Numerical Simulations of Mesoscale Boundary Layer Structure over New York City

Abstract

Roughness length variations and the urban heat island effect are the dominating influences of highly urbanized terrain on boundary layer structure and evolution. Variations in roughness length can alter the surface wind flow by slowing it down, turning it, or a combination of both. The urban heat island effect keeps surface temperatures warmer than surrounding rural areas, leading to a more turbulent nocturnal boundary layer over the urbanized terrain than the surrounding regions. With a pronounced heat flux gradient, surface wind speeds are often enhanced as they flow across the urban regions.

This thesis explores the influences of New York City on the structure and evolution of the boundary layer through a combination of numerical model simulations and observational analysis following the destruction of the World Trade Center buildings on 11 September 2001. Mesoscale processes, such as sea breeze circulations, urban heat island, and terrain modified flows are addressed in this research through the use of observations and several numerical simulations. Surface based observations from the National Weather Services' ASOS network are examined. Additionally, observations from an independent 10 m micrometeorological tower and two Sound Detection and Ranging (SODARS) are used. These observations are also used for model validation.

An observational analysis of 10 m tower data and SODAR data is conducted for an extended study period between 10 September 2001 and 10 December 2001. Tower measurements of wind speed and direction (10 m) and temperature (2 m) are presented. SODAR data of wind speed and direction is also examined. Several different synoptic flow regimes were analyzed during this study period. Aerodynamic roughness length calculations were also made for two independent flow direction sectors. Results from this analysis showed that roughness lengths less than 1 m, if the predominant flow was between 180 to 359 degrees. Twenty-four hour averaged surface temperatures were observed to be warmer over the city center than the surrounding rural areas. Near surface wind speeds were also observed to be lower over the highly urbanized terrain associated with New York City.

Simulations using 1 km grid spacing output from the Advanced Regional Prediction System (ARPS) and PSU/NCAR Mesoscale Model 5 (MM5) are examined during a high ground level pollutant concentration episode in lower Manhattan. The ARPS simulation showed a more defined sea breeze frontal formation and propagation than the MM5 simulation did over lower Manhattan. The ARPS simulation also showed a better defined slowing and turning of the 10 m wind speed over the highly urbanized terrain of lower Manhattan and Brooklyn relative to the MM5 simulation. Since both simulations used the same landuse data and roughness length parameterization, the planetary boundary layer scheme in both models is likely contributing to the observed differences.

The urban heat island effect, urban blocking effect and sea breeze front are analyzed using the ARPS mesoscale model. The sea breeze frontal development and inland propagation agrees well with previous research by Michael (1998) and Bornstein (1994), who showed similar results using WSR-88D imagery and numerical simulations, respectively. Additionally, the turning of the surface (10 m) wind flow agrees well with previous research by Bornstein and Johnson (1977) that showed nighttime conditions during stronger flow regimes (>4 m/s) to be associated with distinctive roughness induced cyclonic turning in the winds over the main core of Manhattan and Brooklyn.