Download Powerpoint

Basic Jet Streak Adjustments
& Frontogenesis
MEA 444
January 13, 2005
The Problem of
Scale Interaction

Definitions of Scales in the Atmosphere:
 (i) as deduced from observations
 (ii) as resulting from sizes of
observational networks
 (iii) as defined by theoretical
considerations (e.g., Lagrangian Time
Scales)
Scale-interactive processes







Synoptic forcing of mesoscale weather phenomena
Generation of internal mesoscale instabilities
Interactions of cloud and precipitation processes with
mesoscale dynamics
Influence of orography, boundary layer, and surface
properties on mesoscale weather system development
and evolution
Feedback contributions of mesoscale systems to
larger-scale processes
Energy budgets associated with mesoscale systems
Mechanisms and processes associated with
statosphere-troposphere exchange.

Ageostrophic Motions Near Jets
Straight Jet
Uniform Jet
 

 0

 x

 u

 0

 s


 p  Vgr   1  KVgr f

1  



Vgr   p K 
 v gr 
f
f

Vgr
Frontogenesis Equation
Vorticity Tendency Eqns.
Vorticity Equations in
Cartesian Coordinates
Stretching Deformation

Confluence and diffluence associated with a jet maximum.
Shearing Deformation

Horizontal Shear in the presence of a positive along-front
temperature gradient.
Shearing
Stretching
Tilting
Diabatic Forcing
Katafronts

The schematic diagram presented below is that of a
katafront. The front-relative flow for a katafront is one in
which the ascent core slopes forward, resulting in all the
precipitation being prefrontal.
Anafronts

The schematic diagram presented below is that of an
anafront. The front-relative flow for an anafront is one in
which the ascent core slopes rearwards over the top of the
cold frontal surface, resulting in all the precipitation being
postfrontal.
Cold Fronts Aloft



A Cold Front Aloft (CFA) can be characterized as "the
leading edge of a transition zone above the surface that
separates advancing cold air from warmer air" (Locatelli,
et al. 1995).
Thus, a CFA is a cold frontal zone that is located in the
lower to middle troposphere above the surface, but which
has become split from the surface front.
For example, this can happen when a front passes over a
mountain range due to the drag from the topography on
the surface front, particularly in combination with strong
sensible heating over the elevated terrain, which can
effectively erase the thermal structure of the low-level cold
front.
Cold Fronts Aloft (cont'd.)

A pronounced rainband is usually associated with the CFA, which
in some instances is capable of producing severe weather
hundreds of kilometers ahead of the surface front. This can
create unanticipated consequences for the forecast process, such
as:



CFAs and attendant convective weather have been documented
during Cold Air Damming events over the cooler air east of the
Appalachians, when traditional logic would argue against the
possibility of significant convection over such stably stratified air.
CFAs have been shown to be responsible for generating lines of
sever thunderstorms to the east of the dryline, and to thereafter
support the continual eastward movement of the squall line for
hundreds of miles ahead of the surface dryline.
Evidence is accumulating that some CFAs are highly unbalanced
phenomena. Thus, they can initiate a geostrophic adjustment
process, during which a major gravity-inertia wave event can
transpire.
History of CFAs





The causes of "prefrontal squall lines" have been mysteries
to meteorologists.
Holzman (1936) and Lichtblau (1936): Most significant
winter precipitation events in the Midwest are associated
with CFAs.
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.
Browning (1985): Suggested that many squall lines in the
Midwest might be associated with split CFAs.
Locatelli et al. (1989): Discovered a CFA rainband that
developed in the lee of the Rocky Mountains and moved
eastward off the Atlantic Coast.
Useful Criteria for Labeling
a Cold Front Aloft




Main precipitation band is well ahead of a
surface trough.
Pronounce temperature gradient (cold advection)
in the mid-troposphere associated with the
band.
Cloud band in satellite imagery well ahead of
surface trough.
Forecast vertical velocity field shows strong
upward motion feature at least 200 km ahead of
surface trough.
Useful Criteria for Labeling
a Cold Front Aloft (cont'd.)



Vertical cross section of θe and winds indicates
the presence of a mid-level front (CFA) ahead of
the surface front.
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"
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.
Suggested that a thermally direct vertical circulation
results from



quasi-geostrophic frontogenesis, and
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.
Split Cold Front

When the surface pressure trough
takes the form of a cold front with a
line of maximum θe running from
the surface front to the base of the
front aloft, we have a split cold
front.
Cold Frontal Rainbands





Strong lifting of the leading edge of the cold front
produces a narrow band of heavy rainfall.
The cold-frontal gradient of the surface is very different
from the conventional synoptic conceptual model.
Wider bands of precipitation exist behind the surface
cold front. These can be associated with local
steepening of the cold front.
These wide bands have some similarities to surge
rainbands such as their movement through the system
and the local enhancement of the baroclinic zone
(although wide bands have weaker circulations).
The wide bands are also oriented along the vertical
shear vector.
Process Leading to the Formation
of Cold Frontal Rainbands