MAMATUS CLOUD

CHARECTARISTIC

Mammatus are most often associated with the anvil
cloud and also severe thunderstorms.

They often
extend from the base of a cumulonimbus , but may
also be found under altocumulus , altostratus ,
stratocumulus, and cirrus clouds, as well as volcanic
ash clouds.

[ citation needed ] In the United States, sky
gazers may be most familiar with the very distinct and
more common cumulonimbus mammatus.

When
occurring in cumulonimbus, mammatus are often
indicative of a particularly strong storm or maybe even
a tornadic storm.

Due to the intensely sheared
environment in which mammatus form, aviators are
strongly cautioned to avoid cumulonimbus with
mammatus.

They also attach to the bottom of other
clouds.

Mammatus may appear as smooth, ragged or lumpy
lobes and may be opaque or translucent.

Because
mammatus occur as a grouping of lobes, the way they
clump together can vary from an isolated cluster to a
field of mammae that spread over hundreds of
kilometers to being organized along a line, and may be
composed of unequal or similarly-sized lobes.

The
individual mammatus lobe average diameters of 1–
3 km and lengths on average of 0.5 km.

A lobe can last
an average of 10 minutes, but a whole cluster of
mamma can range from 15 minutes to a few hours.

They are usually composed of ice, but also can be a
mixture of ice and liquid water or be composed of
almost entirely liquid water.

True to their ominous appearance, mammatus clouds
are often harbingers of a coming storm or other
extreme weather system.

Typically composed primarily
of ice, they can extend for hundreds of miles in each
direction and individual formations can remain visibly
static for ten to fifteen minutes at a time.

While they
may appear foreboding they are merely the
messengers - appearing around, before or even after
severe weather.

Hypothesized formation
mechanisms
The existence of many different types of mammatus
clouds, each with distinct properties and occurring in
distinct environments, has given rise to multiple
hypothesized formation mechanisms [citation needed ],
which are also relevant to other cloud forms.

One environmental trend is shared by all of the
formation mechanisms hypothesized for mammatus
clouds: sharp gradients in temperature, moisture and
momentum (wind shear ) across the anvil cloud/sub-
cloud air boundary, which strongly influence
interactions therein.

The following are the proposed
mechanisms, each described with its shortcomings:
The anvil of a cumulonimbus cloud gradually
subsides as it spreads out from its source cloud.

As air
descends, it warms.

However, the cloudy air will warm
more slowly (at the moist adiabatic lapse rate ) than the
sub-cloud, dry air (at the dry adiabatic lapse rate ).

Because of the differential warming, the cloud/sub-
cloud layer destabilizes and convective overturning can
occur, creating a lumpy cloud-base.

The problems with
this theory are that there are observations of
mammatus lobes that do not support the presence of
strong subsidence in the lobes, and that it is difficult
to separate the processes of hydrometeor fallout and
cloud-base subsidence, thus rendering it unclear as to
whether either process is occurring.

Cooling due to hydrometeor fallout is a second
proposed formation mechanism.

As hydrometeors fall
into the dry sub-cloud air, the air containing the
precipitation cools due to evaporation or sublimation.

Being now cooler than the environmental air and
unstable, they descend until in static equilibrium, at
which point a restoring force curves the edges of the
fallout back up, creating the lobed appearance.

One
problem with this theory is that observations show
that cloud-base evaporation does not always produce
mammatus.

This mechanism could be responsible for
the earliest stage of development, but other processes
(namely process 1, above) may come into play as the
lobes are formed and mature.

There may also be destabilization at cloud base due
to melting.

If the cloud base exists near the freezing
line, then the cooling in the immediate air caused by
melting can lead to convective overturning, just as in
the processes above. However, this strict temperature
environment is not always present.

The above processes specifically relied on the
destabilization of the sub-cloud layer due to adiabatic
or latent heating effects.

Discounting the
thermodynamical effects of hydrometeor fallout,
another mechanism proposes that dynamics of the
fallout alone are enough to create the lobes.

Inhomogeneities in the masses of the hydrometeors
along the cloud-base may cause inhomogeneous
descent along the base.

Frictional drag and associated
eddy-like structures create the lobed appearance of the
fallout.

The main shortcoming of this theory is that
vertical velocities in the lobes have been observed to
be greater than the fall speeds of the hydrometeors
within them; thus, there should be a dynamical
downward forcing, as well.

Another method, that was first proposed by Kerry
Emanuel , is called cloud-base detrainment instability
(CDI), which acts very much like convective cloud-top
entrainment .

In CDI, cloudy air is mixed into the dry
sub-cloud air rather than precipitating into it. The
cloudy layer destabilizes due to evaporative cooling
and mammatus are formed.

Clouds undergo thermal reorganization due to
radiative effects as they evolve.

There are a couple of
ideas as to how radiation can cause mammatus to
form.

One is that, because clouds radiatively cool
( Stefan-Boltzmann law) very efficiently at their tops,
entire pockets of cool, negatively buoyant cloud can
penetrate downward through the entire layer and
emerge as mammatus at cloud-base. Another idea is
that as the cloud-base warms due to radiative heating
from land surface's longwave emission, the base
destabilizes and overturns. This method is only valid
for optically thick clouds.

However, the nature of anvil
clouds is that they are largely made up of ice, and are
therefore relatively optically thin.
Gravity waves are proposed to be the formation
mechanism of linearly organized mammatus clouds.

Indeed, wave patterns have been observed in the
mammatus environment, but this is mostly due to
gravity wave creation as a response to a convective
updraft impinging upon the tropopause and spreading
out in wave form over the entirety of the anvil.

Therefore, this method does not explain the prevalence
of mammatus clouds in one part of the anvil versus
another.

Furthermore, time and size scales for gravity
waves and mammatus do not match up entirely.

Gravity wave trains may be responsible for organizing
the mammatus rather than forming them.

Kelvin–Helmholtz (K-H) instability is prevalent along
cloud boundaries and results in the formation of wave-
like protrusions (called Kelvin-Helmholtz billows) from
a cloud boundary. Mammatus are not in the form of K-
H billows, thus, it is proposed that the instability can
trigger the formation of the protrusions, but that
another process must form the protrusions into lobes.

Still, the main downfall with this theory is that K-H
instability occurs in a stably stratified environment,
and the mammatus environment is usually at least
somewhat turbulent .

Rayleigh–Taylor instability is the name given to the
instability that exists between two fluids of differing
densities, when the denser of the two is atop the less
dense fluid.

Along a cloud-base/sub-cloud interface,
the denser, hydrometeor-laden air could cause mixing
with the less-dense sub-cloud air.

This mixing would
take the form of mammatus clouds.

The physical
problem with this proposed method is that an
instability that exists along a static interface cannot
necessarily be applied to the interface between two
sheared atmospheric flows.

The last proposed formation mechanism is it arises
from Rayleigh-Bénard convection, where differential
heating (cooling at the top and heating at the bottom)
of a layer causes convective overturning. However, in
this case of mammatus, the base is cooled by
thermodynamical mechanisms mentioned above.

As
the cloud base descends, it happens on the scale of
mammatus lobes, while adjacent to the lobes, there is
a compensating ascent.

This method has not proven
to be observationally sound and is viewed as generally
insubstantial.

This plenitude of proposed formation mechanisms
shows, if nothing else, that the mammatus cloud is
generally poorly understood. Detailed observations of
the cloud have been meager and usually occur only by
chance, since mammatus do not pose a meteorological
threat to society. [citation needed ]

SEE MORE : https://en.m.wikipedia.org/wiki/
Mammatus_cloud