The advection of high-resolution tracers by low-resolution winds

The usefulness of any simulation of atmospheric tracers using low-resolutio n winds relies on both the dominance of large spatial scales in the strain and time dependence that results in a cascade in tracer scales. Here, a qua ntitative study on the accuracy of such tracer studies is made using the co ntour advection technique, it is shown that, although contour stretching ra tes are very insensitive to the spatial truncation of the wind field, the d isplacement errors in filament position are sensitive. A knowledge of displ acement characteristics is essential if Lagrangian simulations are to be us ed for the inference of airmass origin. A quantitative lower estimate is ob tained for the tracer scale factor (TSF): the ratio of the smallest resolve d scale in the advecting wind field to the smallest "trustworthy" scale in the tracer field. For a baroclinic wave life cycle the TSF = 6.1 +/- 0.3 wh ile for the Northern Hemisphere wintertime lower stratosphere the TSF = 5.5 +/- 0.5, when using the most stringent definition of the trustworthy scale . The similarity in the TSF for the two flows is striking and an explanatio n is discussed in terms of the activity of potential vorticity (PV) filamen ts. Uncertainty in contour initialization is investigated for the stratospheric case. The effect of smoothing initial contours is to introduce a spinup ti me, after which wind field truncation errors take over from initialization errors (2-3 days). It is also shown that false detail from the proliferatio n of finescale filaments limits the useful lifetime of such contour advecti on simulations to similar to 3 sigma(-1) days, where sigma is the filament thinning rate, unless filaments narrower than the trustworthy scale are rem oved by contour surgery. In addition, PV analysis error and diabatic effect s are so strong that only PV filaments wider than 50 km are at all believab le, even for very high-resolution winds. The minimum wind field resolution required to accurately simulate filaments down to the erosion scale in the stratosphere (given an initial contour) is estimated and the implications f or the modeling of atmospheric chemistry are briefly discussed.