Across the Ohio and middle
Mississippi Valleys and southeastern United States, a number of major precipitation
events (including snowstorms) typically are associated with significant
isentropic lift, entrance regions of jet streaks, frontogenetical
forcing, some degree of instability, and sometimes fronts aloft.
These processes are not separate; they are inter-related in a
non-linear atmosphere. Atmospheric temperature structure also
is crucial. Therefore, we must understand these processes well
in order to produce accurate forecasts.
- Temperature advection, adiabatic
warming/cooling, and diabatic effects can have major influences
on atmospheric temperatures and precipitation type in the cool
season, especially during borderline rain/snow situations.
- Effects of these processes
on temperature: Warm
(cold) air advection
causes a temperature increase
(decrease) at a particular
level or in a layer. Adiabatic
occurs when air rises
in a temperature decrease
(increase). When the
air is stable, then adiabatic cooling due to lift will have a
more substantial effect on temperature than for an unstable atmosphere.
Diabatic effects include diurnal heating/nocturnal cooling, condensation,
evaporative cooling, and melting.
- Diurnal heating/nocturnal
cooling affects temperature
(especially in low levels).
- Condensation produces latent heat release, which
produces warming that can counteract somewhat the effects of
adiabatic cooling from lift. Latent heat release is most noteworthy
cooling occurs as precipitation
falls into relatively dry low levels. The precipitation evaporates
in the drier air which causes cooling in low levels and at the
surface. In borderline rain/snow cases, this can cause frozen
precipitation to remain as such until low-level saturation occurs,
or it could cause liquid precipitation to temporarily change
to frozen or freezing precipitation. Once the low-level air mass
saturates, then evaporative cooling no longer is a factor.
- Melting (of snow to rain aloft or snow on the
ground) causes a small amount of cooling in the atmosphere since
heat from the environment is needed to melt the ice crystals.
If significant melting occurs aloft, then an isothermal layer
at or below 0 deg C could result. A saturated isothermal layer
is important for heavy precipitation production since the layer
will be associated with a larger absolute moisture content than
one in which temperature and mixing ratio decrease with height.
- Advection and vertical motion
often oppose each other. Warm advection usually causes ascent,
which in turn produces adiabatic cooling to at least partially
counteract the warming. However, vertical motion due only to
warm advection likely will not be strong enough to completely
counteract the warming and any latent heat release. Thus, low-level
(e.g., 850 mb) temperatures and thicknesses usually will rise
during warm advection situations. However, occasionally temperatures
and thicknesses may not rise (perhaps even fall) in the face
of warm advection (models can show this). For this to occur,
other forcing mechanisms must be present to produce much stronger
vertical motion (often on a smaller scale), including significant
jet streaks, frontogenesis, and/or CSI/convective instability.
Therefore, models can hint that significant mesoscale processes
may be present to produce strong enough lift and adiabatic cooling
to overwhelm warm advection. This often is a scenario, given
adequate moisture, for heavy precipitation production, such as
was the case during the January 16-17, 1994 snowstorm in Kentucky
in which 1 to 2 feet of snow fell across parts of north-central
Kentucky in less than a 12-hour period.
- Strong adiabatic cooling
could cause precipitation to fall as or change to snow during
the period of maximum lift during borderline rain/snow situations.
- The classic Norwegian Cyclone Model (NCM; Fig. 1), developed
many years ago, involves development of an incipient weather
disturbance along a frontal zone into an mature open wave extratropical
cyclone and then into an occluded system. In this model, steady
precipitation usually occurs along and ahead of a warm front,
with a band of showery precipitation along the cold front.
Fig. 1: Typical frontal structure (open
wave cyclone) and precipitation pattern associated with the Norwegian
Cyclone Model. Precipitation is located along and ahead of the
warm front with a band of showers along the cold front.
- The NCM possesses several
problems, including the fact that it fails to explain detailed
mesoscale frontal and precipitation structures associated with
typical extratropical cyclones. Thus, alternative extratropical
cyclone models have been devised to allow for a more scientific
and correct analysis and evolution of cyclogenesis, fronts, airflow,
and precipitation patterns within winter storms.
