FRONTOGENESIS
- Frontogenesis is a very important
and common atmospheric process that refers to a change in the
magnitude and orientation of a thermal (temperature) gradient
at a level or in a layer due to horizontal changes in the total wind
(i.e., due to patterns of convergence and divergence). This change
(i.e., frontogenetical forcing) alters thermal wind balance,
which forces a vertical motion response in the atmosphere which
can result in mesoscale bands of enhanced precipitation.
- Frontogenesis = an intensification of a temperature
gradient at the surface or aloft.
Frontolysis = a weakening of the gradient.
- During frontogenesis,
a thermally
direct circulation is produced
which is on a smaller scale (lower end of the meso-alpha scale)
than the direct circulation associated with the entrance region
of a jet streak. In low levels, rising motion usually occurs
near the warm side of the low-level temperature gradient (i.e.,
slightly displaced from the maximum frontogenesis area). The
ascent causes adiabatic cooling. Parcels eventually exhibit weaker
sinking and adiabatic warming well on the cool side of the low-level
thermal gradient.
- Frontogenetical zones often
slope upward toward cold air. Thus, the vertical component of
the circulation will be sloped with height toward cold air as
well (complements isentropic lift). The horizontal components
of the frontogenetical circulation consist of an acceleration
of air parcels from cold-to-warm air in low levels and from warm-to-cold
air at upper levels. This circulation acts to weaken the thermal
gradient that frontogenesis attempts to strengthen.
- The smaller-scale direct
thermal circulation forced by frontogenesis usually enhances
the larger-scale direct circulation in the entrance
region of a jet streak, leading to an intensified and more
focused mesoscale upward motion area (i.e., the two circulations
act synergistically). Vertical motion values from frontogenesis can be
on the order of 10s (roughly 20-40) of cm/s (versus 5-10 cm/s for jet dynamics/isentropic
lift). In other words, significant frontogenesis can double or
triple vertical motion values over a smaller-scale from that
due to isentropic lift/warm advection on the synoptic-scale.
Therefore, identifying frontogenetical areas is crucial, since
it can result in a distinct band of heavy precipitation within
surrounding lighter precipitation.
Q VECTORS/F
VECTORS
- We can qualitatively assess
frontogenesis by looking at surface and upper air charts of wind
and temperature. However, frontogenetical
forcing can be assessed more accurately and quantitatively by
looking at "Q
vectors" or "F (frontogenesis) vectors" (Fig. 1). Q
and F vectors are not "real" winds, but they are used
to describe how the geostrophic (Q vectors) or real wind (F vectors)
changes the isotherm/thickness patterns that can lead to vertical
motion.
Fig. 1: Q vectors (or F vectors) in the natural coordinate system. Q is the total vector; Qn and Qs are those components of Q that are directed perpendicular and parallel to isotherms or thicknesses (solid lines), respectively. When Q points from cold to warm air (as shown at left), geostrophic frontogenesis is implied (see text for more details).
- In theory, Q vectors (which
assume use of the geostrophic wind) are related to vertical motion
via the quasi-geostrophic omega equation. F vectors (which employ
the total/actual wind) are not. The total wind can be broken
into two components, the geostrophic (non-divergent part of the)
wind and the ageostrophic (divergent part of the ) wind. By doing
this, equations of motion and other meteorological equations
can be developed to model flow patterns and various processes
in the atmosphere. For the sake of proper theory, the following
discussion will reference Q vectors, which are readily available in the
NWS's Advanced
Weather Interactive Processing System (AWIPS). However, it
should be noted that F vectors (which can be added to the AWIPS Volume
Browser) seem to correlate very well to locations of banded
precipitation. Q vectors also typically show good overall correlation
to banded precipitation patterns.
- The magnitude and direction
of Q vectors describe how the magnitude and orientation of a
thermal gradient are changing, and whether geostrophic frontogenesis
or frontolysis is occurring. When Q vectors point from cold-to-warm (warm-to-cold) air in the low-and-middle levels of
the atmosphere, geostrophic
frontogenesis (frontolysis)
is taking place (Fig.
1).
- Convergence
of Q vectors often is associated
with forcing
for ascent, while divergence of Q typically is associated with descent.
- Q vectors can be broken down
into two vector components (natural coordinates): Qs and Qn (Fig. 1). Qs is the rotational component of Q,
and is directed parallel to isotherms/thicknesses. Qn is the
frontogenetical component of Q, and is directed normal/perpendicular
to the temperature lines.
