We formulate equations for the slow time dynamics of fluid motion that self
consistently account for the effects of the variability upon the mean. The
time-average effects of the fluctuations introduce nonlinear dispersion th
at acts to spatially smooth the transport velocity of the mean flow relativ
e to its circulation or momentum velocity, by the inversion of a Helmholtz
operator whose length scale corresponds to the covariance of the fluctuatio
ns. (C) 1999 Elsevier Science B.V. All rights reserved.