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J. Fischer, H. Leach, and J. D. Woods

Abstract

The distribution of isopycnic potential vorticity (IPV) has been mapped in the seasonal thermocline from a single ship using relative velocity measurements plus absolute ship motion, and density profiles from a CTD mounted on an undulating towed vehicle. The advantage of this technique is that the measurements are quasi-synoptic and no geostrophic assumptions have to be made since the current measurements are independent of the hydrographic data. The data were collected during an 11-day survey at the North Atlantic Polar Front near the Charlie Gibbs Fracture Zone (50°–52°N, 33°–37°W) in summer 1981.

The isopycnic distributions of temperature and potential vorticity are significantly correlated. The synoptic-scale IPV was calculated from the density field first with planetary vorticity alone and then with absolute vorticity (i.e. using the measured relative vorticity). Both versions show a meander structure with a wavelength of about 200 km and 100 km amplitude. A mesoscale front lies between the meander trough and ridge. Although it is possible to detect a mean IPV difference between trough and ridge from hydrographic data alone, the crossfront gradient of IPV was underestimated by excluding the relative vorticity. The contribution of the enstrophy to the variance of potential vorticity is however small, indicating that the Rossby radius of the flow is about 12 km and thus generally smaller than the scales resolved of about 30 km.

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Barrie W. Leach and J. Ian Macpherson

Abstract

A recent paper by Masters and Leise that addresses the problem of correcting inertial navigation data using Loran C describes another variation on the traditional approach that has been used by researchers in the airborne science community for many years. Somewhat disturbing are several inaccurate statements in their paper concerning possible problems in the use of modern Kalman filtering-smoothing techniques in this context. This correspondence attempts to clarify some of the misconceptions and errors expressed in their paper, based on the authors' own positive experiences with applying Kalman filter-smoother algorithms to correct inerfial navigation data being supplied by a modem strapdown navigation system.

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Stevens T. Chan and Martin J. Leach

Abstract

Under the sponsorship of the U.S. Department of Energy and U.S. Department of Homeland Security, a computational fluid dynamics (CFD) model for simulating airflow and dispersion of chemical/biological agents released in urban areas has recently been developed. This model, the Finite Element Model in 3-Dimensions and Massively Parallelized (FEM3MP), is based on solving the three-dimensional, time-dependent Navier–Stokes equations with appropriate physics submodels on massively parallel computer platforms. It employs finite-element discretization for effective treatment of complex geometries and a semi-implicit projection scheme for efficient time integration. A simplified CFD approach, using both explicitly resolved and virtual buildings, was implemented to improve further the model’s efficiency. Results from our model are continuously being verified against measured data from wind-tunnel and field studies. Herein, this model is further evaluated using observed data from intensive operation periods (IOP) 3 and 9 of the Joint Urban 2003 field study conducted in Oklahoma City, Oklahoma, in July 2003. The model simulations of wind and concentration fields in the near and intermediate regions, as well as profiles of wind speed, wind direction, friction velocity, and turbulent kinetic energy (TKE) in the urban wake region, are generally consistent with and compared reasonably well to field observations. In addition, this model was able to reproduce the observed split plume of IOP 3 and the end vortices along Park Avenue in IOP 9. The dispersion results and TKE profiles at the crane station indicate that the effects of convective mixing are relatively important for the daytime release of IOP 3 but that the stable effects are relatively unimportant for the nighttime release of IOP 9. Results of this study also suggest that the simplified CFD approach implemented in FEM3MP can be a cost-effective tool for simulating urban dispersion problems.

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Martin J. Leach and A. A. N. Patrinos

Abstract

The existence of coastal fronts and their effects on deposition patterns in the Washington, D.C. area are presented in this paper. The data are from an experiment conducted from October 1986 to March 1987. An earlier paper by Patrinos et al. presented the details of the deposition patterns. The results presented in that paper were not entirely consistent with the experiment's hypothesis; that is, synoptic-scale southeasterly surface flow would produce excess deposition northwest of the city. Instead, excess deposition was found to the southeast of the urban area. This paper examines the meteorology more closely and shows how small-scale meteorological circulations influence the flow fields and deposition patterns. The wind fields in the experiment area were more northerly to northeasterly in response to coastal circulations, rather than southeasterly as expected from the synoptic conditions. The meteorology and deposition patterns from four cases are presented, with evidence of coastal circulations apparent in three of the four cases.

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Xiaodong Hong, Martin J. Leach, and Sethu Raman

Abstract

Variable vegetation cover is a possible trigger for convection, especially in semiarid areas due to differential surface forcing. A two-dimensional numerical model with explicit cloud physics and a detailed vegetation parameterization scheme is used to investigate the role of vegetation differences in triggering convective cloud formation. The ground surface in all simulations includes two irrigated vegetation areas with a dry steppe in the center of the domain. The effects of atmospheric stability, ambient moisture profile, and horizontal heating scale are investigated.

Atmospheric stability controls the growth of convective circulations. Thermal circulations form at the interfaces between the vegetated areas and the dry steppe. In the more stable environment, two distinct convective cells persist; they merge into one cell in the less stable cases. The existence of low-level moisture controls the timing and persistence of clouds that form. An interesting result is the earlier dissipation of clouds in less stable cases, as greater mixing with drier air from aloft leads to the dilution of the cloud water. Since the largest thermal forcing exists at the interfaces, length of the steppe interacts with the stability to control the merger of the cells. The two cells merge quickly into one for narrow horizontal heating. For the widest heating scale studied, no merger occurs.