- Qs vectors basically describe
temperature advection patterns, and force vertical motion on
the synoptic scale. Qs describes how the orientation of the isotherms/thicknesses
is changing with time due to horizontal changes in the geostrophic
wind. Qs vectors are most pronounced in areas where the wind
is tending to rotate isotherms significantly, i.e., in areas
of warm and cold advection. The longer the Qs vectors, the greater
the temperature advection pattern and forcing for synoptic scale
vertical motion. Typically, ahead of (behind) a storm system, Qs convergence (divergence) will be present, which represents forcing
for ascent
(descent) associated
with warm
(cold) air advection
(Fig. 2). Forcing fields associated with Qs often
are relatively broad and weaker than that for Qn.
Fig. 2: Example of Qs vectors, Qs divergence (solid lines), and Qs convergence (dashed lines) in the 850-700 mb layer. Convergence is shown over the Ohio Valley where low-level warm advection (not shown) was occurring. Thus, this is an area of forcing for synoptic-scale ascent. Divergence over the Plains was coincident with cold advection, i.e., forcing for descent.
- Qn vectors can be very important
(the dominant term), and forces vertical motion on the mesoscale/frontal
scale. Qn describes how the magnitude of the thermal gradient
is changing, i.e., the gradient is becoming stronger (frontogenesis)
via confluence or weaker (frontolysis) via difluence. Qn vectors
are longest where the thermal gradient is changing the most,
not necessarily where the tightest thermal gradient exists. Convergence of Qn vectors represents forcing for ascent; divergence
of Qn forces descent (Fig. 3). Typically,
Qn convergence fields (especially from 1000-700 mb) usually are
linearly aligned and often associated with enhanced banded precipitation
given sufficient moisture.
Fig. 3: Example of Qn vectors, Qn divergence (solid lines), and Qn convergence (dashed lines) in the 850-700 mb layer. Qn pointed from cold to warm air indicating a frontogenetical situation. Strong convergence over Kentucky and Tennessee was forcing for significant frontogenetical forcing and ascent. This example is from the January 17, 1994 snowstorm over Kentucky. Heavy snow was occurring at this time over central Kentucky.
- Identifying frontogenetical areas, especially in the cool season, is very important, because these are areas where significant upward vertical motion and banded precipitation likely will be located, given adequate moisture and saturation. Qn vectors can be used to assess frontogenesis. The longer the Qn vectors, the greater is the geostrophic frontogenesis (i.e., Qn vectors are longest where the temperature gradient is strengthening the most), and the more the vertical motion response must be. Ascent in low-levels is near the warm side of the frontogenesis maximum with the frontogenetical zone and area of ascent usually tilted toward cold air aloft (Fig. 4).
- Qn vectors should be evaluated
in the 1000-850 mb layer when boundary layer convergence/forcing
is occurring or expected (e.g., along/near surface/low-level
cold fronts, or possibly during convective situations when boundary
layer forcing is vital). In isentropic lift/overrunning situations,
850-700 mb layer Qn vectors (e.g., Figs 2 and 3)
should be evaluated (e.g., during winter situations and/or along/ahead
of warm fronts). Qn vectors from 700-500 mb also can be important
in isentropic lift situations. Qn vectors from 500-300 mb may
be reflective of upper-level frontogenesis associated with shortwaves,
jet streaks, or possibly even stratospheric intrusions of high
potential vorticity air.
- While observed and gridded
model data fields are very useful, they cannot fully diagnose
frontogenesis in the real atmosphere. However, data can provide
valuable information concerning frontogenetical trends and locations,
from which additional processes (e.g., jet streaks, isentropic
lift, diabatic effects, moisture) can be evaluated to construct
a reasonable precipitation forecast.
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Fig. 4: Frontogenesis produces a mesoscale direct thermal circulation that is sloped with height toward cold air. Q/F vectors are longest where frontogenesis is the greatest. On the periphery of this area, Q/F vectors are shorter given weaker frontogenesis. Thus, Q/F vector convergence (forcing for lift) occurs on the southern/eastern periphery of the maximum frontogenesis area (as shown at left). A steeply sloped frontogenetical zone in low-to-middle levels can produce a definitive band of heavy precipitation (rain or snow) superimposed on broader, lighter precipitation in cool season. In the warm season, low-level frontogenesis also can force the lift needed to promote deep convective development and subsequent heavy rainfall. |
CONDITIONAL SYMMETRIC
INSTABILITY (CSI)
- CSI is a type of moist symmetric
instability (MSI). When adequate moisture and lift is present
to release this instability, moist slantwise convection will
result which apparently produces slanted mesoscale circulations
of saturated air parcels. CSI results from the combined effect
of vertical (gravitational) and horizontal (inertial) forces.
