A study of the development of extratropical cyclones with an analytic model. Part II: Sensitivity to tropospheric structure and analysis of height tendency dynamics
Abstract
An analytic quasigeostrophic model is used to examine the sensitivity of type B cyclogenesis to the vertical structure of the troposphere given a particular stratospheric temperature configuration. It is found that there is
an optimal tropospheric configuration that produces the largest negative height tendency at the center of the 1000-mb model cyclone. Based on the response of the 1000-mb height tendencies, altering the baroclinicity in
the model planetary boundary layer (PBL) does not significantly affect the instantaneous quasigeostrophic
dynamics of the deep atmosphere. Rather, the PBL temperature anomalies affect the development of lowertropospheric
model lows by hydrostatically shifting or steering the cyclone centers to locations beneath more
(or less) favorable deep atmospheric quasigeostrophic conditions for development. Diagnostic analyses of three individual stratospheric-tropospheric model configurations are also performed
to examine the dynamics that drive the height (pressure) tendency field. Generally, the analytic model findings
confirm previous observational and numerical investigations of height tendency mechanisms and support the
notion of a stratospheric level of insignificant dynamics. In the optimal development case, the 1000-mb low is
located almost directly underneath the region of strongest 200-mb temperature advection associated with a
tropopause undulation (potential vorticity anomaly). This strong lower-stratospheric warm advection instantaneously
overwhelms adiabatic cooling in the stratosphere and troposphere so that there are height falls over and
downstream of the 1000-mb low. When the static stability is lowered in the troposphere and raised in the
stratosphere to realistic "warm-sector" values, the vertical motion increases, and the local warming in the
stratosphere and cooling in the troposphere decrease. The reduced tropospheric cooling results in larger net local
column warming that intensifies the 1000-mb height falls. The intensified vertical circulation also acts to amplify
the tropopause undulation. As the amplitude of the undulation increases, characteristics of the occlusion process
can be identified.
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