- Cold fronts are not all homogeneous,
i.e., they vary in associated temperature contrast, wind shift,
and precipitation regimes. Two basic groups of cold fronts have
fronts (Figs. 2a and 2b) are associated with relatively deep/sharp troughs
aloft, i.e., the upper flow is roughly parallel to the surface
front. In these cases, the surface fronts often possess sharp
temperature changes and wind shifts, and significant vertical
motion. System-relative airflow exhibits a sloping rearward
ascent of warm, moist air which can result in a line of showers
or thunderstorms along the front and extensive post-frontal precipitation.
Fig. 2a (far
left): Typical 500
mb (dashed height lines) and surface low and frontal pattern
associated with a ana-cold front.
Fig. 2b (near left): Common precipitation pattern with ana-cold
fronts. A line of convection may be along the front with post-
frontal precipitation due to "system- relative" front-to-rear
sloped ascent behind the front.
fronts (Figs. 3a and 3b) are not as well-defined as ana fronts, and usually
possess weak-to-moderate temperature gradients and scattered
showery precipitation along the front. Kata fronts occur with
a less amplified flow pattern (i.e., more westerly momentum)
aloft, which results in a system-relative forward sloping
ascent and a band or area of more significant precipitation ahead
of the surface front.
Fig. 3a (far
left): Typical 500 mb (dashed height lines)
and surface low and frontal pattern associated with a kata-cold
3b (near left): Common
frontal, precipitation, and flow (large hatched arrow) regime
associated with a kata-cold front. Precipitation occurs along
a cold front aloft (CFA; line U-U) located ahead of the surface
front with drier air aloft behind the CFA. Precipitation also
occurs along/ahead of the warm front. Scattered showers are possible
along the surface cold front due to low-level moisture and any
- Precipitation patterns associated
with kata fronts have been explained via a split cold front or cold
front aloft (CFA) model.
front aloft (Figs. 3b, 4a, and 4b) is associated with the leading edge
of cold advection aloft (often evident at 700 mb and on satellite
imagery), as well as with frontogenesis and convergence between
warm, moist air ahead of the CFA and cooler, drier air behind
it. This typically produces significant lift and a band or area
of stratiform and/or convective precipitation along and ahead
of the CFA within the surface warm sector. This is one explanation
of why we often see "pre-frontal" squall lines.
Fig. 4a: Vertical cross-section of clouds and precipitation
patterns associated with a CFA (dashed line). Rain and possible
convection occurs along and ahead of the CFA, with scattered
showers and possible thunderstorms behind the CFA and along the
surface cold front.
Fig. 4b: A 3-D view of a CFA and surface cold front/trough
and warm front. An area of rain and possible thunderstorms occurs
along and ahead of the CFA with drier air aloft overtop low-level
moisture behind the CFA. Scattered showers and thunderstorms
are possible along the surface cold front assuming ample instability
- At the same time, in general
a less active surface boundary/cold front exists behind the CFA,
especially in the cool season when only limited instability may
be present. However, in the warm season, some sunshine could
occur between the CFA and surface boundary (as drier air moves
in aloft overtop low-level moisture). The convective instability
may then be released through surface/low-level convergence along
the surface boundary leading to development of a line of strong
convection, especially if additional upper-level dynamics are
- It is very important to monitor
fronts aloft via satellite imagery, the leading edge of 700/500
mb cold advection, and shortwaves (perhaps subtle) in model data.
Sometimes, model data suggest fronts aloft via their relative
humidity pattern, although many times, they indicate too high/widespread
RH and precipitation duration forecasts versus that observed.
- An isentropic process is an adiabatic
process (i.e., no parcel
heat exchange with its environment). For synoptic scale weather
systems, air parcels generally move along constant potential
temperature/theta (i.e., isentropic) surfaces, NOT constant pressure
(isobaric) surfaces (Figs.
5a and 5b). In other words,
air moves in 3 dimensions, not on horizontal pressure surfaces.
Fig. 5a: Example of an isentropic surface in 2-dimensions.
Bold solid (dashed) lines are lines of constant pressure/isobars
(mixing ratio/isohumes) while the bold arrow is wind direction
on the surface. Flow is from higher-to-lower values of pressure
and moisture. Thus, ascent/upward moisture transport is occurring.
Fig. 5b: Same as Fig. 5a except the isentropic surface
is shown in 3 dimensions to more clearly show ascent and upward
- Isentropic analysis allows
the ability to attain quantitative estimates of vertical motion
and coherently track air flow and the 3-dimensional transport
of moisture in space and time (unlike pressure coordinates).