When CSI exists, the atmosphere often is weakly stable to both
vertical (upright convection) and horizontal (inertial instability)
displacements, but potentially unstable to slanted movement.
In other words, a parcel displaced vertically or horizontally
eventually would come back to its original position (be stable
with respect to the ambient environment), although a parcel displaced
slantwise would result in a tilted upward acceleration (be unstable
compared to the environment).
- CSI/MSI usually is most common
in the presence of frontogenesis and near anticyclonically curved
entrance regions of jet streaks, especially when there is significant
"along-stream variation" in the flow.
- Several qualitative conditions
must be met to produce CSI,
including 1) near saturated atmosphere, 2) near neutral stability (temperature curve in a
sounding is nearly parallel to the moist adiabats), 3) strong vertical speed shear (baroclinic environment),
and 4) large scale forcing to produce upward
parcel displacements.
- When CSI is present, it can
be "sandwiched" between a convectively unstable (CU)
area to the south and weak symmetric stability (WSS) to the north.
In other words, a "spectrum of potential instabilities"
exists from south to north (Fig. 5). In
areas of CU, equivalent potential temperature (theta-e) lines
fold back on themselves, i.e., theta-e decreases with height.
Farther north, in the CSI area, there is no convective instability
and little or no CAPE but near neutral stability exists (i.e.,
theta-e lines are nearly vertical). Finally, north of the CSI,
WSS would exist, where the air is a little more (but not too)
stable, i.e., theta-e lines still show good slope but are not
vertical.
Fig.5: Sample cross-section from south (right side) to north (left side of diagram) of momentum (M; solid lines) and theta-e (dashed lines) surfaces. Convective instability (CU), CSI, and weak symmetric stability (WSS) are indicated depending on the slope of the M versus theta-e surfaces (see text).
- Meteorologists typically
use spatial-height cross-sections of absolute momentum ("M" in m/s) and equivalent potential temperature (theta-e) to assess CU, CSI, and WSS (Fig. 5). In reality, this method may be a test
for potential symmetric instability (PSI), although CSI will
still be used here due to their similarity. Cross-sections of
observed or model forecast data should be made roughly perpendicular
to the 1000-500 (or 850-300) mb thickness lines. CSI apparently
is present in saturated areas where the slope of the theta-e
lines is steeper than that of the M surfaces (i.e., theta-e lines
are more vertical than M lines; Fig. 5).
In other words, near neutral stability and significant vertical
speed shear exist.
- CSI results
in mesoscale circulations (rolls) superimposed on the synoptic
forcing field. CSI seems to cause a scale reduction in enhanced
vertical motion from that associated with frontogenesis. In addition,
CSI-related
ascent values may be on the order of several m/s (versus cm/s for synoptic scale/general
isentropic ascent). Therefore, charge separation is possible
resulting in elevated slanted convection with thunder and lightning
(although it is more likely with elevated convective instability).
At the very least, the release of CSI usually results in the
development of multiple
small-scale bands of heavy precipitation within general areas of winter storm (cool season)
precipitation, producing zones of higher rainfall or snowfall
amounts. Lighter precipitation between heavier bands may be associated
with the weak downward component of the CSI-induced circulations.
The enhanced precipitation bands normally are 50-250 miles in
length and oriented parallel to the thickness field. While released
CSI and frontogenetical forcing focus and enhance lift to produce
banded precipitation, it is quite difficult to determine the
exact role or relative importance of these processes in these
situations.
- South of the CSI area, CU can result in upright convection/thunderstorms
due to the release of gravitational instability; upward vertical motions
can be on the order of 10-20 m/s.
This unstable air may feed northward due to the low-level jet.
North of the CSI area, WSS exists. However, in the presence of frontogenesis, updrafts
in the WSS area can become concentrated with downdrafts weak and diffuse. Thus, banded
precipitation is possible in WSS areas, although thunderstorms
are not likely.
- More information concerning CSI can be found on the CSI Home Page.
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