Extratropical Synoptic-Scale
Processes and Severe
Convection
John Monteverdi
Doswell, C.A. III, and L.F. Bosart, 2001: Extratropical synoptic-scale processes
and severe convection. Severe Convective Storms, Meteor. Monogr ., 28, no.
50, Amer. Meteor. Soc., 27-69.
Extratropical Synoptic-Scale
Processes and Severe
Convection
• Introduction
• Brief Overviews of Synoptic-scale and Subsynoptic
Scale Processes
– Quasigeostrophic Principles
– Upper Level Jet Streak Dynamics
End of First Part
• Vertical Wind Shear
• Planetary Boundary Layer Processes
– Eckman Theory
– Diurnal and Baroclinic Effects
Introduction
• Traditional view says synoptic-scale processes
simply provide setting for severe convection, and
that convection does not have an impact on the
synoptic scale
• Traditional view is that the mesoscale provides
the source of parcel lift or local destabilization at
the convective scale
• Current view is that there are feedbacks
between the synoptic-scale, high end mesoscale
(aka subsynoptic) and the convective (low end
mesoscale) scales.
Introduction
• What needs to be kept in mind is that nature
does not really know these rigid definitions of
scale
• There are considerable interactions between the
scales
• Compartmentalizing the discussion on the bases
of these scales is still useful.
Brief Overviews of Synoptic-scale and
Subsynoptic Scale Processes
Quasigeostrophic Principles
• Deep, moist convection (DMC) requires three
ingredients
– Moisture
– Low Static Stability
– Ascent of parcels to LFC
• Extratropical cyclones (ETC) play role in first
two, but, by definition, do not provide parcel lift
• ETC provide the environment favoring the
formation/development of mesoscale processes
(e.g. related to drylines, fronts etc.)
Brief Overviews of Synoptic-scale and
Subsynoptic Scale Processes
Quasigeostrophic Principles
• The synoptic-scale vertical motion and pressure
fields associated with ETC can be diagnosed
and understood on the basis of quasigeostrophic
(QG) theory
– The QG-height tendency equation can be used to
understand the development of pressure fields
– The QG-omega equation can be used to diagnose the
vertical motion fields
The Relation of QG-Theory to
Understanding the Synoptic Scale
Environments of Severe Convection
• The value of these equations is not really in the
prediction, but qualitative understanding of midlatitude,
synoptic-scale processes associated with ETC
• Great effort is made by Doswell and Bosart to explore
the importance of the static stability parameter
The Relation of QG-Theory to
Understanding the Synoptic Scale
Environments of Severe Convection
•  is assumed to be a constant, which introduces
error (why?)
• Rising motion favors decrease in stability and
sinking motion increases stability (why?)
• Emanuel et al (1987) parameterizations, show
that ascent is localized and intense, while
descent is weaker and widespread and this is
due to the first assumption above, which is
erroneous (why?)
• Static stability is important in formation of ETC
(why?)
Jet Streaks
• Conceptual model
used to diagnose
ascent areas
• Ascent favorable for
cyclogenesis and
organized DMC
• Jet streaks often
coupled to low level
jet streams, advecting
moist, unstable air
Figure 2
Vertical Wind Shear
• Important factor is determining severity of
convection
• Geostrophic vertical wind shear
associated with thermal advection
• Strong vertical wind shear is major factor
in supercell convection
Vertical Wind Shear
• Strong vertical wind shear has been
viewed as inhibiting factor for convection,
as it tends to reduce intensity of updrafts
• Actually promotes new cell development
by interaction of existing updrafts and
shear
• For synoptically-evident, major outbreaks
favorable wind shear widespread
• But wind shear parameters vary
substantially in synoptic scales
• End of Part 1
Upper Level Jet Streaks
• Jet streams have along-flow variation in
wind speed that cannot be only curvature
effects, called jet streaks
Planetary Boundary Layer
• Defined as the tropospheric layer where
the effects of the surface are important
• PBL not synoptic because time scale of
processes as small as an hour or less
• Still interacts with synoptic-scale systems
Diurnal Variations in PBL
• On sunny days, PBL has inversion that ascends
and weakens
• The erosion of the stable layer is one reason
why DMC usually begins in afternoon
• Movement of well-mixed, dry layer over a cooler
layer can create capping inversion, suppressing
convection
• Capping can promote convection elsewhere by
“storing” parcels with high CAPE
Diurnal Variation in PBL
• Decoupling of surface and atmosphere
creates nocturnal boundary layer wind
maximum, especially on sunny days
• Diurnal cycle in horizontal temperature
gradient, makes for poleward flow
increasing with height at night
• Combo of both makes low-level jet stream
• Diurnal changes in PBL wind profile can
modify potential for severe convection