Local Circulations Developed in the Vicinity of Both Coastal and Inland Urban Areas: A Numerical Study with a Mesoscale Atmospheric Model

Abstract

A numerical study was conducted of the effects of two nearby urban areas, one coastal and one inland, on the local circulations and transport of urban pollutants, using a 3D mesoscale incompressible and hydrostatic atmospheric model. The Lagrangian Particle Dispersion Model was applied in the atmospheric model to study the transport of pollutants. The main experimental results are as follows: 1) A ?chain flow,? which flows downward from the upper layer over the coastal urban area to the lower layer over the inland urban area, is formed as the sea-breeze front moves inland. 2) If an inland urban area exists, the induced sea-breeze?heat-island circulation system is stronger and is maintained longer in time. This feature becomes clearer as the distance between the two urban areas widens. 3) The chain flow efficiently transports coastal urban pollutants into the lower layer over the suburban area, located between the two urban areas, and also to the farther-inland urban area, prior to the arrival of the sea-breeze front. 4) As the circulation is deflected by the Coriolis force, the amount of coastal urban pollutants that reach the inland urban area becomes dependent on the distance between the two urban areas. 5) Nonlinear effects increase the penetrating speed of the sea-breeze front. This effect becomes more evident as the size of the inland urban area increases. The chain flow is generated by the combination of the sea-breeze?heat-island circulations over the coastal urban area and the heat-island circulation over the inland urban area. Although this flow basically can be represented as the sum of the two circulations, nonlinear effects intensify the chain flow. This intensification occurs earlier in time and over a broader region as the inland urban area increases in size.