Cold tracer parameterizatio of heterogeneous ozone loss in a global NWP model

The ability to accurately forecast ozone is becoming more important as Numerical Weather Prediction (NWP) models are extended upward to include the stratosphere. Homogeneous (gas-phase) ozone chemistry can be parameterized in terms of odd-oxygen production and loss rates applied locally to advected ozone mixing ratios. A key feature of heterogeneous chemistry, however, is that as air becomes photochemically "activated" it can cause non-local ozone destruction via advection. A recent approach to this problem incorporates an additional tracer, the so-called "cold tracer," which tags and traces processed air. Initially absent, cold tracer is created rapidly when stratospheric temperatures fall below the threshold temperature for nitric acid trihydrate (NAT) formation. This newly created cold tracer is then advected and, if not subject to further sub-NAT temperatures, progressively erodes with a time scale of several days when advected into sunlight. As the cold tracer decays in sunlight it also reduces local ozone mixing ratios via parameterized ozone loss chemistry.

Forecasts (hindcasts) of stratospheric ozone run with parameterized ozone chemistry, including a cold tracer, are examined for several case studies that include regions of sub-NAT temperatures. The NWP model used is a high-altitude version of the Navy's operational global forecast model, NOGAPS-ALPHA (Navy Operational Global Atmospheric Prediction System- Advanced Level Physics and High Altitude). The model is run with a top at 0.005 hPa (near 85 km), with vertical resolution of 2 km throughout the middle atmosphere, and with spectral horizontal resolution of T79 and T239 (~1.5 and 0.5 degrees respectively). Accurate polar temperatures are needed for modeling NAT formation. Results show that the model's polar temperatures are sensitive to the sub-grid scale gravity wave drag (GWD) parameterization scheme and the choice of tuning parameters in the GWD scheme. Results from the cold tracer forecasts show that the cold tracer parameterization is capable of realistically modeling heterogeneous ozone loss in the NWP model.