- Vertical motion on an isentropic
surface is determined via pressure advection, which is analogous
to temperature advection on a constant pressure surface (e.g.,
850 mb). In other words, an area of warm air advection at 850
mb likely also is an area of isentropic lift.
ascent and upward moisture transport
(Fig. 5) are present in areas where winds on
the theta surface cross isobars and isohumes (mixing ratio lines)
from higher-to-lower values of pressure (similar to warm advection
on a pressure surface) and mixing ratio.
- For descent, wind flow is
from lower-to-higher pressure, and often from lower-to-higher
values of mixing ratio which produces drying.
- The stronger the winds, the
tighter the pressure gradient (i.e., the steeper the slope of
the isentropic surface), and the more perpendicular the winds
are to the isotherms and isohumes, the stronger the upward motion
and moisture transport will be. This can lead to significant
- Vertical motion values associated
with isentropic lift usually are "synoptic-scale" values,
i.e., on the order of several (perhaps 5-10) cm/s.
- Divergence within entrance
and exit regions of jet streaks (see below) can increase the
flow along isentropic surfaces and isentropic lift.
- Significant diabatic effects,
e.g., latent heat release or diurnal heating/cooling, and isentropic
analysis near the ground are limitations and can make accurate
isentropic analysis difficult.
- A "jet streak"
refers to a portion of the overall jet stream where winds along
the jet core flow are stronger than in other areas along the
jet stream. Entrance and exit regions of jet streaks
are very important in terms of vertical motion, surface pressure
systems, and organized precipitation given sufficient low-level
regions are where air parcels
"exit" out of a jet streak and decelerate downstream
from the jet core (Fig
6). Entrance regions are where parcels "enter" into a jet
streak and accelerate upstream from the jet core (Fig. 6).
Fig. 6a: Idealized example of the entrance and exit
regions of a straight jet streak. Highest winds are within the
streak along the line/arrow labeled "jet." Divergence
(div) usually occurs within the left exit and right entrance
regions, while convergence (conv) normally occurs in the right
exit and left entrance regions.
Fig. 6b: Example of the entrance and exit regions of
a straight jet streak. Dashed lines are lines of equal wind speed
(isotachs); solid lines are height lines along which the total
wind blows. The small arrows denote a component of the ageostrophic
wind due to jet streaks that results in divergence and convergence
in exit and entrance regions.
- Within the exit and entrance
regions of jet streaks, air parcels moving at different speeds
become out of balance with the existing thermal (temperature)
gradient in these regions. Thus, the atmosphere attempts to restore
(thermal wind) balance through vertical motion. The vertical
motion is attained through ageostrophic winds. Thus, vertical
motion is required within entrance and exit regions of jets.
- In general, the more the
variation of the total wind"
within an exit and entrance region (i.e., the faster the winds
are accelerating within an entrance region or decelerating in
an exit region per unit area along the flow; Fig. 6b),
the greater the vertical motion must be to restore thermal wind
balance. The "cross-stream
variation" (i.e., how
quickly wind speeds change in a plane perpendicular to the jet
axis) also is important in promoting vertical motion fields.
- Within jet entrance and exit
regions, the cross-stream
component of the inertial advective part of the ageostrophic
wind (i.e., the small arrows
in Fig. 6b) dictates the amount of divergence/convergence
(and subsequent vertical motion) due to jet streak dynamics. Upper-level divergence (convergence) often is
associated with upward (downward) vertical motion in the atmosphere.
- However, even without the
presence of a jet streak, curvature in the flow (i.e., upper-level
troughs and ridges) results in divergence/convergence (and subsequent
vertical motion) due to the along-stream component of the inertial advective
part of the ageostrophic wind (Fig. 7). The
the curvature (i.e., the
more amplified the jet pattern) and the shorter the wavelength between a trough and ridge axis aloft, the greater the upper-level
divergence pattern will be
due to the along-stream component of the ageostrophic wind.
Fig. 7: Example of an upper-level flow pattern showing
trough and ridge axes. Solid lines are constant height lines.
Arrows are a component of the ageostrophic wind due to troughs
and ridges that results in convergence (con) and divergence (div)
- Thus, upper-level divergence
(of the ageostrophic wind) is caused by 1) jet streak entrance
and exit regions, and 2) curvature and wavelength of the overall
flow (troughs and ridges). It is very important to consider both
these phenomena. It can be advantageous to look at the two components
of the upper-level ageostrophic wind individually to assess which
phenomena is most important to the production of vertical motion.
