Influence of Surface Heterogeneities on the Boundary Layer Structure and Diffusion of Pollutants
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2002-01-02
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Boundary layer processes are the dominating factors in the diffusion and transport of pollutants. Air pollution dispersion is known to be controlled by several boundary layer factors. Eddy diffusion, described by the amount of turbulent kinetic energy, is the main influence on how quickly a parcel of polluted air expands and boundary layer depth determines the extent of vertical mixing potential. This thesis explores the influence that mesoscale surface related features have on the boundary layer meteorology and air pollution dispersion through a combination of numerical model simulations and observations. Mesoscale processes including sea breeze and land breeze circulations, vegetation and soil type gradient induced circulations, urban heat island and terrain modified flows are addressed in this research through the use of various numerical simulations. Surface based observations from a meso-network and ground based remote sensing observations using two SODARs are examined. These observations are also utilized for model validation.
An observational analysis of 10 m micrometeorological tower measurements and collocated SODAR measurements is conducted for two different boundary layer events, which correspond to near neutral and convective conditions. SODAR data analysis of reflectivity and wind speed profiles is presented along with tower measurements of wind speed/direction (at 2 m, 5 m and 10 m levels) and the temperature difference between 10 m and 2 m. The analysis indicated dramatically different boundary layer structures during these events.
The near-neutral case shows that the boundary layer properties remained relatively constant during the 24-hour period, as indicated by the SODAR reflectivity that consisted of homogeneous echoes up to a height of about 100 m. The static stability, implied by the observed 10 m - 2 m temperature difference (?NT10m-2m), supports a neutral, well-mixed boundary layer. The wind speed profile indicated time dependent fluctuations in magnitude associated with shear induced boundary layer eddies.
The convective case shows a rapidly increasing boundary layer during the early morning, just after sunrise. Strong fluctuating periods of high reflectivity during the day allude to convective boundary layer thermals. At the same time, near surface temperature gradient (?NT10m-2m) shows strong buoyant instability. The overall convective pattern and associated boundary layer momentum is much different than that of the neutral case.
Numerical simulations, using the Advance Regional Prediction System (ARPS) includes an idealized tropical seabreeze simulation over India, a regional scale simulation over eastern North Carolina and a city scale simulation over Raleigh, North Carolina. Using both the seabreeze and city scale meteorological simulations, coupled ARPS-CALPUFF dispersion simulations are conducted to investigate the influence of the boundary layer meteorology on dispersion patterns.
The 5 km resolution seabreeze simulation shows that the boundary layer properties are distinctly different across a region that is affected by seabreeze circulations. The dispersion experiment from a point source near the coastline indicated that the seabreeze has a very noticeable effect on atmospheric dispersion overland. Higher surface concentrations are modeled during the seabreeze episode and the post seabreeze period later in the evening.
The regional numerical simulation (6 km grid spacing) over eastern North Carolina combined several interesting boundary layer scenarios. The typically warmer Sandhills region and some of the larger urban centers are simulated as warmer surfaces. Turbulence is also higher over these warmer locations. Landuse is shown to have a definite impact on the degree of simulated turbulence as well as the boundary layer height.
The high-resolution atmospheric simulation conducted over Raleigh, North Carolina is for the same case presented by the regional simulation. The 1 km model forecast shows large distinctions within the microscale boundary layer structure over the city scale domain. The boundary layer variations are directly related to the landuse parameterization. During the night, the model is sensitive to terrain variations. Over the more elevated areas the wind speed was overall higher, while noticeably weaker over low-lying areas. A simulated down slope flow at night is detected and associated temperatures are less in the low-lying areas. The data from both SODARs compares well with the simulated profiles.
Dispersion patterns utilizing the high-resolution meteorology are influenced by the landuse in several respects. The 1 km ARPS simulation showed microscale convergence zones develop along strong surface temperature and sensible heat flux gradients, which are mostly a result of vegetation differences between the urban areas in/around Raleigh and surrounding rural areas. In the dispersion simulation, these convergence zones are directly related to higher surface concentrations.
The elevation is shown to influence the wind field at night, therefore modifying the concentrations field. More elevated areas, specifically the higher north-south ridge in the western part of the domain is associated with slightly stronger wind at night and therefore lower concentrations. The lower lying areas are more stable as a cooler slope flow is established at night. This drainage flow allows pollutants to collect over some of the highly populated suburbs around Raleigh.
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MS
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Marine, Earth and Atmospheric Sciences
