Space and Time Variations in Turbulence during the Manhattan Midtown 2005 Field Experiment

The Manhattan Midtown-2005 field experiment (MID05) collected turbulence observations at 12 street-level sites (at 3-m height) and at 5 rooftop sites (at 220-m average height). The MID05 observations of 30-min averaged standard deviations of wind speed components and temperature and of sensible heat and momentum flux are found to be consistent with the authors' previously reported averaged observations in similar tall-building surroundings in the Oklahoma City Joint Urban 2003 (JU2003) and Manhattan Madison Square Garden 2005 (MSG05) field experiments. The main focus of this paper, though, is on the magnitudes of the space and time variations of the 30-min averaged turbulence values. Wind tunnel experiments and computational fluid dynamics model outputs would suggest large variations, but the full-scale urban observations show that the standard deviations of the space and time variations are usually less than 50% of the averages. Some individual observations are tabulated and the minimum and maximum listed, showing a typical range at street level for, say, sigma(v) of about +/-10%-20% in time and about +/-40% in space. It is suggested that the reason for the observed lack of large variations in turbulence is the large amount of mixing generated by (i) the 20 degrees-40 degrees meanders in wind direction over the 30-min periods, which cause a "flopping'' of building wakes, and (ii) the strong vertical mixing around the tall buildings.