# Longitudinal rolls

From AMS Glossary

## longitudinal rolls

(

*Also called*rolls, roll vortices, organized large eddies.) Atmospheric coherent structures in the form of persistent organized counterrotating roll vortices that are approximately aligned with the mean wind and span the depth of the planetary boundary layer.Longitudinal rolls are frequently present in the atmospheric boundary layer in near-neutral to moderately unstable stratification. They are believed to be the result of nonlinear equilibration of mixed convective–dynamic normal mode instabilities of the mean boundary flow. Longitudinal rolls produce a nonlocal transport not only of momentum, but also of scalar quantities that mix the boundary layer more efficiently than local turbulent diffusion. The quasi-two-dimensional longitudinal rolls generate a mean secondary circulation that organizes the smaller-scale three- dimensional turbulent eddies into linear patterns. The existence of longitudinal rolls significantly changes the fluxes within the boundary layer and at the surface. Flux profiles also differ between the updraft and downdraft regions of longitudinal rolls. In favorable thermodynamic conditions cloud streets (linear boundary layer cloud patterns) form in the updraft regions between the rolls.

*See*coherent structures, Langmuir circulation, two-dimensional eddies, horizontal convective rolls.Etling, D., and R. A. Brown 1993. Roll vortices in the planetary boundary layer: A review. Bound.-Layer Meteor.. 65. 215–248.