Velocity deformation for LRT

[sahil.bhandari]

I get why velocity deformation is not imp for short distances. Why is velocity deformation for LRT a good measure for horizontal turbulence? I would have thought the large (compounding) uncertainties in velocity farther away would have lured us to use a standard model for large distances (and prevent simulation iteration from very large deviations)?

[alicec]

I'm not sure I understand your question. Here is some more information on this option from the user guide.

https://www.ready.noaa.gov/hysplitusersguide/S625.htm
KDEF defines the way the horizontal turbulence is computed. The default approach is to compute the horizontal mixing in proportion to the vertical mixing using one of the methods defined above (see the technical documentation for details). The original computation was to compute the mixing from the deformation of the horizontal wind field. The limitation of this method is that for shorter-range dispersion simulations (<100 km) the deformation parameterization used in conjunction with larger scale meteorological fields will not reflect the diurnal variations in horizontal turbulence. Using diurnal sensitive methods will not effect longer-range calculations because the particles are distributed over many meteorological grid cells where variations in the transport vector dominate the horizontal dispersion process.

You can also see the technical document
https://www.arl.noaa.gov/documents/reports/arl-224.pdf