The temporal behavior of numerically simulated multicell-type storms.
Part II: The convective cell life cycle and cell regeneration
Robert G. Fovell and Pei-Hua Tan
Monthly Weather
Review
March, 1998 (vol 126, p. 551)
These pages present a brief overview of the paper that can be understood (hopefully!) by an individual with a basic background in meteorology. Only a portion of the paper's content is covered below. Please direct comments, questions, and/or reprint requests to Robert Fovell.
Here is a replacement for Figure 9, which did not print well in PDF format.
Links to sections on this page:
Links to sections on the next page:Multicell storms are organized clusters of individual short-lived convective cells. Here, a cell is a localized region of intensified convective activity (vertical motion, vapor condensation, etc.), such as an individual cumulus cloud. The average cell undergoes a life cycle, spanning initiation, maturation, and decay, that typically spans an hour or less, though the parent storm can survive for many hours. The longevity of the multicell storm is largely due to its ability to continually generate new, replacement cells as older ones decay.
First, however, we examine the broad circulation upon which the convective cells are embedded. In the image below, the storm is propagating eastward (to the right), so the east side will be called the storm's forward side. In this case, storm movement is due to the weight of its dense cold air pool. The cold pool's leading edge is called the gust front.
As the cold pool spreads, it collides with air that is warmer, more moist and less dense, and forces that air to rise up and over the cold pool. As the less dense air rises in the storm updraft, it saturates, forming the cloud. Precipitation particles in the cloud fall into the subsaturated subcloud region, whereupon some of the particles evaporate, cooling the surrounding air. This is what formed, and serves to maintain, the cold pool. The cold pool is fed by dry air originating in the middle troposphere; this helps keep the evaporation cooling going.
The convective cells reside in this general storm updraft. The picture below is from a two-dimensional (2D) simulation made with a numerical cloud model. Cloud models attempt to resolve individual convective clouds (cells) but do not try to capture these clouds' constituent cloud droplets or raindrops. The contoured field is vertical velocity (solid contours for upward motion; dashed for downward) and the colored field is perturbation potential temperature, computed relative to the model's initial condition. Areas that have been warmed are marked in red, while the blue areas point to locations that have experienced cooling. The cloud outline is superimposed, marked by the thick black line.
A vertical cross-section reveals a "family" of cells within (and essentially comprising) the updraft, each in a different stage of the cell life cycle. At this time, there is a strong updraft located above x=32 km, representing a convective cell that is just reaching its mature stage. Note the air at the top of the cell has been warmed. This has been caused by the release of latent heat when vapor condenses, and this heating fuels the storm.
The maturing cell is just starting to produce heavy precipitation at this time. Above x=24 is the remnants of a decaying cell. Several minutes earlier, this cell had been strong and the dominant updraft in the storm. Just ahead of the gust front is a shallow, new cloud that will evolve into the next strong cell within this storm. Also to be noted is the updraft located at the gust front, formed as a result of the collision between the cold and less dense air. The cold pool and its forced updraft are crucial to this storm's survival, because that is how moist inflow is lifted into the storm.
The figure linked here shows a time sequence of fields spanning a period of about 9 min, showing how a cell moves within the storm as it evolves. The simulation is nearly periodic, and so you can "loop" the panels to track a particular cell. The subsequent weakening and disorganization of the mature cell, and explosive growth of the new cell, are well captured in the sequence.
Note the new cell grows out of the low-level updraft at the gust front, and is subsequently swept toward's the storms rear (at left) as it ages. As it moves, the intensifying cell becomes cut off from the gust front's forced updraft. Once cut off, the cell's eventual demise is ensured, and storm survival requires the establishment of yet another new cell updraft by the cold pool's forced updraft. There's a link to a movie loop of this phenomenon in the next section.
New cell generation and episodic dilution
A somewhat different view is afforded by the picture below. Now the
vertical velocity and cloud outline fields are superimposed on a colored
field of equivalent potential temperature
(qe).
This property is nearly
conserved apart from mixing, and thus represents a good tracer, showing how
air is moved around within the storm.
In an undisturbed atmosphere,
qe tends to be high both near the ground and
in the upper troposphere, with a distinct minimum in between (see x > 45
km in figure below). Further to the left, it is seen that
the storm lifts high qe
air from near the surface even as it causes low qe
air from midlevels to descend. That is, the storm is mixing up the lower to
middle troposphere; that's its job. The high qe air is lifted
upon collision with the storm's gust front. The low qe
air descended in the storm's downdrafts. In saturated air, a vertical decrease of
qe with
height indicates convectively unstable air, so by putting high qe air over low qe air,
the storm is removing the atmosphere's instability.
As the high qe air is lifted over the gust front, it is subjected to
mixing with the lower qe air located both in the cold pool and in the
middle troposphere east of the storm. Only a small part of the inflow
remains undiluted.
Note that within the mature cell located above x=32 is a small pocket of
undiluted, high qe
air. This is the same location where the largest degree of warming was
found at this same time (see above). Unsurprisingly, the greatest amount
of latent heating is generated by the richest, least diluted air.
The figure linked here is an animated GIF image
showing a time sequence of vertical velocity and qe fields.
