Constraining tropospheric mixing timescales using airborne observations and numerical models

[摘要] A technique is demonstrated for estimating atmospheric mixing time-scales fromin-situ data, using a Lagrangian model initialised from an Eulerian chemicaltransport model (CTM).This method is applied to airborne tropospheric CO observations taken during seven flights of the MediterraneanIntensive 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 underlyingLagrangian model.The method uses the fact that in Lagrangian tracer transportmodelling, the mixing spatial and temporal scales are decoupled: the spatialscale is determined by the resolution of the initial tracer field, and the timescale by the trajectory length.The chaotic nature of lower-atmospheric advection results in the continuous generation of smaller spatial scales, aprocess terminated in the real atmosphere by mixing.Thus, a mix-down lifetime can be estimated byvarying trajectory length so that the model reproduces the observed amount ofsmall-scale tracer structure.Selecting a trajectory length is equivalent tochoosing a mixing timescale. For the cases studied, the results are very insensitive to CO photochemical changecalculated 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, sincethe 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 relativelyhomogeneous regions, i.e. it is not too important to account for changes in aircraftaltitude 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 shorttimescale 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 numbersrefer to a mixing spatial scale of 2.8°, defined here by the resolution of the Eulerian grid from which tracer fieldswere interpolated to initialise the Lagrangian model.For the flight of 3 August, theobserved concentrations result from a complex set of transport histories, andthe models are used to interpret the observed structure, while illustrating where more caution is required with this method of estimatingmix-down lifetimes.

[学科分类] 大气科学