| Unstable Cloud Forms | ||
|
|||||||||
|
Cumuliform clouds form in conditions of atmospheric instability through
the formation of rising thermals. These are envelopes of rising air
from a locally warm surface. The air in contact with the heated surface
forms a bubble of warm air which breaks away and rises through the surrounding
cooler air. The thermals tend to be roughly hemispherical or mushroom-shaped,
as shown above, varying in width of tens to hundreds of meters across.
As the thermal rises and expands, friction between the rising air within the thermal and the relatively still air outside it causes a tumbling movement that appears to be constantly turning the atmosphere inside out. Thermals are normally invisible below the condensation level, where they are known as dry thermals, and are a source of uplift used by glider pilots. Above the condensation level the water vapor condenses into a visible cloud of tiny water droplets. The condensation causes a release of latent heat which further heats the thermal and helps to offset the reduction in buoyancy arising from the mixing with cooler air on the leading edge of the thermal together with the evaporation of droplets (causing cooling) on the margins of the cloud. At the colud-base the thermals are normally rising at speeds of 1 m/s (2 kts), but may reach 5 m/s (10 kts) or more within the cumulus clouds, while within thunderstorms the upcurrents may attain speeds of at least 30 m/s (60 kts). Large cumulus clouds usually contain a number of thermals, which rise one after the other through the main body of the cloud, partly composed of the remnants of former thermals. The newer, more vigorous thermals will break through the top or flanks of the cloud as growing turrets, giving rise to the typical cauliflower-like appearance of the cumulus clouds, until arrested by a high-level stable layer, such as an inversion, which will cause the cloud to spread out laterally beneath it (e.g. the anvil at the top of a thunderstorm cloud). If the air is only unstable for a small depth above the condensation level, small 'fair weather' cumuli result, with no precipitation. When the air is unstable to great heights, the cumulus will grow both vertically and laterally, and may well develop into a cumulonimbus cloud producing precipitation. |
||
1. The straight, dotted lines sloping from the bottom right to top left are DALR lines. 2. The convex curves represented by dashed lines sloping away
from bottom right to top left
3. The straight, thin, dashed lines which slop from bottom left
to top right are humidity mixing
Plotting and Analysis 1. E-E' represents the plot of temperature (ELR) in relation to pressure level (altitude). 2. d-d' represents the plot of dew-point temperatures in relation to pressure level. 3. Using DALR lines, draw a line parallel to these from the surface
air temperature (line D-D');
4. Using the mixing ratio lines, draw a line parallel to these
through the surface dew-point
5. The intersection of these two lines indicates the level at
which the rising parcel of air becomes
6. Using the SALR lines, interpolate an SALR line from the point
of intersection CB, upwards;
Weather Systems
|
||
Stability Definitions - Eta Model
Lifted Index - is calculated by lifting (frontal, orographic,
upper air dynamics, etc.) a parcel of air
dry adiabatically while conserving moisture until it reaches saturation.
At that point the parcel is
lifted moist adiabatically up to 500 mb. The Lifted Index is
the ambient air temperature minus the
lifted parcel temperature at 500 mb. If the parcel is warmer
than the environment (negative L.I.),
it has positive buoyancy, and will tend to continue to rise, favoring
convection. L.I. values less than
-5 C indicate very unstable conditions. A positive L.I. value
indicates negative parcel buoyancy,
and the parcel will tend to sink. This is representative of stable
conditions where convection is
unlikely. Increasingly negative numbers correspond to increasing
instability and likelihood of severe
weather. At times, very high (stable) lifted index values in
cold air are indicative of frozen
precipitation verses rain during warm advection events. The extreme
stability results in cold air
"damming", which restricts the advance of warm air at the surface.
CAPE - Convective Available Potential Energy, a measure
of the cumulative buoyancy of a
parcel as it rises, in units of Joules per Kilogram. CAPE values
larger than 1,000 J/Kg represent
moderate amounts of atmospheric potential energy. Values exceeding
3,000 J/Kg are indicative
of very large amounts of potential energy, and are often associated
with strong/severe weather.
CINH - Convective INHibition, a measure of negative buoyancy
below the layer of positive
buoyancy (if it exists), in Joules per Kilogram. Below the "positive
area" which defines the CAPE,
there can exist some negative area, where the parcel is cooler than
the environment. The
atmosphere in these situations are sometimes referred to as "capped".
In these cases, either lifting
of a parcel through some forcing mechanism, or heating of the lower
atmosphere to eliminate the
negative buoyancy area is need for initiation of convection.
Dynamically, once the parcel gets
through this negative area it is free to rise through the positive
area. Thus, occasionally a sounding
may have more than one negative region, but only the lowest negative
area is considered the
Convective Inhibition. Since CINH is not reported unless some
CAPE is present, the CINH
values are typically fairly low. Any CINH values well up into
the hundreds is considered significant
inhibition.
Forecast Product Development Team (FPDT)
NOAA - 1997