These two components also explain 1) why 4-cell divergence patterns
associated with straight jet streaks become 2-cell patterns for
curved jets, and 2) why divergence values for curved jet streaks
usually are stronger than that for straight jets (see below).
- For STRAIGHT
jet streaks, a 4-cell
pattern of divergence aloft
and vertical motion usually occurs (Fig. 8a).
Fig. 8a: Four-cell pattern of divergence/vertical motion
associated with a straight jet streak. In the dashed left exit
(i.e, upper right portion of image) and right entrance jet regions
(lower left portion of image), divergence aloft and upward motion
usually occur. Convergence and downward motion usually prevail
in the solid line right exit (lower right) and left entrance
(upper left) regions of a straight jet streak.
Fig. 8b: Two-cell pattern of divergence/upward motion
(dashed; jet exit region, i.e., right half of image) and convergence/downward
motion (solid; jet entrance region, i.e., left half of image)
associated with a cyclonically-curved jet streak. Values often
are greater than those with a straight streak.
Fig. 8c: Same as Fig. 8b except for an anticyclonically-curved
jet streak. Divergence aloft and upward motion occur in the jet
right entrance region (left half of image) with convergence and
descent in the jet right exit region (right half of image).
- Within entrance
regions, a thermally
direct secondary circulation
(Fig. 9a) occurs associated with the ageostrophic
wind. Warm air usually rises within the right entrance (right
rear) region while cold air sinks in the left entrance region.
To complete the circulation, horizontal ageostrophic winds often
flow from warm-to-cold air at upper levels, and from cold-to-warm
air at low levels. The circulation is on the order of approximately
400-600 km in horizontal extent.
- Within exit
regions, a thermally
indirect secondary circulation
(Fig. 9b) occurs. Cold air rises in the left exit
(left front) region and warm air sinks in the right exit region.
The horizontal ageostrophic components include flow from warm-to-cold
air at low levels and from cold-to-warm air at upper levels.
Fig. 9a (far
left): Idealized "box" direct
thermal circulation in the entrance region of jet streaks. Warm
air rises in the right entrance region; cold air sinks in the
left entrance region. Horizontal ageostrophic flow occurs from
colder to warmer air in low levels.
Fig. 9b (near left):
Idealized indirect thermal circulation in the exit region
of jet streaks. The circulation is opposite that shown in Fig.
- However, circulations associated
with jet streaks are not "boxes" as shown in the examples
above. Typically, vertical
components are sloped along isentropic surfaces (Fig. 10). Thus, jet streak circulations and isentropic
surfaces are not independent. In other words, in jet entrance
and exit regions, enhanced upper-level divergence may lead to
enhanced flow and vertical motion along isentropic surfaces.
Fig. 10: Cross-section of an east-west jet streak exit
region. The core of jet is directed into the page so that the
right (left) side of the image is the right (left) exit region.
A more realistic sloped ascent (bold arrow) roughly along isentropic
surfaces (sloped thin solid lines) occurs toward the level of
maximum upper-level divergence.
- For CURVED
jet streaks, the "classic" 4-cell (Fig. 8a)
vertical motion pattern can be more complicated, and usually
becomes a 2-cell vertical motion pattern.
- For a cyclonically-curved jet (Fig.
8b), maximum upper divergence
values and subsequent ascent usually are found along and to the
left of the core of the exit region, with descent along and to
the left of the entrance region.
- For an anticyclonically-curved jet (Fig. 8c),
upper divergence and ascent are strongest along and to the right
of the entrance region, with descent along and to the right of
the exit region.
- Ascent/descent values usually
are greatest for cyclonically-curved jet streaks, second greatest
for anticyclonically-curved jet streaks, and relatively weakest
(but still significant) for straight jets assuming adequate along-stream
variation in the wind (Fig. 8).
- If varying temperature patterns
(isotherms) are superimposed on jet streaks, different thermal
advection pattern will result aloft. This can cause the location
of maximum divergence and convergence to shift slightly with
respect to the jet core.
- Occasionally, the ascending
branches of two separate jet streaks may be coincident over one
location. This merger (coupling) is associated with the ascending
branches of the direct circulation in the entrance region of
one jet and the indirect circulation in the exit region of a
second jet. This interaction maximizes upper-level divergence
and vertical motion.
look closer at entrance
regions of jet streaks, since they are important to significant precipitation
events (including snowstorms) in the Ohio Valley.