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Chapter 6
Cloud Development and Forms
Four mechanisms lift air so that
condensation and cloud formation
can occur:
1. Orographic lifting, the forcing of air above a mountain barrier
2. Frontal lifting, the displacement of one air mass over another
3. Convergence, the horizontal movement of air into
an area at low levels
4. Localized convective lifting due to buoyancy
Orographic lifting
The upward displacement of air that leads to adiabatic
cooling is called orographic uplift (or the orographic effect).
When air approaches a topographic barrier, it can be
lifted upward or deflected around the barrier.
Downwind of a mountain ridge, on its leeward side,
air descends the slope and warms by compression
to create a rain shadow effect, an area of lower precipitation.
Fronts
Fronts are transition zones in which great temperature
differences occur across relatively short distances.
Air flow along frontal boundaries results in the widespread development
of clouds in either of two ways. When cold air advances toward warmer
air (cold front), the denser cold air displaces the lighter warm air ahead
of it (a). When warm air flows toward a wedge of cold air (warm front),
the warm air is forced upward in much the same way that the
orographic effect causes air to rise above a mountain barrier (b).
Horizontal convergence
Pressure differences set the air in motion in the
effect we call wind. When a low-pressure cell
is near the surface, winds in the lower
atmosphere tend to converge on the center of
the low from all directions. Horizontal movement
toward a common location implies an
accumulation of mass called horizontal convergence,
or just convergence for short.
Convection
Convection results from heating the air near the
surface. The result is an updraft which is often strong
enough to form clouds and precipitation.
Static stability
The air’s susceptibility to uplift is called its static stability.
Statically unstable air becomes buoyant when lifted and
continues to rise if given an initial upward push;
statically stable air resists upward displacement and
sinks back to its original level when the lifting mechanism
ceases. Statically neutral air neither rises on its own
following an initial lift nor sinks back to its original level;
it simply comes to rest at the height to which it was displaced.
Static stability
The stability depends on the temperature of the lifted air parcel
compared to its environment.
Statically unstable is warmer than the environment after lifting;
statically stable is colder than the environment; and
statically neutral has the same temperature as the environment.
Static stability
The stability depends on the temperature of the lifted air parcel
compared to its environment.
Statically unstable is warmer than the environment after lifting;
statically stable is colder than the environment; and
statically neutral has the same temperature as the environment.
Adiabat
z
T
Parcel curve
Static stability
The stability depends on the temperature of the lifted air parcel
compared to its environment.
Statically unstable is warmer than the environment after lifting;
statically stable is colder than the environment; and
statically neutral has the same temperature as the environment.
Adiabat
Unstable
environment
z
T
Parcel curve
Static stability
The stability depends on the temperature of the lifted air parcel
compared to its environment.
Statically unstable is warmer than the environment after lifting;
statically stable is colder than the environment; and
statically neutral has the same temperature as the environment.
Adiabat
Stable
environment
Unstable
environment
z
T
Parcel curve
Static stability: unsaturated air
Dry and unsaturated air follows the dry adiabat, and
stability is relative to DALR
DALR
Stable
environment
Unstable
environment
z
T
Parcel curve
Static stability: saturated air
Saturated air follows the saturated adiabat, and
stability is relative to SALR
SALR
Stable
environment
Unstable
environment
z
T
Parcel curve
Absolutely unstable air
When a parcel of unsaturated or saturated air is lifted
and the Environmental Lapse Rate (ELR)
is greater than the dry adiabatic lapse rate (DALR),
the result is absolutely unstable air.
Absolutely stable air
When a parcel of unsaturated or saturated air is lifted
and the Environmental Lapse Rate (ELR) is less than
the saturated adiabatic lapse rate (SALR), the result
is absolutely stable air and the parcel will resist lifting.
Conditionally unstable
When the ELR is between the dry and saturated
adiabatic lapse rates the air is said to be
conditionally unstable, and the tendency for a lifted
parcel to sink or continue rising depends on whether or
not it becomes saturated and how far it is lifted.
The level of free convection is the height to which a
parcel of air must be lifted for it to become buoyant
and to rise on its own.
