  The probability distribution function (PDF) tails of the zonal flow structure
formation and the PDF tails of momentum flux by incorporating effect of a shear
flow in ion-temperature-gradient (ITG) turbulence are computed in the present
paper. The bipolar vortex soliton (modon) is assumed to be the coherent
structure responsible for bursty and intermittent events driving the PDF tails.
It is found that stronger zonal flows are generated in ITG turbulence than
Hasegawa-Mima (HM) turbulence as well as further from marginal stability. This
suggests that although ITG turbulence has a higher level of heat flux, it also
more likely generates stronger zonal flows, leading to a self-regulating
system. It is also shown that shear flows can significantly reduce the PDF
tails of Reynolds stress and structure formation.
