k is effective diffusivity
l is strain rate
Lfil = Ö(k/l)
Typical values:
   k=5m2s-1, l=5x10-6 s-1
   Lfil~1km
   Converges in ~ 1-2 days
m/l<2.5:  as exponentially growing/inert tracer
m/l>2.5:  width dependent on growth rate
                larger than for exponential growth
Lfil ~ Ö(km/l2)