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Transient: deviations from time mean |
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Eddy: deviations from zonal mean |
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Why transients/eddies matter to zonal and time
means? |
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Three components contribute to the zonal- and
time-mean transport: |
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Mean Meridional Circulation (such as the
three-cell circulation) |
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Stationary planetary Waves (such as the
wavenumber 1-3 eddies in the Northern Hemisphere). |
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Transient Eddies (such as the weather systems =
midlatitude cyclones and anticyclones). |
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Stationary and transient eddies are important to
the poleward fluxes of temperature, moisture, energy, and angular momentum. |
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Transient eddy fluxes dominant the meridional
flux of temperature except in the Northern Hemisphere during winter, when
stationary eddies contribute up to half of the flux. |
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The low-level maximum in the troposphere is
associated with the structure of growing mid-latitude cyclones (I.e.,
weather systems). |
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Both the mean meridional circulation and eddies
transport water and play an important role in determining the nature of the
hydrological cycle. |
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Moisture convergence in the tropics is dominated
by the transport provided by the mean meridional circulation. |
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The subtropcs serves as source regions for water
vapor. |
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Eddies remove water from the tropics and supply
it to middle and high latitudes. |
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In the tropical easterlies, eastward angular
momentum is transferred from Earth to the atmosphere via frictional forces
and mountain torque. |
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This westerly angular momentum is transported
upward and then polarward into the Hadley Cell. |
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Eddies then transport angular momentum polarward
and downward into mid-latitude westerlies. |
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In the mid-latitude, the westerly momentum is
returned to the Earth. |
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Stationary: These waves do not move around much
and are fixed in certain geographic locations. |
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Planetary: These waves have large wavelengths,
one the order of several thousands of kilometers. |
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Wave: Their structures vary in the zonal
direction. |
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Stationary planetary waves are forced by
large-scale mountains (such as Himalaya and Rocky mountain ranges) and heat
contrasts between continents and oceans. |
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Stationary planetary waves are stronger in
winter than in summer. |
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Mid-latitude cyclone and anticyclone are the
major transient eddies that play an important role in meridional transports
of heat, momentum, and moisture. |
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These mid-latitude weather systems grow from the
baroclinic instability associated with the strong north-south temperature
gradients in mid-latitudes. |
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Mid-latitude cyclones have typical spatial
scales of wavenumbers 5-6 and have typical time scale of 7-10 days. |
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Mid-latitude cyclones are marked by well-defined
fronts separating the warm air mass from the south and the cold air mass
from the north. (Very different from tropical hurricanes, which do not have
frontal features). |
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The wettest air is called maritime air, while
the driest is called continental. |
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Air deemed, from warmest to coolest, tropical, polar,
or arctic |
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Once formed, air masses migrate within the
general circulation. |
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Upon movement, air masses displace residual air
over locations thus changing temperature and humidity characteristics. |
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Further, the air masses themselves moderate from
surface influences. |
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Fronts separate air masses and bring about
changes in temperature and humidity as one air mass is replaced by another. |
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There are four general types of fronts
associated with mid-latitude cyclones with the name reflective of the
advancing air mass. |
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Cold Fronts |
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Cold fronts form when cold air displaces warm
air. |
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Indicative of heavy precipitation events,
rainfall or snow, combined with rapid temperature drops. |
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Extreme precipitation stems from rapid vertical
lifting associated with the steep cold front boundary profile. |
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Because cold air is dense, it spills across the
surface producing a steeply inclined leading edge. |
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Warm moist air ahead of the front is forced
aloft with great vertical displacement. |
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This
accounts for large vertical cumulonimbus clouds and heavy precipitation. |
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Such
sharp transitions between the colder, drier air behind the front and the
warmer, moisture air ahead of the front, can be easily detected on
satellite images and radar composites. |
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When two fronts meet, the warm air mass between
them is displaced aloft resulting in an occluded front. |
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This typically occurs when a cold front closes
on, and meets a warm front as it circulates about the low pressure center
of a mid-latitude cyclone. |
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Cold air now occupies the surface completely
around the low while warmer air is displaced aloft. |
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When two unlike air masses remain side by side,
with neither encroaching upon the other, a stationary front exists. |
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Fronts may slowly migrate and warmer air is
displaced above colder. |
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Fronts are zones of transition rather than sharp
boundaries. |
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Bjerknes, the founder of the Bergen school of
meteorology, developed polar front theory to describe interactions between
unlike air masses and related aspects of the mid-latitude cyclone. |
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Cyclogenesis |
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Mature
Cyclone |
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Occlusion |
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The hurricane is characterized by a strong
thermally direct circulation with the rising of warm air near the center of
the storm and the sinking of cooler air outside. |
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The warm core of the hurricane serves as a
reservoir of potential energy, which is continuously being converted into
kinetic energy by the thermally direct circulation. |
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Hurricanes, the most powerful of all storms,
have sustained winds of 120 km/hr (74 mph). |
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Although of lesser intensity than tornadoes, the
much larger size and longer life span makes hurricanes much more
devastating. |
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Average diameters are approximately 600 km (350
mi) and central pressure averages about 950 mb but may be as low as 870 mb. |
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Most energy attained by hurricanes stems from
latent heat release in the cloud formation process. |
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Hurricanes occur where warm waters abound and
during the times of highest SSTs. |
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For the N.H., August and September are the most
active months. |
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Conditions Necessary for Hurricane Formation |
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Hurricanes form only over deep water layers with
surface temperatures in excess of 27 oC. |
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Energy is derived from latent heat release and
associated evaporation of water |
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Poleward of about 20o, water
temperatures are usually below this threshold |
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Hurricanes are most frequent in late summer and
early autumn during high SST times |
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Coriolis force is an important contributor, and
as such, hurricanes do not form equatorward of 5o. |
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