Stratosphere–Troposphere Exchange:

Isabel Ramos Álvaro

Simulations for an

Ozone-like Tracer

Valdebenito Laura 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., 1992. A zonal average model of the stratospheric contributions to the tropospheric ozone budget. J.

Geophys. Res. , 97, 18047-18060.

Robertson, L., Langner, J., and Engardt, M. 1999. An Eulerian limited-area atmospheric transport model. J. Appl. Met. 38, 190-210.

Simmons A.J. and J.K. Gibson, 2000. The ERA-40 Project Plan, ERA-40 Project Report Series No. 1, ECMWF, Reading RG29AX, UK., 63 pp.,

available from www.ecmwf.int/publications/

Undén et al, 2002. HIRLAM-5 Scientific documentation. (Available at http://www.knmi.nl/hirlam/)

Ozone Like Tracer ... Non Trivial Lateral Boundaries

CTOP=300 ppb CE

AS

T=

1 ppbC

WE

ST

=1

ppb

Deposition Flux

ECMWF (1x1) Nov 1989(Dirichlet BC)

Rapanui

Tololo

Deposition Flux

Hirlam (0.1x0.1) Oct 1999(Higher Resolution, Dirichlet BC)

CW

ES

T=

1 pp

b CE

AS

T=

1 ppb

CTOP=300 ppb

Tololo

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

Deposition Flux

Tololo

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

Deposition Flux

Flux Flux

Tololo