Split Fronts and Cold Fronts Aloft

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Split Fronts andCold Fronts Aloft

• Steven Koch
• North Carolina State University

COMAP 99 Monday, 9 August 1999
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History

• The causes for prefrontal squall lines have
  been mysteries to meteorologists
• Holzman (1936) and Lichtblau (1936)most
  significant winter precipitation events in the
  Midwest are associated with cold fronts aloft

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• Crawford (1950) no prefrontal instability lines
  of any importance over the southeastern states
  exist without a warm tongue at 850 mb and strong
  cold advection at 700 mb
• Newton (1950) reputed the notion that the
  prefrontal squall line is a result of a cold
  front aloft on the basis of lack of evidence.
  This view received widespread acceptance for the
  next 35 years despite mounting evidence to the
  contrary!

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• Browning (1985) suggested that many squall lines
  in the United Kingdom and the Midwest U.S. might
  be associated with split fronts
• Locatelli et al. (1989) observed a Cold Front
  Aloft (CFA) rainband to develop in the lee of the
  Rocky Mountains and move eastward to the Atlantic
  Coast

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Useful Criteria for Labeling a Feature a
CFA(Hobbs et al. 1990)

• A broad mid-level cloud band in satellite
  imagery found at least 200 km ahead of and nearly
  parallel to surface cold front
• Pronounced dry/wet bulb temperature gradient
  (cold advection) in the mid-troposphere
  associated with the band
• Forecast vertical velocity field shows strong
  upward motion feature along mid-tropospheric
  temperature gradient
• Main precipitation band is well ahead of the
  surface front

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Useful Criteria for Labeling a Feature a
CFA(Hobbs et al. 1990)

• Vertical cross section of ?e and horizontal winds
  indicates pronounced backing in association with
  ?e gradient
• Geostrophic wind along the suspected CFA has a
  concentrated region of vertical and lateral shear
  revealed by the field of absolute momentum
• Zero isodop in the radial velocity display from
  WSR-88D shows mid-level backward S pattern
  above a low-level S

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Absolute Momentum Gradients

• Cold fronts are characterized by
• Hyperbaroclinicity
• Strong static stability
• Large absolute vorticity

ug geostrophic wind along the front
y cross-front direction
Under semi-geostrophic balance, the following
product isolates the presence of a front, since
it is the product of the cross-front temperature
gradient and the absolute geostrophic vorticity
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• Hobbs et al. (1990, 1996) and Locatelli et al.
  (1995)
• The Cold Frontogenesis Aloft (CFA) Model
• Rocky Mountains block eastward progress of cold
  air at low levels and destroy thermal contrast
  due to strong sensible heating. Cold air
  continues to advance at mid levels ahead of
  surface trough.
• A thermally direct vertical circulation results
  from
• quasi-geostrophic frontogenesis
• ageostrophic isallobaric forcing at low levels
  due to the changing pressure gradient caused by
  cold advection aloft
• The midlevel zone of frontogenesis well ahead of
  surface trough is shown to be capable of
  triggering prefrontal squall lines

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Variants on the CFA Theme

• When the surface pressure trough takes the form
  of an occluded warm front, the situation reduces
  to the warm occlusion model of Bjerknes and
  Solberg (1922).

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Variants on the CFA Theme

• When the surface pressure trough takes the form
  of a cold front with a line of maximum ?e running
  from the surface front to the base of the front
  aloftThe split cold front model of Browning and
  Monk (1982).

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• Koch and Moore (1998) Split front triggered
  convective band over cold air damming region in
  southeastern U.S. Event analyzed with mesoscale
  model and WSR-88D suggests the following stepwise
  procedure for real-time detection of split fronts
  and CFAs
• Model fields suggested by Hobbs et al. (1990)
  model cross sections of ageostrophic transverse
  circulation and horizontal winds
• Radial velocity zero isodop signature of
  cold-over-warm advection vertical cross
  sectional analysis of reflectivity and radial
  winds
• VAD and 88D hodograph backing-over-veering
  behavior
• VAD thermal retrievals for quantifying CFA cold
  advection

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Temperature Gradient Equation

• Term 1 Vertical wind shear
• Term 2 Curvature effect
• Term 3 Vertically differentiable curvature
  effect
• Term 4 Vertically differentiable acceleration

Assuming geostrophic wind shear, only Term 1
remains. V??T is then the VAD-derived
temperature advection.
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For more information and for discussion of the 19
Dec 95 split front case with thermal retrievals

• http//courses.ncsu.edu/classes-a/
• mea/mea715_info/CFA_Tutorial.htm

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The End