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B. W. Leach and J. I. MacPherson

Abstract

Airborne wind measurement techniques currently being used onboard the National Aeronautical Establishment (NAE) Twin Otter Atmospheric Research Aircraft are described and their fundamental limitations are discussed. In particular, a recently acquired LTN-90-100 strapdown Inertial Reference System (IRS) exhibits significant low frequency errors in its velocity components (primarily Schuler oscillation errors that can attain peak values of 2 to 3 m s−1), actually degrading wind computation accuracy compared with older techniques. A new wind measurement technique, based on a Kalman filter integrated navigation approach, is shown to mitigate this problem and provide wind computation accuracy superior to previous methods. Preliminary results, based on applying the Kalman filter to Twin Otter flight test data, indicate that inertial velocity accuracies of 0.3 m s−1 rms (per axis) are attainable under ideal conditions, with a corresponding improvement in the accuracy of earth-referenced wind components.

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V. Fiekas, H. Leach, K-J. Mirbach, and J. D. Woods

Abstract

Mesoscale instabilities have been found with a wavelength of about 85 km growing along the meandering jet of the North Atlantic Intergyre Front at about 52°N, 33°W. They show the signature of baroclinic instabilities and are typified by vertical motions of up to 5 m d−1, which have been diagnosed using the ω equation. The effect of the earth's curvature on motion on these scales is shown to be negligible.

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H. Leach, S. J. Bowerman, and M. E. McCulloch

Abstract

Mesoscale eddies in the northeast North Atlantic were investigated using the SeaSoar towed CTD and ADCP data from the 1991 Vivaldi cruise. These data cover an area of 1700 km × 1500 km between 39° and 54°N and between 35° and 10°W. To maximize statistical significance, but retain the possibility of determining north–south gradients, statistics of eddy quantities were calculated separately for the northern and southern halves of the cruise area. The mean flow in the south is essentially zero; in the north the flow is dominated by the North Atlantic Current (NAC) with a mean speed of 6.5 cm s−1. The eddy kinetic energy in the south, 205 cm2 s−2, is, however, only slightly less than in the north, 272 cm2 s−2. The eddy momentum transports, or Reynolds stresses, uυ, show a poleward decrease, corresponding to an acceleration of the mean eastward flow associated with the NAC of 0.03 cm s−1 day−1. The eddy heat transports, uT, are not significantly different from zero in the south but show a clear poleward transport in the north of 5.5 K cm s−1, or 0.1 PW for the 365-m layer 1500 km wide. The depth-averaged eddy potential vorticity fluxes, uq, show a convergence toward the source region of the low-potential-vorticity eastern North Atlantic Central Water west of Biscay. The residual or rectified eddy transport velocity implied by the eddy potential vorticity flux, u* = −uq/q, is 0.7 cm s−1 toward the southwest in the south, while in the north it is 0.9 cm s−1 toward the northwest crossing the property isolines. The directions correspond to a divergence from the formation region of the eastern North Atlantic Central Water. An assessment of the overall volume transport of the region suggests that the westward eddy volume transport (∼4 Sv; Sv ≡ 106 m3 s−1) is almost balanced by an eastward geostrophic flow (∼3 Sv) with the remainder being supplied by a smaller contribution leaving the northward-flowing eastern boundary current (∼1 Sv).

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Frank J. Gouveia, Martin J. Leach, Joseph H. Shinn, and William E. Ralph

Abstract

Although measured vertical profiles of wind, turbulence, and tracer concentrations are critical for understanding the urban boundary layer, it is problematic to field a sounding system or a tall structure to support anemometers in a densely populated area. Anemometers mounted on an existing building may be measuring flow distorted by that building. During the Joint Urban 2003 field experiment in Oklahoma City, the authors solved these problems by using a large crane to support a cable and crossarm framework holding a vertical array of eight 3D sonic anemometers. The highest level was over 80 m above the surface; the lowest was just under 8 m. The open-lattice structure of the crane boom and skeletal array framework did not substantially alter the airflow to the sensors. Review of the spectra shows that there are no consistent oscillations in the wind data. Data were accepted from all azimuths, although the flow was from the south 80% of the time. Profiles of wind show pronounced curvature, indicating that the higher levels may be affected by rougher surfaces at a great distance from the crane. This crane-lofted system was safely erected and disassembled in a few hours, stood undisturbed for 34 days, and collected over 1500 separate profiles of 30-min averages.

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A. A. N. Patrinos, M. J. Leach, R. M. Brown, R. L. Tanner, and F. S. Binkowski

Abstract

A field study in the Washington, D.C. area explored the impact of urban emissions and mesoscale meteorology on precipitation chemistry. The study was a follow-up to an earlier, considerably more industrialized, study in the Philadelphia area; emissions along the Delaware Valley were found to affect the deposition of nitrate and sulfate on the urban mesoscale. The Washington studies were designed to complement and enhance the earlier study with an expanded sampling domain, sequential precipitation sampling and airborne measurements. Four storms were sampled successfully between October 1986 and April 1987. Results appear to confirm the conclusions of the Philadelphia study, although the upwind-downwind contrast in nitrate and sulfate deposition is not as pronounced. This difference is attributed to the area's widely distributed emission patterns and to the prevailing theories regarding the production of nitric acid and sulfuric acid on the relevant time and space scales. The importance of mesoscale meteorology and hydrogen peroxide availability is highlighted in at least two of the sampled storms.

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