sinks, generation and dissipation. The inability of the models to produce the observed increase in eddy kinetic energy is consistent with the underprediction of the horizontal flux convergence. The predicted flux values are affected by model boundary errors which are expected to propagate into the domain at the rate of 20 to 30° longitude per day. This implies the influence of boundary errors on the flux maximum associated with the western jet core as early as the 24 h forecast. Generation by cross-contour flow tends to be negatively correlated with the horizontal flux convergence, but also is affected by latent heat release. Latent heat release is found to enhance positive generation in both the observed and moist model results as compared to the dry model results.

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Philip J. Smith and Chih-Hua Tsou

Abstract

The role of static stability (σ) is diagnosed for an intense extratropical cyclone that developed over the central United States during 9–11 January 1975. Results indicate that minimum σ, values occurred in the lower troposphere at 0000 UTC 10 January 1975, during the period of slow cyclone development, and then increased as rapid development proceeded. Further, the upward advection of smaller static stabilities in the cyclone area, a forcing process in the height tendency equation, resulted in a significant reduction of height falls attributed to vorticity advection, thermal advection, and latent heat release.

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Song Chin Lin and Philip J. Smith

Abstract

Diabatic heating by latent heat release and longwave radiation and corresponding generation of available potential energy have been computed for a tornado-producing cyclone system. Also, the contributions of two subareas, encompassing the immediate cyclone vicinity and the convection area, to the cyclone system generation are examined applying the concepts of limited region baroclinic and barotropic generation.

Heating values, which are dominated by convective latent heat release, are generally in good agreement with observed weather features and cyclone development. The resulting positive cyclone system generation is comparable with previous studies and is strongly influenced by the contributions made by the two subareas. In turn, the latter are dominated by their. barotropic components, indicating that the heating fields in these subareas contribute more effectively to increasing or maintaining the baroclinicity of the cyclone system than of the subareas themselves.

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Matthew J. Carrier, Hans E. Ngodock, Philip Muscarella, and Scott Smith

Abstract

The assimilation of surface velocity observations and their impact on the model sea surface height (SSH) is examined using an operational regional ocean model and its four-dimensional variational data assimilation (4DVAR) analysis component. In this work, drifter-derived surface velocity observations are assimilated into the Navy’s Coastal Ocean Model (NCOM) 4DVAR in weak-constraint mode for a Gulf of Mexico (GoM) experiment during August–September 2012. During this period the model is trained by assimilating surface velocity observations (in a series of 96-h assimilation windows), which is followed by a 30-day forecast through the month of October 2012. A free-run model and a model that assimilates along-track SSH observations are also run as baseline experiments to which the other experiments are compared. It is shown here that the assimilation of surface velocity measurements has a substantial impact on improving the model representation of the forecast SSH on par with the experiment that assimilates along-track SSH observations directly. Finally, an assimilation experiment is done where both along-track SSH and velocity observations are utilized in an attempt to determine if the observation types are redundant or complementary. It is found that the combination of observations provides the best SSH forecast, in terms of the fit to observations, when compared to the previous experiments.

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Philip Muscarella, Matthew J. Carrier, Hans Ngodock, Scott Smith, B. L. Lipphardt Jr., A. D. Kirwan Jr., and Helga S. Huntley

Abstract

The Lagrangian predictability of general circulation models is limited by the need for high-resolution data streams to constrain small-scale dynamical features. Here velocity observations from Lagrangian drifters deployed in the Gulf of Mexico during the summer 2012 Grand Lagrangian Deployment (GLAD) experiment are assimilated into the Naval Coastal Ocean Model (NCOM) 4D variational (4DVAR) analysis system to examine their impact on Lagrangian predictability. NCOM-4DVAR is a weak-constraint assimilation system using the indirect representer method. Velocities derived from drifter trajectories, as well as satellite and in situ observations, are assimilated. Lagrangian forecast skill is assessed using separation distance and angular differences between simulated and observed trajectory positions. Results show that assimilating drifter velocities substantially improves the model forecast shape and position of a Loop Current ring. These gains in mesoscale Eulerian forecast skill also improve Lagrangian forecasts, reducing the growth rate of separation distances between observed and simulated drifters by approximately 7.3 km day−1 on average, when compared with forecasts that assimilate only temperature and salinity observations. Trajectory angular differences are also reduced.

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Matthew J. Carrier, Hans Ngodock, Scott Smith, Gregg Jacobs, Philip Muscarella, Tamay Ozgokmen, Brian Haus, and Bruce Lipphardt

Abstract

Eulerian velocity fields are derived from 300 drifters released in the Gulf of Mexico by The Consortium for Advanced Research on Transport of Hydrocarbon in the Environment (CARTHE) during the summer 2012 Grand Lagrangian Deployment (GLAD) experiment. These data are directly assimilated into the Navy Coastal Ocean Model (NCOM) four-dimensional variational data assimilation (4DVAR) analysis system in a series of experiments to investigate their impact on the model circulation. The NCOM-4DVAR is a newly developed tool for data analysis, formulated for weak-constraint data assimilation based on the indirect representer method. The assimilation experiments take advantage of this velocity data along with other available data sources from in situ and satellite measurements of surface and subsurface temperature and salinity. Three different experiments are done: (i) A nonassimilative NCOM free run, (ii) an assimilative NCOM run that utilizes temperature and salinity observations, and (iii) an assimilative NCOM run that uses temperature and salinity observations as well as the GLAD velocity observations. The resulting analyses and subsequent forecasts are compared to assimilated and future GLAD velocity and temperature/salinity observations to determine the performance of each experiment and the impact of the GLAD data on the analysis and the forecast. It is shown that the NCOM-4DVAR is able to fit the observations not only in the analysis step, but also in the subsequent forecast. It is also found that the GLAD velocity data greatly improves the characterization of the circulation, with the forecast showing a better fit to future GLAD observations than those experiments without the velocity data included.

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