| Detailed analysis of mesoscale transport of ozone across the tropopause over East Asia during the spring of 2001 is conducted using regional simulations with the University of Wisconsin Nonhydrostatic Modeling System (UWNMS), in situ flight data, and a new, two-scale approach to diagnosing this ozone flux. From late February to early April, synoptic activity regularly deformed the tropopause, leading to observations of ozone-rich (concentration exceeding 80 ppbv) stratospheric intrusions and filaments at tropospheric altitudes. Ozone flux across a material surface at scales where diabatic heating is negligible is difficult to interpret in a numerical simulation, since model resolution is generally not sufficient to capture detailed small-scale mixing processes. However, an upper-bound on the flux can be established by assuming that there exists a dynamical division by spatial scale, above which the wind advects large scale structures, while below it the wind leads to irreversible transport through nonconservative random strain. A formulation for this diagnosis is given and applied to ozone flux across the dynamical tropopause. Three case studies are chosen to correspond with flights over east Asia during the TRACE-P (TRansport and Chemical Evolution over the Pacific) campaign. Local and domain-averaged flux values using this method agree with other numerical and observational studies during similar synoptic environments. Sensitivity to numerical resolution, prescribed divisional spatial scale, and potential vorticity (PV) level is investigated. Divergent residual flow in regions of high ozone and PV gradients tended to maximize flux magnitudes, with the direction of the flux dependent upon asymmetries in the divergent anomalies, the orientation of the tropopause, and the fine scale structure of the ozone field. An estimate of the flow of ozone out of the lower-most stratosphere is given by computations of ozone flux divergence. A spectral analysis of the wind field lends support to the theory that random strain is the main mechanism for the initial development of mixing across a dynamical interface. |
