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B. A. Elliott and T. B. Sanford

Abstract

One remarkable result of the Local Dynamics Experiment in POLYMODE is the discovery of a large number of small energetic eddies. Using CTD data, SOFAR float tracks, and profiles of absolute velocity, we describe one of these features—the subthermocline eddy D1. The feature has an advective time scale of 1 day, and its primary mode of movement is advection by the ambient flow field. The eddy is resolved into a coordinate system moving with the velocity of the ambient flow, which ranges from 11–16 cm s−1. Its phase speed is less than 2 cm s−1.

The eddy is a lens centered at 1500 db. Velocity profiles show the lens has no detectable signal in and above the main thermocline or below about 3000 db. Its subsurface velocity maximum is 28.6 cm s−1 at 1500 db, 15 km from the center. The radial variation of the azimuthal velocity is Gaussian inside the velocity maximum but decays as e br beyond. There is little evidence of the eddy beyond a radius of 25 km.

Inside the velocity maximum, the eddy is characterized by a salinity minimum of 34.965‰ over a potential temperature range of 3.838°–3.917°C and an oxygen maximum of 6.1 ml l−1. The freshest water is located in the lower half of the eddy about 10 km from the dynamic axis. However, the eddy is uniformly fresher than the ambient water over most of its density range.

The water property data on any density surface show radial and azimuthal structure. Well mixed, 50–100-db thick layers may indicate active mixing in the interior. Hence this eddy may have at one time been colder and fresher than observed here. Outside the velocity maximum to a radius of 25 km the water is a mixture of eddy water and the surrounding water. Based on water properties, the eddy core is predominantly of Labrador Sea origin.

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B. A. Elliott and T. B. Sanford

Abstract

The dynamics of a subthermocline lens observed during the POLYMODE Local Dynamics Experiment are examined using density data and measurements of the velocity field obtained by an absolute velocity profiler. It is shown that the momentum balance is nonlinear. The lens' potential vorticity contours are closed in the horizontal and vertical, trapping low-salinity water at the lens core. The lens's dynamics are explained by a series of elementary models based on the classical Bessel-tunction vortex. The models show that nonlinearity enters in two ways, through the nonlinear momentum balance and through the finite character of the stretching vorticity. The models suggest a lens anatomy: the core; a boundary layer at the velocity maximum; a buffer zone; and a geostrophic region. The first two terms are self-explanatory. The buffer zone extends from the velocity maximum to a radius we term the geostrophic radius; at which there is a salinity front. On either side of this front the character of the mixing processes is quite different. At larger radii the momentum balance is geostrophic, and the lens remains a coherent structure through finite stretching vorticity.

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R. O. Reid, B. A. Elliott, and D. B. Olson

Abstract

In order to clarify some inconsistencies in the literature, on ocean energetics, the evaluation of the available potential energy (APE) is reconsidered. Attention is focused on the baroclinic APE under conditions in which the hydrostatic approximation prevails. Following Margules and Lorenz (ML), we define this as the difference between the total enthalpy of a given sample state of the fluid and that of an isentropically leveled state having the same mass. It is partly gravitational potential energy and partly elastic internal energy. For emphasis, we denote this as the ML APE. It differs from the gravitational part (APEg) by an amount that depends on whether the elastic internal energy is released (as in the atmosphere) or stored (as in the ocean) during an isentropic leveling process. In the ocean the ML APE is found to be somewhat less than the APEg, in contrast to the atmosphere where the ML APE greatly exceeds the APEg. This difference is ascribed to the temperature dependency of compressibility, which has opposite sign as well as different magnitude for sea water as compared to air.

A series expansion of the ML APE which explicitly accounts for mass conservation is given for a general binary fluid. The leading term in this expansion is positive definite for a stably stratified reference state and is equivalent to an approximation of APE frequently employed in ocean modeling studies. It is shown that this is a reasonable approximation (±10%) even for large perturbations, as in Gulf Stream rings.

A similar series expansion of the APEg shows that for seawater the difference from the ML APE should not exceed ∼20%. On the other hand, estimates based on the unexpanded form of the APEg are vulnerable to serious error unless special care is taken to assure that the reference state is consistent with mass conservation. Some examples of erroneous estimates for Gulf Stream rings, in which the APE is too large by a factor of 3 or more, are identified. The correct estimates of the APE are comparable to the kinetic energy for the case of mesoscale eddies in the sea.

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Timothy A. Coleman, Kevin R. Knupp, James Spann, J. B. Elliott, and Brian E. Peters

Since the successful tornado forecast at Tinker AFB in 1948 paved the way for the issuance of tornado warnings, the science of tornado detection and forecasting has advanced greatly. However, tornado warnings must be disseminated to the public to be of any use. The Texas tornado warning conferences in 1953 began to develop the framework for a modern tornado warning system and included radar detection of tornadoes, a spotter network, and improved communications between the U.S. Weather Bureau, spotters, and public officials, allowing more timely warnings and dissemination of those warnings to the public.

Commercial radio and television are a main source of warnings for many, and the delivery methods on TV have changed much since 1960. NOAA Weather Radio (NWR) was launched after the 1974 Super Outbreak of tornadoes, with the most important feature being the tone alert that allowed receivers to alert people even when the radio broadcast was turned off. Today, NWR reaches most of the U.S. population, and Specific Area Message Encoding technology has improved its warning precision. Outdoor warning sirens, originally designed for use in enemy attack, were made available for use during tornado warnings around 1970.

