Split Fronts and Cold Fronts Aloft Steven Koch North Carolina State University COMAP 99 Monday, 9 August 1999

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

 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 35 years  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!

 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

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

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”

Absolute Momentum Gradients Cold fronts are characterized by: Hyperbaroclinicity Strong static stability Large absolute vorticity u g =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:

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

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).

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 aloft: The split cold front model of Browning and Monk (1982).

 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

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.

For more information and for discussion of the 19 Dec 95 split front case with thermal retrievals: mea/mea715_info/CFA_Tutorial.htm

The End