(It should loop continuously; hit the
back button to return here after viewing.) Again, the development
of a new cell from the gust front updraft and its subsequent propagation
towards the storm's rear is obvious. Note that as a new cell begins to
form, a "ribbon" of high qe air rise upward into
the storm. Soon, however, low qe air finds its
way into the ribbon, and the growing cell is cut off at this point,
leaving only an isolated patch of undiluted, high qe air at the cell updraft's center.
Tracer analysis (discussed in the paper)
demonstrates that the low qe air originates in
both the cold pool as well as in the middle troposphere on the storm's
forward side. This process operates in each newly established cell, and
can be described as an episodic dilution.
Later, this episodic dilution is interpreted as being the
inevitable consequence of the cell's own induced local circulation.
Convective cell motion through the storm
The figure below shows the vertical velocity, perturbation potential
temperature and storm-relative vector
wind fields at an instant of time during the storm's mature phase.
In the upper panel several updrafts, each in a different stage of the cell
life cycle, may again be seen. Feature 1 is the gust front forced lifting, which
is in the process of growing vertically and is ready to spawn a new cell.
Feature 2 is a mature cell near the time of its maximum intensity.
Features 3, 4 and 5, farthest to the rear, are the remnants of earlier
cells. The latter two are convectively inactive.
Note the warmest air at this time resides on the rear side of
Feature 2, the mature cell. The vector airflow field shows that the cells
represent substantial perturbations upon the generally rising updraft
airflow (though the vertical scale has been exaggerated somewhat).
Again, these features may be thought of as the different stages in the life
cycle of a single convective cell. How and why the cell updraft moves depends
upon the stage the cell is in. Note the cells are embedded in air that is
generally moving from west to east (i.e., towards the storm's rear).
Thus, part of the reason why the cells move relative to the storm is simply
horizontal advection. However, closer inspection shows that the mature
cell (feature 2) actually moves rearward more slowly than would be expected
by advection. In contrast, convectively inactive updrafts 4 and 5 are
moving rearward much faster than expected by advection alone.
Features 4 and 5 bear the characteristics of stable, westward (rearward)
propagating gravity
waves. Such waves move relative to their embedding media. At the
time shown, these updrafts are propagating rearward at about 50 m/s, much
faster than the flow (0-15 m/s) at those locations. This is the fate of a
convective cell, reached after its instability is exhausted.
In contrast, convectively active cell 2 is moving rearward at roughly half
the speed of the embedding flow [1]. The figures below help explain why. The
local vertical velocity acceleration (LWDT) indicates how the vertical
velocity field is changing at a particular grid point. (Example: if the
motion is upward, but LWDT is negative, the upward motion is slowing at
that grid point and at that time.) LWDT is the combination of advection
(WADV) and parcel vertical acceleration (PWDT). A large
part of WADV comes from the rearward-directed airflow "pushing" the
updraft rearward. Therefore, we would expect to see positive values of
WADV to exist on the rearward-facing (west) side of all of the updrafts,
which is what is seen in the top panel below. (The shaded area shows where
WADV is positive.)
PWDT, on the other hand, shows the sense of instantaneous
parcel acceleration. It is itself the
combination of the vertical pressure acceleration and the buoyancy
acceleration. The pressure and buoyancy accelerations are typically (but
not always) in
opposition, a kind of tug-of-war. If the buoyancy force is directed upward and is stronger than
the (assumed opposing) pressure force, then a parcel at that location will
experience a net upward push, and PWDT will be positive.
In gravity wave features 4 and 5, both WADV and PWDT operate
in the same sense; they are both trying to drive the updraft rearward.
Thus, the features move faster than pure advection would be able to
accomplish. In the mature cell, feature 2, however, WADV and PWDT are in
opposition. WADV is trying to bring the cell rearward, but PWDT shows that
the updraft is dissipating on its west side and growing on its east side
as it moves. The net result -- given by LWDT -- is still rearward movement in this case,
but at a slower rate than expected by advection alone. This is seen in
the LWDT field, the top panel of the figure below; all of the updrafts are
tending rearward (westward).
The bottom panel above shows the TVPT field, the combination of
adiabatic (expansion and compression) and diabatic (condensation warming and
evaporative cooling) effects. Rising air
experiences adiabatic expansion cooling. If this is the only active
process,
then upward motion would be colocated with negative values of TVPT (with
positive values in the downdrafts). This occurs in gravity wave features 4
and 5 because they are convectively inactive -- no diabatic effects are
present. In the mature cell, feature 2, TVPT is positive on the east side
of the updraft. There, diabatic heating from condensation production
exceeds adiabatic expansion cooling. This is driving parcels upward
there (i.e., they are positively buoyant). On feature 2's west side,
however, TVPT is negative. There is some condensation generation there
(not shown, see the paper's Fig. 8), but it is insufficient to counter
expansion cooling. The cell is dissipating (regenerating) on its west
(east) side, opposing the advective tendency, because diabatic effects
dominate on the east side while adiabatic effects are larger on the west
side.
The net result of all this is that, following establishment, updrafts move
more slowly than the flow. However, as the updrafts age and become
convectively inactive, they accelerate substantially, even though the
airflow figure shows the embedding flow has slackened in strength.
[1] These speeds were computed by tracking the updraft center.
In the case of feature 2, the shape of the updraft is changing with time;
this confuses the issue a little.
Page created May, 1998, by
Augmented October, 1998.
Robert Fovell
