3-DIMENSIONAL CHEMICAL FORECASTING - A METHODOLOGY

During the recent ASHOE (Airborne Southern Hemisphere Ozone Experiment ) and SESAME (Second European Stratospheric Arctic and Middle-Latitude Experiment) field campaigns we produced a number of 10-day chemical f orecasts of the stratosphere to aid in flight planning and interpretat ion of the observations. A general circulation model was integrated at high resolution to produce a 10-day meteorological forecast. These fo recast fields were then used to specify the circulation in an off-line three-dimensional chemical transport model to produce a medium-range chemical forecast of the stratosphere. The chemical model has a detail ed stratospheric chemistry scheme, including heterogeneous reactions o n polar, stratospheric clouds and sulfate aerosols. The NASA ER-2 high -altitude research aircraft made in situ measurements of chemical trac ers in the lower stratosphere during the ASHOE campaign, which took pl ace in four deployments from March to October 1994. This paper present s examples from the chemical forecasts made during the second and four th deployments. We show that the ability to indicate the likely compos ition of the atmosphere is an important improvement over the standard meteorological forecasts, especially in air perturbed by chemical proc essing on polar stratospheric clouds. Of particular interest was the a bility to forecast the chemical composition of filaments, which had be en eroded from the vortex edge. Furthermore, immediate post-flight com parison between observed data and synoptic model data gave useful chem ical interpretation of the observations.