The Structure and Evolution of the Urban Boundary Layer During an Extreme Heat Event

The urban boundary layer dynamics stimulated by urban microclimates has important effects on controlling processes such as precipitation, cloud formation, and air quality and dispersion. It is thus important to understand the evolution of boundary layer dynamics. However, given its importance the effect of the urban environment on the distribution of key weather variables in the boundary layer is still not well-understood. In this paper, an extreme heat event with a maximum lapse rate of about 14.5 K/km was analyzed. For comparison, the non-heat wave period considered experienced a maximum of about 6 K/km. Surface conditions (thermal, moisture, and momentum) during both periods were defined using an aggregate of surface station data across New York City. Vertical distributions of the thermal and moisture variables, such as temperature and water vapor density, were measured from microwave radiometer measurements at the City College of New York. Background wind profiles measured by a Doppler wind radar located at the Liberty Science Center in nearby Jersey City provided the magnitude and direction of prevailing winds during both periods. Traditional convective mixed layers usually exhibit a uniform distribution in the potential temperature and other conserved variables. These urban profiles however showed non-instantaneous mixing, with a very noticeable minimum in potential temperature at about ~250-500m into the mixed layer. The almost simultaneous warming from both the surface and from warm air aloft worked to counteract the mixing thus enabling that distinct minimum to persist through the hours of maximum insolation.