Estimating the height of the nocturnal urban boundary layer for dispersion applications

We analyzed three data sets to examine methods to estimate the height of the stable boundary layer (SBL) in the complex setting of an urban area. We focused on a data set from a field study that we conducted in Riverside, California, in 2011 in which surface micrometeorological variables were measured using a sonic anemometer. In addition, profiles of temperature and relative humidity were measured with a tethersonde up to a height of 500 m. These measurements, supplemented with data from other field studies, were used to evaluate representative diagnostic and prognostic models for the SBL height. As expected, these models performed poorly in the spatially inhomogeneous setting of the urban area. The time integrated friction velocity (u*), with time starting at sunset, provided the best estimate of the height. This formulation is useful because we show that the surface friction velocity can be estimated with a measurement of wind speed at one height, and an estimate of the surface roughness length.