P REDICTION O F F RONTOGENETICALLY F ORCED P RECIPITATION B ANDS. PETER C. BANACOS NWS / Storm Prediction Center WDTB Winter Weather Workshop IV Boulder, CO ~ 23 July 2003. O UTLINE. Frontogenesis …where it fits in the forecast process kinematics and dynamics of frontogenesis
PETER C. BANACOS
NWS / Storm Prediction Center
WDTB Winter Weather Workshop IV
Boulder, CO ~ 23 July 2003
(S. Petterssen 1936)
(Keyser et al. 1988, 1992)
Examine separate contributions of
horizontal divergence, deformation,
and vorticity to the field of
Flow fields involving deformation acting frontogenetically are prominent in the majority of banded precipitation cases.
Need to consider orientation of isotherms relative to axis of dilatation.
Pure vorticity acts to rotate isotherms, cannot tighten or weaken them.
The kinematic field, and deformation in particular, plays the most prominent role in the 2-D frontogenesis aloft.
Other processes such as diabatic heating and tilting effects may also contribute to frontogenesis.
Flow field dominated by deformation.
Ageostrophic circulation develops as a response to increasing temperature gradient.
TWO CLASSES OF BANDS:
NW of surface cyclone --“wrap around precipitation”
Date: 15 October 2001 (Case #1)
Large-scale deformation field - eastern KS and western MO
18z 15 OCT 01
18z mosaic base reflectivity and surface observations
18z 600mb Frontogenesis
0 hr ETA 12z
6 hr ETA 18z
6hr ETA forecast valid
18z 15 OCT 01
The plane of the cross-section should be taken perpendicular to the mid-level (850-500mb) thermal wind vector or thickness lines.
Nature of environmental wind profile may be conducive to “seeder-feeder” mechanism and rapid precipitation generation / elongation of bands during initial development phase.
Col point aloft
Frontogenesis coincident with col point / straight shear
Training thunderstorms, in gravitationally unstable environment
Two problems for heavy precip:
Moisture starved, and moving fast
Note strong curvature to the shear vector with height. This tends to preclude coherent banding, even in the presence of frontogenesis.
Unlike Case #1, this case shows narrow multiple banded precipitation. Lower stability likely played a role.
Near neutral or unstable lapse rates (with respect to a moist adiabat) implies multiple narrow and intense (maybe 5-10 km or so in width), bands. Resulted in 2-3”/hr snowfall rates on Nov 9, 2000.
As gravitational or symmetric stability decreases, the horizontal scale of the band decreases while the intensity of the band increases. Multiple bands become established in an unstable regime.
An example from Moore and Lambert (1993)
2-D 750mb frontogenesis
21z RUC Forecast valid at 00z
0018Z 22 Oct 02
700mb (thick dashed line)
600mb (thin dashed line)
0018Z 22 Oct 02
Multiple bands exist here in negative EPV regime over Montana.
Great Falls, MT
700-500mb lapse rate: 5.1 C/km
700-500mb lapse rate: 6.7 C/km
850-500mb lapse rate: 3.5 C/km
BOS 13 UTC 1 1/2SM –SN
BOS 14 UTC 1/2 SM SN
BOS 15 UTC 1/2 SM SN SNINCR 1/ 2
BOS 16 UTC 1/2 SM SN SNINCR 1/ 3
BOS 17 UTC 1/2 SM SN SNINCR 2/ 4
BOS 18 UTC 1/4 SM +SN SNINCR 2/ 6
BOS 19 UTC 1/4 SM +SN SNINCR 2/ 8
BOS 20 UTC 1/4 SM +SN SNINCR 2/10
BOS 21 UTC 1/4 SM SN SNINCR 1/10
BOS 22 UTC 1/4 SM -SN
BOS 23 UTC 2 SM –SN
BOS 00 UTC 10 SM