Constraining tropospheric mixing timescales using airborne observations and numerical models P. Good1, C. Giannakopoulos1, F. M. O’Connor2, S. R. Arnold3, M. de Reus4, and H. Schlager5 1National Observatory of Athens, Greece 2Centre for Atmospheric Science, Cambridge, UK 3School of the Environment, University of Leeds, UK 4Max-Planck-Institute for Chemistry, Atmospheric Chemistry Department, Mainz, Germany 5German Aerospace Center, Institute of Atmospheric Physics, Oberpfaffenhofen, Germany
Abstract. A technique is demonstrated for estimating atmospheric mixing time-scales from
in-situ data, using a Lagrangian model initialised from an Eulerian chemical
transport model (CTM). This method is applied to airborne tropospheric CO observations taken during seven flights of the Mediterranean
Intensive Oxidant Study (MINOS) campaign, of August 2001. The time-scales derived, correspond to mixing applied at the spatial scale of the CTM grid.
They are relevant to the family of hybrid Lagrangian-Eulerian models, which impose Eulerian grid mixing to an underlying
Lagrangian model. The method uses the fact that in Lagrangian tracer transport
modelling, the mixing spatial and temporal scales are decoupled: the spatial
scale is determined by the resolution of the initial tracer field, and the time
scale by the trajectory length. The chaotic nature of lower-atmospheric advection results in the continuous generation of smaller spatial scales, a
process terminated in the real atmosphere by mixing. Thus, a mix-down lifetime can be estimated by
varying trajectory length so that the model reproduces the observed amount of
small-scale tracer structure. Selecting a trajectory length is equivalent to
choosing a mixing timescale. For the cases studied, the results are very insensitive to CO photochemical change
calculated along the trajectories. That is, it was found that if CO was treated as a passive tracer, this did not affect the mix-down timescales derived, since
the slow CO photochemistry does not have much influence at small spatial scales.
The results presented correspond to full photochemical calculations. The method is most appropriate for relatively
homogeneous regions, i.e. it is not too important to account for changes in aircraft
altitude or the positioning of stratospheric intrusions, so that small scale structure is easily distinguished.
The chosen flights showed a range of mix-down time upper limits: a very short
timescale of 1 day for 8 August, due possibly to recent convection or model error, 3 days for 3 August,
probably due to recent convective and boundary layer mixing, and 6-9 days for 16, 17, 22a, 22c and 24 August. These numbers
refer to a mixing spatial scale of 2.8°, defined here by the resolution of the Eulerian grid from which tracer fields
were interpolated to initialise the Lagrangian model. For the flight of 3 August, the
observed concentrations result from a complex set of transport histories, and
the models are used to interpret the observed structure, while illustrating where more caution is required with this method of estimating
Citation: Good, P., Giannakopoulos, C., O’Connor, F. M., Arnold, S. R., de Reus, M., and Schlager, H.: Constraining tropospheric mixing timescales using airborne observations and numerical models, Atmos. Chem. Phys., 3, 1023-1035, doi:10.5194/acp-3-1023-2003, 2003.