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  • Author or Editor: M. J. Leach x
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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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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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Hung-Neng S. Chin, Martin J. Leach, Gayle A. Sugiyama, John M. Leone Jr., Hoyt Walker, J. S. Nasstrom, and Michael J. Brown

Abstract

A modified urban canopy parameterization (UCP) is developed and evaluated in a three-dimensional mesoscale model to assess the urban impact on surface and lower-atmospheric properties. This parameterization accounts for the effects of building drag, turbulent production, radiation balance, anthropogenic heating, and building rooftop heating/cooling. U.S. Geological Survey (USGS) land-use data are also utilized to derive urban infrastructure and urban surface properties needed for driving the UCP. An intensive observational period with clear sky, strong ambient wind, and drainage flow, and the absence of a land–lake breeze over the Salt Lake Valley, occurring on 25–26 October 2000, is selected for this study.

A series of sensitivity experiments are performed to gain understanding of the urban impact in the mesoscale model. Results indicate that within the selected urban environment, urban surface characteristics and anthropogenic heating play little role in the formation of the modeled nocturnal urban boundary layer. The rooftop effect appears to be the main contributor to this urban boundary layer. Sensitivity experiments also show that for this weak urban heat island case, the model horizontal grid resolution is important in simulating the elevated inversion layer.

The root-mean-square errors of the predicted wind and temperature with respect to surface station measurements exhibit substantially larger discrepancies at the urban locations than their rural counterparts. However, the close agreement of modeled tracer concentration with observations fairly justifies the modeled urban impact on the wind-direction shift and wind-drag effects.

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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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