TRANSPORT BETWEEN THE TROPICAL AND MIDLATITUDE LOWER STRATOSPHERE - IMPLICATIONS FOR OZONE RESPONSE TO HIGH-SPEED CIVIL TRANSPORT EMISSIONS

Several recent studies have quantified the air exchange rate between t he tropics and midlatitudes in the lower stratosphere using airborne a nd satellite measurements of chemical species. It is found that the mi dlatitude air is mixed into the tropical lower stratosphere with a rep lacement timescale of 13.5 months (with 20% uncertainty) for the regio n from the tropopause to 21 km [Volk et al., 1996] and at least 18 mon ths for the region of 20-28 km [Schoeberl et al., 1997]. These estimat es are used to adjust the horizontal eddy diffusion coefficients, K-yy , in a two-dimensional chemistry transport model. The value of K-yy pr eviously used to simulate the subtropical barrier, 0.03 x 10(6) m(2)/s , generates an exchange time of about 4 years, and the model without s ubtropical barrier (K-yy = 0.3 x 10 m(2)/s) has an exchange time of 5 months. Adjusting the K-yy to 0.13 x 10(6) m(2)/s is from the tropopau se to 21 km and 0.07 x 10(6) m(2) is above 21 km produces the exchange timescales which match the estimates deduced from the measurements. T he subtropical barrier prevents the engine emissions of the high-speed civil transport (HSCT) aircraft from being transported into the tropi cs and subsequently lifted into the upper atmosphere or mixed into the southern hemisphere. The model results show that the calculated ozone response to HSCT aircraft emissions using the K-yy adjusted to observ ed mixing rates is substantially smaller than that simulated without t he subtropical barrier.