A urban boundary layer and dispersion parameterization for the LLNL NARAC modeling system: preliminary tests with the Joint Urban 2003 Field Project data set

The Lawrence Livermore National Laboratory (LLNL) National Atmospheric Release Advisory Center (NARAC) modeling system addresses the transport and dispersion of toxic and hazardous materials from a variety of source types and accounts for a wide range of physical processes. To provide real-time forecasts for emergency response applications, NARAC runs ADAPT, a diagnostic meteorological model, to generate gridded non-divergent wind, turbulence, and other meteorological fields. These are then used by NARAC's Lagrangian dispersion model, LODI, to predict the dispersion patterns. This paper describes modifications to the algorithm used to generate the boundary layer wind and turbulence profiles within ADAPT to parameterize urban scale effects on the urban boundary layer (UBL) wind and turbulence profiles.

The UBL consists of four sublayers including the urban canopy layer (UCL), the roughness sublayer, the inertial sublayer, and the “outer” layer. The key surface parameters describing roughness effects (displacement height, roughness length, mean building height and the average fractional frontal area) are obtained from building data corresponding to the urban core of Oklahoma City, where the Joint Urban 2003 Field Project took place. Given these parameters, a reference wind speed, surface heat flux, and UBL height, the mean wind and turbulence profiles can be generated. Within the UCL, the mean wind is parameterized by an exponential profile based on a “canopy” model, whereas above the UCL, the wind is given by the Monin-Obukhov (MO) similarity profile that accounts for stability effects; the UCL and MO profiles are matched at the canopy top to ensure the continuity of the profile.

Meteorological observations and tracer sampling data from the Joint Urban 2003 Field Project are used to assess the new approach. In particular, the ability of the parameterization to reconstruct the urban vertical profiles of wind and turbulence are evaluated. The impact on the fate and transport of a contaminant released in an urban area is also examined.

This work was performed under the auspices of the US Department of Energy by University of California, Lawrence Livermore National Laboratory under Contract W-7405-Eng-48. UCRL-ABS-230874.