Observed winds, turbulence, and dispersion in built-up downtown areas of Oklahoma City and Manhattan

Abstract

Wind and tracer data from the Oklahoma City Joint Urban 2003 (JU2003) and the Manhattan Madison Square Garden 2005 (MSG05) urban field experiments are being analyzed to aid in understanding air flow and dispersion near street-level in built-up downtown areas. The mean winds are separately calculated for groups of anemometers having similar exposures such as “near street level” and “on building top”. Several general results are found, such as the scalar wind speed at street level is about 1/3 of that at building top. Turbulent standard deviations of wind speed components and temperature, and vertical fluxes of momentum and sensible heat, are calculated from sonic anemometers near street level at 20 locations in JU2003 and five locations in MSG05, and from two rooftop locations in MSG05. The turbulence observations are consistent with observations in the literature at other cities, although the JU2003 and MSG05 data are unique in that many data are available near street level. For example, it is found that the local (i.e., at the measuring height) $\sigma_{w}/u_{\ast}$ averages about 1.5 and the local $u_{\ast}/u$ averages about 0.25 in the two cities, where $\sigma_{w}$ is the standard deviation of vertical velocity fluctuations, $u_{\ast}$ is the friction velocity, and u is the wind speed. The ratio of temperature fluctuations to temperature scale, $\sigma_{T}/T_{\ast}$ , averages about −3 in both cities, consistent with similarity theory for slightly unstable conditions, where $\sigma_{T}$ is the standard deviation of temperature fluctuations, and $T_{\ast}$ is the temperature scale. The calculated Obukhov length, L, is also consistent with slightly unstable conditions near street level, even at night during JU2003. The SF₆ tracer concentration observations from JU2003 are analyzed. Values of $uC_{max}/Q$ for the continuous releases are calculated for each release and arc distance, where $C_{max}$ is the 30-min average arc maximum concentration, Q is the continuous source emission rate, and u is the spatial-averaged wind speed in the downtown area. The basic characteristics of the JU2003 plot of averaged $uC_{max}/Q$ agree reasonably well with similar plots for other urban experiments in Salt Lake City and London (i.e., at $x < 1000 m, C_{max}/Q = Ax^{-2})$ . A is found to be about 3 during the day and about 10 during the night.