Download The PBL is covered by a layer of warmer air, creating what is known

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Climate Dyn.; J. CLim; Proceeding Roy. Soc. Atmospheric Chemistry and Physics (An Interactive Open
Access Journal of the European Geosciences Union; http://www.atmospheric-chemistry-andphysics.net/home.html)
http://www.britannica.com/EBchecked/topic/1364303/planetary-boundary-layer-PBL
The PBL is covered by a layer of warmer air, creating what is known as a temperature inversion. The
boundary between the cooler PBL below and the warmer layer above can be visually marked by the
base of the clouds in the area.
The wetter the air advected into the region and the greater the additional water added by evaporation
and transpiration, the lower the height of the top of the PBL. For every 1 °C (1.8 °F) increase in daily
maximum surface temperature for a well-mixed PBL, the top of the PBL is elevated 100 metres (about
325 feet). … Over deserts, the PBL may extend up to 4,000 or 5,000 metres (13,100 or 16,400 feet) in
altitude. In contrast, the PBL is less than 1,000 metres (3,300 feet) thick over ocean areas, since little
surface heating takes place there because of the vertical mixing of water.
The cooling of the lifting air initiates the condensation of water vapour and the development of
miniscule particles of liquid water called cloud droplets. The small clouds just above the PBL are known
as planetary boundary layer clouds.
http://www.britannica.com/EBchecked/topic/586707/temperature-inversion
Under normal conditions air temperature usually decreases with height. Temperature inversion, a
reversal of the normal behaviour of temperature in the troposphere (the region of the atmosphere
nearest the Earth’s surface), in which a layer of cool air at the surface is overlain by a layer of warmer
air.
The following subsidence refers to large-scale downdraft from high above the PBL onto the PBL. It is
probably the reason why under the EAWM, the PBL is capped by the “warmer” layer above it, and lowlevel stratus clouds are formed just above the PBL:
A subsidence inversion develops when a widespread layer of air descends. The layer is compressed and
heated by the resulting increase in atmospheric pressure, and as a result the lapse rate of temperature
is reduced. If the air mass sinks low enough, the air at higher altitudes becomes warmer than at lower
altitudes, producing a temperature inversion. Subsidence inversions are common over the northern
continents in winter and over the subtropical oceans; these regions generally have subsiding air because
they are located under large high-pressure centres.
As the offshore wind blows across the China Coastal Current front, the following probably describes how
slanted clouds are formed above the ocean front:
A frontal inversion occurs when a cold air mass undercuts a warm air mass and lifts it aloft; the front
between the two air masses then has warm air above and cold air below. This kind of inversion has
considerable slope, whereas other inversions are nearly horizontal. In addition, humidity may be high,
and clouds may be present immediately above it.
http://www.britannica.com/EBchecked/topic/122305/cloud
High clouds, which are found at mean heights above the ground of 13 to 5 km (42,500 to 16,500 feet),
are cirrus, cirrocumulus, and cirrostratus. Middle clouds, 7 to 2 km (23,000 to 6,500 feet), are
altocumulus and altostratus. Low clouds, 2 to 0 km (6,500 to 0 feet), are stratocumulus, stratus, and
nimbostratus. A cloud that extends through all three heights is called a cumulonimbus. A cloud at the
surface is called a fog.
Precipitation in significant amounts generally falls only from nimbostratus and cumulonimbus clouds.
Steady rain that lasts for an entire day or longer is usually produced by the nimbostratus variety. A
cumulonimbus is a common sign of a thunderstorm and on rare occasions is associated with tornadoes.
PBL as defined in various models:
http://www.met.tamu.edu/class/metr452/models/2001/PBLproject.html
Planetary Boundary Layer;
Horizontal and Vertical Diffusion

The Planetary Boundary Layer, a Definition

Recognizing the PBL

Vertical Mixing

Horizontal Mixing and Diffusion

Conclusions
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References