Assume the ELR is 0.7 °C/100 m and the air is
unsaturated. As a parcel of air is lifted, its temperature
is less than that of the surrounding air,
so it has negative buoyancy.
A parcel starts off unsaturated but cools to the LCL,
where it is cooler than the surrounding air. Further lifting
cools the parcel at the SALR. At the 200-m level, it is
still cooler than the surrounding air, but if taken to 300 m,
it is warmer and buoyant.
Tutorial ”Stability” Ch 6 (ed4)
The ELR can be changed by the advection of air with a different temperature
aloft. In (a), the winds at the surface and the 100 m level bring in air with
temperatures of 10 °C and 9.5 °C, respectively, yielding an ELR of 0.5 °C/100 m.
In (b), the surface winds still bring in air with a temperature of 10 °C.
The wind direction at the 100 m level has shifted to northeasterly,
and the advected air has a temperature of 9.0 °C.
The ELR changes when a new air mass
replaces one that has a different lapse rate.
Location A has a steeper ELR than does B.
As the air mass over Location A moves over B,
it brings to that location the new temperature profile.
Air that is unstable at one level may
be stable aloft. The solid line depicts
a temperature profile that is unstable
in the lowest 500 m but capped by an
inversion. An unsaturated air parcel
displaced upward would cool by the
DALR (dashed line), making it initially
warm and buoyant relative to the
surrounding level. After penetrating
the inversion layer, the rising air is no
longer warmer than the surrounding
air, and lifting is suppressed. The
parcel continues upward due to its
momentum. It cools more rapidly than
the surrounding air and becomes
relatively dense. After stopping, the
air parcel sinks and eventually comes
to rest at some equilibrium level.
Entrainment
An air parcel has no barrier to prevent it from mixing with
its surroundings. As air rises, considerable turbulence is
generated, which causes ambient air to be drawn into the
parcel. This process, called entrainment, is especially
important along the edges of growing clouds. Entrainment
suppresses the growth of clouds because it introduces
unsaturated air into their margins and thus causes some
of the liquid droplets to evaporate.
Inversions
Situations in which the temperature
increases with altitude are called
inversions. Air parcels rising through
inversions encounter ever-warmer
surrounding air and have strong
negative buoyancy. Inversions are
extremely stable and resist vertical
mixing. Radiation inversions result
from cooling of the surface.
Frontal inversions exist at the
transition zone separating warm and
cold air masses. Subsidence
inversions result from sinking air.
Frontal Inversion
Subsidence Inversion
The Basic Cloud Types
High clouds - cirrus, cirrostratus, and cirrocumulus
Middle clouds - altostratus and altocumulus
Low clouds - stratus, stratocumulus, and nimbostratus
Clouds with vertical development - cumulus and cumulonimbus
High clouds are generally above 6000 m (19,000 ft).
The simplest of the high clouds are cirrus,
which are wispy aggregations of ice crystals.
Low clouds have bases below 2000 m. Stratus are layered
clouds that form when extensive areas of stable air are lifted.
Usually the rate of uplift producing a stratus cloud is only a few
tens of centimeters per second, and its water content is low.
Low, layered clouds that yield light precipitation are called nimbostratus.
Seen from below, these clouds look very much like stratus,
except for the presence of precipitation.
Stratocumulus are low, layered clouds with some vertical development.
Their darkness varies when seen from below because their thickness
varies across the cloud. Thicker sections appear dark, and thinner
areas appear as bright spots.
Intensely developed clouds are cumulus congestus. They consist
of multiple towers, and each tower has several cells of uplift.
This gives them a fortress-like appearance with numerous columns
of varying heights. Their strong vertical development implies
that these clouds form in unstable air.
Cumulonimbus are the most violent of all clouds and produce the
most intense thunderstorms. In warm, humid, and unstable air,
they can have bases just a few hundred meters above the surface
and tops extending into the lower stratosphere. A cumulonimbus is
distinguished by the presence of an anvil composed entirely of ice crystals
formed by the high winds of the lower stratosphere that extend the cloud forward.