Rahul B. Mahajan and Gregory J. Hakim
Department of Atmospheric Sciences, University of Washington,Seattle, WA
Journal of the Atmospheric Sciences, 68, submitted.
The spatial spreading of infinitesimal disturbances
superposed on a turbulent baroclinic jet is explored. This
configuration is representative of analysis errors in an idealized
midlatitude storm track and the insight gained may be helpful to
understand spreading of forecast errors in numerical weather
prediction models.
This problem is explored through numerical experiments of a
turbulent baroclinic jet that is perturbed with spatially localized
disturbances. Solutions from a quasi-geostrophic model for the
disturbance fields are compared against those for a passive tracer to
determine whether disturbances propagate faster than the basic-state
flow. Results show that the disturbance spreading rate is sensitive
to the structure of the initial disturbance. Disturbances that are
localized in potential vorticity (PV) have far-field winds that allow
the disturbance to travel downstream faster than disturbances that
are initially localized in geopotential, which have no far-field
wind. Near the jet, the spread of the disturbance field is observed
to exceed the tracer field for PV-localized disturbances, but not for
the geopotential-localized disturbances. Spreading rates faster than
the flow for localized-geopotential disturbances are found to occur
only for disturbances located off the jet axis.
These results are compared with those for zonal and
time independent jets to qualitatively assess the effects of
transience and non-linearity. This comparison suggests that the
average properties of localized perturbations to the turbulent jet
can be decomposed into a superposition of dynamics associated with a
time-independent parallel flow plus a ``diffusion'' process.