“Storm based” warnings, adopted by the National Weather Service in 2007, replaced countybased warnings and greatly reduce the warning area. As communications advances continue, tornado warnings will eventually be delivered to precise locations, using GPS and other location technology, through cellular telephones, outdoor sirens, e-mails, and digital television, in addition to NWR.

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M. M. Schreier, B. H. Kahn, A. Eldering, D. A. Elliott, E. Fishbein, F. W. Irion, and T. S. Pagano

Abstract

The combination of multiple satellite instruments on a pixel-by-pixel basis is a difficult task, even for instruments collocated in space and time, such as the Moderate Resolution Imaging Spectroradiometer (MODIS) and Atmospheric Infrared Sounder (AIRS) on board the Earth Observing System (EOS) Aqua. Toward the goal of an improved collocation methodology, the channel- and scan angle–dependent spatial response functions of AIRS that were obtained from prelaunch measurements and calculated impacts from scan geometry are shown within the context of radiance comparisons. The AIRS spatial response functions are used to improve the averaging of MODIS radiances to the AIRS footprint, and the variability of brightness temperature differences (ΔTb) between MODIS and AIRS are quantified on a channel-by-channel basis. To test possible connections between ΔTb and the derived level 2 (L2) datasets, cloud characteristics derived from MODIS are used to highlight correlations between these quantities and ΔTb, especially for ice clouds in H2O and CO2 bands. Furthermore, correlations are quantified for temperature lapse rate (dT/dp) and the magnitude of water vapor mixing ratio (q) obtained from AIRS L2 retrievals. Larger values of dT/dp and q correlate well to larger values of ΔTb in the H2O and CO2 bands. These correlations were largely eliminated or reduced after the MODIS spectral response functions were shifted by recommended values. While this investigation shows that the AIRS spatial response functions are necessary to reduce the variability and skewness of ΔTb within heterogeneous scenes, improved knowledge about MODIS spectral response functions is necessary to reduce biases in ΔTb.

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Bradley G. Illston, Jeffrey B. Basara, Christopher A. Fiebrich, Kenneth C. Crawford, Eric Hunt, Daniel K. Fisher, Ronald Elliott, and Karen Humes

Abstract

Soil moisture is an important component in many hydrologic and land–atmosphere interactions. Understanding the spatial and temporal nature of soil moisture on the mesoscale is vital to determine the influence that land surface processes have on the atmosphere. Recognizing the need for improved in situ soil moisture measurements, the Oklahoma Mesonet, an automated network of 116 remote meteorological stations across Oklahoma, installed Campbell Scientific 229-L devices to measure soil moisture conditions. Herein, background information on the soil moisture measurements, the technical design of the soil moisture network embedded within the Oklahoma Mesonet, and the quality assurance (QA) techniques applied to the observations are provided. This project also demonstrated the importance of operational QA regarding the data collected, whereby the percentage of observations that passed the QA procedures increased significantly once daily QA was applied.

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Renee A. McPherson, Christopher A. Fiebrich, Kenneth C. Crawford, James R. Kilby, David L. Grimsley, Janet E. Martinez, Jeffrey B. Basara, Bradley G. Illston, Dale A. Morris, Kevin A. Kloesel, Andrea D. Melvin, Himanshu Shrivastava, J. Michael Wolfinbarger, Jared P. Bostic, David B. Demko, Ronald L. Elliott, Stephen J. Stadler, J. D. Carlson, and Albert J. Sutherland

Abstract

Established as a multipurpose network, the Oklahoma Mesonet operates more than 110 surface observing stations that send data every 5 min to an operations center for data quality assurance, product generation, and dissemination. Quality-assured data are available within 5 min of the observation time. Since 1994, the Oklahoma Mesonet has collected 3.5 billion weather and soil observations and produced millions of decision-making products for its customers.

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J. C. Doran, S. Abbott, J. Archuleta, X. Bian, J. Chow, R. L. Coulter, S. F. J. de Wekker, S. Edgerton, S. Elliott, A. Fernandez, J. D. Fast, J. M. Hubbe, C. King, D. Langley, J. Leach, J. T. Lee, T. J. Martin, D. Martinez, J. L. Martinez, G. Mercado, V. Mora, M. Mulhearn, J. L. Pena, R. Petty, W. Porch, C. Russell, R. Salas, J. D. Shannon, W. J. Shaw, G. Sosa, L. Tellier, B. Templeman, J. G. Watson, R. White, C. D. Whiteman, and D. Wolfe

A boundary layer field experiment in the Mexico City basin during the period 24 February–22 March 1997 is described. A total of six sites were instrumented. At four of the sites, 915-MHz radar wind profilers were deployed and radiosondes were released five times per day. Two of these sites also had sodars collocated with the profilers. Radiosondes were released twice per day at a fifth site to the south of the basin, and rawinsondes were flown from another location to the northeast of the city three times per day. Mixed layers grew to depths of 2500–3500 m, with a rapid period of growth beginning shortly before noon and lasting for several hours. Significant differences between the mixed-layer temperatures in the basin and outside the basin were observed. Three thermally and topographically driven flow patterns were observed that are consistent with previously hypothesized topographical and thermal forcing mechanisms. Despite these features, the circulation patterns in the basin important for the transport and diffusion of air pollutants show less day-to-day regularity than had been anticipated on the basis of Mexico City's tropical location, high altitude and strong insolation, and topographical setting.

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