Impact of Uncertainties in the Atmospheric Boundary Layer Height on the Numerical Simulation of Chemical Species

Uncertainties in the atmospheric boundary layer height (ABL-H) are associated either with the uncertainties in meteorological fields or the definition of the ABL-H. When the ABL-H is involved in ABL parameterization schemes these uncertainties can have impacts on numerical simulation of chemical species. The impacts are examined numerically by employing a 1-D diffusion model with the K-profile scheme and 3-D air quality forecast model with the turbulent kinetic energy (TKE)-based scheme. The dependency on the ABL-H in the two schemes is very different. Sensitivity tests with the 1-D model show that the over-estimated/under-estimated ABL-H leads to the decrease/increase of concentration of tracers under both stable and unstable conditions due to the increase/decrease of the volume of tracers within the ABL. Under unstable conditions, the over-estimated/under-estimated ABL-H also enhances/weakens the vertical diffusivity, and leads to the decrease/increase of the concentration of tracers for the negative/positive vertical gradient of tracer. These impacts result in a little change with height within the ABL. The impacts of the ABL-H through the counter-gradient term and entrainment flux are much smaller than the impact through changing the volume of the ABL and vertical diffusivity in the ABL. Sensitivity tests with the 3-D numerical model with the TKE scheme show that over urban areas the over-estimated/under-estimated ABL-H leads to the increase/decrease of TKE in the whole ABL but does not always lead to stronger/weaker diffusivity and lower/higher concentrations of chemical species.