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Talking Points
Title slide
Explanation of CSTAR project
Topics to be covered
Motivation: Forecast details matter to the public
Motivation: IFPS allows for band depiction; modern datasets allow for
comprehensive study
Training objectives
Study definitions
Study methodology overview
Definition of a single band
Climatology topic slide
Geographical band distribution
Note events are generally scattered across the study domain
Cyclone-relative distribution
Note predominance of single-banded events in the northwest quadrant
relative to the surface cyclone center
Single-banded event duration
Most events are of short duration (<6 h), but a few persist for > 14 h
Forcing mechanisms topic slide
Forcing Scales
Observed phenomena occur on different scales and therefore are
diagnosed through different diagnostics.
QG thinking diagnoses features on the cyclone scale
Frontogenesis diagnoses features associated with a frontal zone
CSI, gravity wave diagnostics explain individual cells
We will focus on the frontal environment in this work – therefore
frontogenesis is our focus
Frontogenesis definition
Frontogenesis can result from deformation, differential vertical motion,
and differential diabatic heating. The first order mechanism is
deformation.
Definition of deformation
Example of how a deformation flow can contribute to frontogenesis
Given this midlevel flow and temperature gradient where would you
expect frontogenesis?
Answer: in the diffluent flow ahead of the cut-off low at the edge of the
warm air advection
Frontogenesis is important because of the ageostrophic response it
initiates. Frontogenesis diagnoses a region where the geostrophic flow is
taking the atmosphere out of thermal wind balance. The ageostrophic
response in the form or a direct circulation restores thermal wind balance
through ascent and associated adiabatic cooling on the warm side of the
front, and descent and associated adiabatic heating on the cold side of the
front.
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Figure from Emanuel (1985) which shows that the weaker the stability
the stronger and more focused the ascending branch of the direct
circulation. Note that instability is not needed to established focused
strong ascent (resulting in a band)
Synoptic context topic slide
Composite methodology
Explanation of composite datasets
Note that we are resolving the environment in which bands form, not the
individual bands.
Series of composite summary maps of the northwest composite from 12
hours before band initiation (T = -12 ) to 12 hours after band initiation (T
= +12). Note the composite cyclone is undergoing cyclogenesis and
develops a deformation zone northwest of the cyclone center as the
midlevel circulation becomes established at the time of band initiation.
Frontogenesis is maximized at this time and correlates well with the
composite band location
Series of composite summary maps of the east composite from 12 hours
before band initiation (T = -12 ) to 12 hours after band initiation (T =
+12). Note the composite cyclone is undergoing cyclogenesis. Although
a closed midlevel circulation forms, it is not associated with a
deformation zone, rather deformation is found in diffluent flow ahead of
the cyclone. Frontogenesis is maximized within this deformation zone
and is coincident with the composite band location.
Series of composite summary maps of the nonbanded composite from 12
hours before band initiation (T = -12 ) to 12 hours after band initiation (T
= +12). Note the composite cyclone does not exhibit cyclogenesis as it is
located in the confluent entrance region of a jet. Accordingly the cyclone
does not form a midlevel closed circulation. Some deformation and
associated frontogenesis is found in confluent flow ahead of the cyclone,
however no band was observed.
Graph comparing composite surface cyclone development
The banded composite cyclone exhibits deepening in the 12 h preceding
band initiation while the nonbanded composite cyclone does not
Comparison of cross sections taken through the midlevel frontogenesis
maxima at T = 0.
Both banded composite cross sections are marked by a deep layer of
frontogenesis and weak conditional stability. The ascent field exhibits a
narrow, strong updraft. The nonbanded composite cross section exhibits
weaker frontogenesis and stronger conditional stability. The ascent field
in this environment is broad and weak.
Conceptual models of banded and nonbanded systems
Note the location and magnitude of frontogenesis
Conceptual models of cross-sectional frontal environments of banded and
nonbanded systems
Note the depth and magnitude of frontogenesis as well as the conditional
stability differences
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Nicosia and Grumm (1999) conceptual model
Note the common location of the midlevel frontogenesis maxima
northwest of the cyclone and presence of weak conditional stability
Moore et al. (2002) conceptual model of banded case in central United
States
Note the common location of the frontogenesis maxima northwest of the
surface cyclone
Also note that the trowal airstream contributes to deformation northwest
of the surface cyclone as part of the midlevel flow turns cyclonic (trowal
airstream) and part turns anticyclonically (warm conveyer belt).
Forecast strategy topic slide
Forecast framework
Downscale approach to forecasting bands both in space and time
Parameters to assess one to two days in advance of anticipated event
Parameters to assess 6-24 h in advance of anticipated event
Parameters to assess 0-6 h in advance of anticipated event
Strategy application topic slide
Application of forecast strategy to the 6-7 January 2002 snowstorm
Series of maps showing a summary of forecast guidance available to the
forecaster at 00 UTC 6 January 2002 with parameters to assess along the
left margin. Are you concerned for potential band development? Why or
why not?
Series of maps showing a summary of forecast guidance available to the
forecaster at 12 UTC 6 January 2002 with parameters to assess along the
left margin. Are you concerned for potential band development? Why or
why not?
Series of maps showing a summary of forecast guidance available to the
forecaster at 18 UTC 6 January 2002 with parameters to assess along the
left margin. Are you concerned for potential band development? What
action may you take
Cross sections through forecast frontogenesis maxima from the Eta and
RUC models valid at 00 UTC 7 January 2002. Do you expect band
development
Series of maps showing a summary of short-term model guidance
available to the forecaster at 21 UTC 6 January 2002 with parameters to
assess along the left margin. Where do you expect band formation to
occur and where will it move? What action do you take?
Series of maps and radar images showing the observed evolution of the
system. How well did you forecast?
Snowfall map from the 6-7 January 2002 snowstorm
Conclusion and CSTAR homepage URL
Additional resources and acknowledgements