Modeling and Observations of the Structure and Evolution of the Urban Boundary Layer of New York City

The boundary layer dynamics of urban microclimates has important effects on precipitation, cloud formation, and air quality and dispersion processes. It is thus relevant to understand the evolution of boundary layer dynamics. Despite this importance, urban effects on the distributions of key weather variables in the boundary layer are not well-understood. Herein, an extreme local convective event (ELCE) with a maximum lapse rate of 14.5 K/km was observed and analyzed along with seasonal averages of vertical profiles. Seasonal aggregates of the winter (December 2014, January 2015, and February 2015) and summer (June, July, and August 2015) vertical profiles are created for the daytime and nighttime. The non-heat event period considered experienced a maximum of about 6 K/km. Vertical distributions of the thermal and moisture variables, such as temperature and water vapor density, were measured using a microwave radiometer at the City College of New York. The instrument used was the Radiometrics Profiling Radiometer model MP-3000A, a passive multi-frequency microwave radiometer (MWR). The vertical profiles contain 58 levels using a non-uniform spacing, with a finer resolution in the boundary layer. Traditional understanding of convective mixed layers usually exhibit a uniform distribution in the potential temperature and other conserved variables above the surface layer. These urban profiles however showed non-instantaneous mixing, with a very noticeable minimum in potential temperature at about ~250-500m into the mixed layer. Observations of the vertical profiles of virtual potential temperature standard deviations of the wintertime and summertime profiles were ~10 K and ~5 K, respectively. When the ELCE was compared against the average diurnal cycle, the ELCE virtual potential temperature magnitude was ~6K warmer than the average. Summer time planetary boundary layer estimates are highest in the summer, 1200m, compared to the winter at 600m. These observations were compared against an urbanized version of the Weather and Research Forecasting (WRF) model. The urbanized-WRF underestimated the amount of mixing and thus missed the warm layers that occur in the upper levels of the atmosphere during the ELCE.