Whitaker, J. S., and C. A. Davis, 1995:
Cyclogenesis in a saturated environment. J. Atmos. Sci.,
52, 889-907.
ABSTRACT
Cyclogenesis in a saturated environment
Jeffrey S. Whitaker
CIRES, University of Colorado, Boulder, CO
Christopher A. Davis
National Center for Atmospheric Research, Boulder, CO
Abstract
The dynamics of baroclinic wave growth in a saturated environment is examined
using linear and nonlinear models employing a parameterization of latent heat
release that assumes all rising air is saturated, and saturation equivalent
potential temperature is conserved on ascent. Piecewise potential vorticity
(PV) diagnostics are used to interpret the results.
When the stability to vertical displacements in saturated air is allowed to
increase with height, as it must in an atmosphere with a constant, positive
lapse rate of potential temperature, the growth rates of the most unstable modes
of the Eady problem grow only marginally faster than the modes of the dry
problem. The vertical variation of moist static stability produces a gradient
of moist potential vorticity in the rising air, eliminating the short wave
cutoff present in the dry Eady problem. The destabilization of the short waves
is shown to be associated with the interaction between surface potential
temperature anomalies and diabatically generated lower tropospheric potential
vorticity anomalies.
Nonlinear primitive equation simulations, starting from normal-mode initial
conditions, show that while the dry wave grows at nearly the linear growth rate
until maximum amplitude is reached, the moist wave grows signficantly faster
than the linear growth rate at finite amplitude. This enhanced growth is
associated with the rapid amplification of a mesoscale PV anomaly generated by
latent heat release at the warm front. The rapid amplification of the surface
cyclone results from the superposition of the circulation associated with this
mesoscale PV anomaly upon the circulation associated with the surface and upper
boundary potential temperature anomalies. Additional integrations with finite
amplitude initial conditions more typical of atmospheric conditions exhibit
similar behavior.
It is suggested that many of the rapid cyclogenesis events that occur as
upper-tropospheric PV anomalies cross the east coasts of continents may arise
from the rapid generation of PV anomalies by condensational heating in the moist
maritime lower troposphere.