Stratosphere–Troposphere Exchange: Isabel RamosÁlvaro

Simulations for an Ozone-like Tracer ValdebenitoLaura Gallardo

We simulate the intrusion and dispersion of ozone-like tracer subject to dry deposition and photolysis deposition according to the method described in Follows and Austin (1992). For that purpose we apply the Multiscale Atmospheric Transport and Chemistry model (MATCH, Robertson et al, 1999). We focus on the treatement of the stratospheric influx and the subsequent dispersion of the intruded tracer. The photolysis rates are derived on-line using the procedures included in the photochemical scheme of MATCH, designed to provide a good description under high and low reactive nitrogen regimes. The dry deposition flux is proportional to the tracer's concentration and the inverse of the sum of the aerodynamic and surface resistance (See Robertson et al, 1999 for details).

We use two set of meteorological data to drive the dispersion model: 1) Reanalysis fields from the European Center for Middle Range Weather Forecast (ECMWF). These fields have a horizontal resolution of about 1° or 125 km in the horizontal and 31 levels in the vertical from the surface up to about 10 hPa. 2) Dynamically interpolated ECMWF reanalysis fields using the dynamical model HIRLAM (High Resolution Limited Area Model (Undén et al, 2000). These fields have a horizontal resolution of 0.1° (~ 11 km), every 3 hours. The stratospheric influx is represented in the model as a boundary condition (BC), either as a fixed mixing ratio at the top and lateral boundaries (Dirichlet) or as a mass flux at the upper and lateral boundaries (Neumann). At the bottom the only boundary condition is dry deposition.

Performed simulations of intrusions of an O3 like tracer at the tropopause are very sensitive to the treatment of lateral BC. These problems are further enhanced by the steep Andean topography. We have performed several runs testing different model set ups: 1) Large-scale fields (ECMWF), using Dirichlet BC at the top and the lateral boundaries, and zero initial conditions (IC) 2) High-resolution fields (HIRLAM), using Dirichlet BC at the top and the lateral boundaries, and zero initial conditions (IC) 3) High-resolution fields (HIRLAM), using Dirichlet BC at the top and the lateral boundaries, and NON-zero initial conditions (IC) 4) High-resolution fields (HIRLAM), using Dirichlet BC at the top and Neumann BC at the lateral boundaries, and zero initial conditions (IC) The output fields obtained using large-scale fields are smooth and selfconsistent and show no numerical instabilities or noise. When we increase the resolution instabilities and noise appear. At first we thought that this was due to the use of Dirichlet BC at lateral boundaries or the use of zero-initial conditions. This does not seem to be the case. Reasons that may explain this are connected to the narrow (E-W) domain, and the effects of the Andean slope perturbing the upper westerly flow. To be continued...Any suggestions?

References Follows, M.J., and Austin, J.F., A zonal average model of the stratospheric contributions to the tropospheric ozone budget. J. Geophys. Res., 97, Robertson, L., Langner, J., and Engardt, M An Eulerian limited-area atmospheric transport model. J. Appl. Met. 38, Simmons A.J. and J.K. Gibson, The ERA-40 Project Plan, ERA-40 Project Report Series No. 1, ECMWF, Reading RG29AX, UK., 63 pp., available from Undén et al, HIRLAM-5 Scientific documentation. (Available at

Ozone Like Tracer... Non Trivial Lateral Boundaries

CTOP=300 ppb CEAST=1 ppb CWEST=1 ppb Deposition Flux ECMWF (1x1) Nov 1989 (Dirichlet BC) Rapanui Tololo

Deposition Flux Hirlam (0.1x0.1) Oct 1999 (Higher Resolution, Dirichlet BC) CWEST=1 ppb CEAST=1 ppb CTOP=300 ppb Tololo

Hirlam (0.1x0.1) Oct 1999 Non – Zero Initial Conditions Deposition Flux Tololo

Hirlam (0.1x0.1) Oct 1999 (Lateral Boundaries Conditions as Fluxes, Neumann) Deposition Flux Flux Tololo