Evaluation of a Comprehensive Meteorological/Dispersion Model with Data from a Shoreline Urban Field Study

We evaluate estimates of concentration estimates from a comprehensive model, The Air Pollution Model (TAPM), with observations from two field studies conducted in 2004 and 2005 at Wilmington, a coastal urban area located south of Los Angeles. In 2004, the tracer, SF6 was released at ground-level, and in 2005, the tracer was released both inside the stack of a power plant (buoyant elevated release) and outside the stack (non-buoyant elevated release). The meteorological conditions that governed dispersion were measured with sonic anemometers, a minisodar, and a temperature profiler. TAPM, developed by Australia's CSIRO (Commonwealth Scientific and Industrial Research Organization), is typical of the class of models based on the numerical solution of the governing mass, momentum, and energy equations. It provides estimates of the meteorological as well as the concentration fields.

Both field studies pointed to the importance of the shoreline thermal internal boundary in determining ground-level concentrations. This was particularly important during the 2005 study when ground-level concentrations were governed by the rate of entrainment of the buoyant elevated plume by TIBL. The performance of TAPM in describing the results from these field studies was compared with that of a semi-empirical dispersion model, which assumes a Gaussian concentration profile in the horizontal, and uniform mixing in the vertical of the plume entrained into the TIBL. The height of the TIBL height was computed with a simple model in which the height grows as the square root of the distance from the shoreline. The meteorological inputs for the semi-empirical model were constructed from measurements made at the site.

TAPM's performance in explaining concentration estimates during the 2004 surface releases was comparable to that from the semi-empirical model. However, it did not provide adequate concentration estimates during the 2005 study. One reason for this is the inability of TAPM in simulating the growth of TIBL with distance from the shoreline. The paper provides a detailed examination of other factors that contribute to the performance of TAPM.