CARIBIC Observations of Rapid Mixing Across the Extra-tropical Tropopause: A Case Study

Using the CARIBIC Boeing 767 aircraft, a suite of trace gases and aerosols was measured
between Germany and the Maldives in June 2000 at altitudes between 9.4 and 10 km. In the
extra-tropics, the flight track was located in the tropopause region. A large variability of trace
gases and ultra-fine aerosol concentrations was observed while the aircraft intercepted air
masses from the upper troposphere and the lowermost stratosphere, as well as outflow of deep
convection. The correlations of alkanes (C2 − C5) observed in the non-convective areas point
to relatively rapid mixing across the tropopause within about a day. Unusually high mixing
ratios of short-lived alkanes (C4 − C6) in the convective areas indicate rapid transport of
boundary layer air masses to cruising altitude. Using the ratios of the mixing ratios of alkanes
(C3 − C5) observed in the convective and non-convective areas, we estimate the age of air
masses in the tropopause region to be 24(±6) days for this event. This timescale is similar to
that of vertical transport within the troposphere. Altogether our observations provide further
evidence that the extra-tropical tropopause is often not a very effective mixing barrier.ing barrier.