Monday, 15 January 2007
Investigation on the circulation of Urban Boundary Layer induced by urbanized land surface
Exhibit Hall C (Henry B. Gonzalez Convention Center)
In this study, idealized MM5 simulations are conducted to examine the structural change of wind and temperature within the urban boundary layer (UBL) as urban surface characteristics change. The urbanization and accompanying concentration of heat distribution nearby urban area are becoming important issue in its own right and also becoming important for understanding global warming trend. This study investigates the relationship between surface fluxes from urban land surface and atmospheric circulation within UBL in terms of urban canopy dynamics and physics. In MM5, urban properties are characterized by albedo (15%), moisture availability (10%), emissivity (88% at 9μm), roughness length (80cm), and thermal inertia (0.03 cal cm-2 k-1 s-1/2) which are specified in land use type. Modifying the values specified in land use type carefully, we could examine the sensitivity of meteorological variables on the change of urban surface properties. To assess the direct impact of urban modification, we eliminated possible external time dependent forcing such as the time dependent lateral boundary condition and the diurnal variation of solar insolation. The short wave radiation was fixed as noon condition. Windless and sunny day over Korean peninsula in summer (July 23, 2005) was selected as initial condition. With this model setup, surface energy balance equation which is directly related with the temperature and wind within UBL was examined in detail. As the land-use type changes from grass to urban, temperature of near-surface becomes 1.4°C warmer, and vertical velocity, usually observed over the downwind side in urban area, becomes the maximum 0.07m/s stronger within UBL. Among physical parameters characterizing urban surface, roughness length was found to be most influencing factor for the near surface temperature and wind within UBL. It was found that the roughness length is tightly related to static stability near sub-UBL. As roughness length increases (25cm, 50cm, 100cm and 200cm), the static stability values within sub-UBL increased -0.00404K/m, -0.00470K/m, -0.00560K/m, and -0.00757K/m Related to this, the vertical motion was enhanced within the UBL. These results are consistent with other studies such as Bornstein et. al (2000), Thielen et. al. (2000).