Transcript Document

Introduction to
METEOROLOGY
Philip Bedient
Civil and Environmental Eng
Rice University
Severe Storms
Cirrus Clouds
• Feathery or fibrous
• High: Over 7 km or 23,000 ft
• Composed entirely of ice crystals
Altocumulus Clouds
• Fluffy, cotton-ball like clouds
• Middle: 2-7 km, 6500-23,000 ft
Stratus Clouds
• Stratified or in layers
• Low: Below 2 km or 6,500 ft.
• Extremely thin
Cumulonimbus Clouds
• Gray rain clouds of vertical development
• Base below 2 km or 6,500 ft.
• Can extend over 30 to 40,000 ft
HAIL STORMS
Heat Exchange
Latent heat is required to
evaporate water, melt ice
or sublimate ice (convert
ice directly to vapor).
Note, it requires
approximately 600 cal of
heat to evaporate one
gram of water.
Requires 80 cal to melt 1
g of ice into water
Vapor Pressure
• V.P. generally
increases with
Temperature
• Higher over liquid vs
ice for same T
• e = es for Sat V.P.
when air is saturated
• Gulf coast is most
humid area in U.S.
Vapor Pressure
• e =  RT / 0.622
• where
e = V.P. in mb
vapor density in g/cm3
R = dry air gas const
= 2.87 x 103 mb cm3/goK
T = absolute T (0K)
Humidity
• Warm air holds more
moisture than cold air
• RH = 100 e/es
• 50% RH implies that
the air contains 50%
of the max value
• Picture to the right has
what level of RH?
SOLAR ENERGY
The rate at which energy
is emitted from each
square centimeter of
surface as a function of
wavelength is very much
like that for an ideal or
black body at 6000 0K.
Sun
O.3 - 0.8 mm
Earth is at 300 0K.
Measured in Cal/m2
0.8 - 20 mm
Earth
SOLAR ENERGY
The clear atmosphere is essentially
transparent between 0.3 and 0.8 um, where
most of the solar (short wave) radiation occurs.
Between 0.8 and 20 um, where much of the
terrestrial (long wave) radiation is emitted,
there are several bands of moderate
absorptivity by water vapor, carbon dioxide,
and other trace gases.
Absorption of Radiation
UV
Ozone Hole
Infrared
Temperature Distribution in the Vertical
In the lowest layer, the troposphere, the temperature
decreases with height at an average of 6.5 degrees C/km (3.5
deg. F/1000 ft). The air in this layer is well stirred due to
vertical convection currents. Almost all clouds are found in
this layer.
The second layer, called the stratosphere, has an upper
boundary of about 50 km. The temperature is relatively
constant in the lower part, but it increases with height in the
upper.
The mesosphere is the zone between 50 and 85 km in which
the temperature decreases rapidly with height, reaching
about -95 deg. C at the mesopause (the coldest point in the
atmosphere).
In the thermosphere the temperature increases with height.
Earth’s Heat Balance
THE EARTH’S HEAT BALANCE
Figure 3.4 shows earths energy or heat balance.
69% is lost by the earth and its atmosphere to space.
Although there is a heat balance for the planet as a
whole, all parts of the earth and its atmosphere are not
in radiative balance.
It is the imbalance between incoming and outgoing
energy over the earth that leads to the creation of wind
systems and ocean currents that act to alleviate the
surpluses and deficits of heat.
Relates to formation of severe storms and hurricanes
July
January
Mean Surface T
Seasonal Variation of the Earth’s
Energy
The fact that the oceans act as heat reservoirs is illustrated
by the January and July mean air temperature maps seen
in the previous figure.
There is a greater variation in the temperature between
seasons in middle and high latitudes over the continents
than over the ocean.
Also note how the isotherms dip equatorward over the
oceans in summer and poleward in the winter, indicating
that the ocean is cooler than the land in summer and
warmer than the land in winter.
Daily
Annual
Temperature Lag
Temperature Lag
Earth loses heat continuously through radiation. During
some months, the incoming energy exceeds the
outgoing energy of the Earth. The temperature will
increase because the airs heat content will be rising.
The max temperature will occur at the time when the
incoming energy ceases to exceed the outgoing. When
the outgoing energy is greater than the incoming, the
temperature will fall until the two are again in balance.
At the point where a “surplus” of energy begins to
appear, the lowest temperature will have occurred.
General Circulation
vv
General Circulation of the Atmosphere
The horizontal flow ot the Earths surface is shown in the
center of the diagram; the net meridian circulation, at the
surface and aloft is depicted around the periphery.
The component of the flow along meridians has a speed
on average of less than 0.1 of that along latitude circles
The effect of Corioles force tends to bend streamlines to
the right in the Northern Hemisphere.
Jet streams were only discovered in 1946 and are
important drivers of major weather and air mass systems.
They flow at hundreds of MPH and dominate U.S. weather
especially in the winter.
High
Low
Vertical Motion and its Relation to Fronts
An important cause of vertical motion is the divergence
and convergence of air currents circulating around the
great anticyclones and cyclonic spins of the atmosphere.
Air currents spiraling inward at low levels of cyclones
converge, in other words, they move toward the center.
Since the horizontal area occupied by a volume of air
must decrease with time, the vertical depth must
increase.
Frontal Passage
Warm
Cold
Winds increase as cold front approaches
THE SIBERIAN EXPRESS
The simplified National Weather Service map
shows an intense winter cold wave caused by
an outbreak of frigid continental arctic air. This
event brought subfreezing temperatures as far
south as the Gulf of Mexico. Temperatures on
NWS maps are in degrees Fahrenheit.
Houston temperature dropped to 8 degrees F
for more than 8 hours and caused massive
damage to plumbing systems in 1989.
Development of a Wave Cyclone in the
Northern Hemisphere
The genesis stage of the wave cyclone normally takes
between 12 and 24 hours. Subsequent development of the
wave, shown in the previous figures, takes an additional
two or three days.
As the wave breaks, the cold front begins to overtake the
warm front. This process is called occlusion and the
resulting boundary is call an occluded front.
Vertical Cross Sections of Occlusions
The vertical cross sections of Figure 5.10 illustrate that
the cold front can either move up over the warm front
(warm-front type occlusion) or it can force itself under
the warm front (cold-front type). The occluded front is
the boundary that separates the two cold air masses.
COLD FRONT
Hurricanes - Andrew’s Path in 1992
Minimum Pressures
Hurricane Andrew
Hurricane Andrew
Figure 5.17 shows the variation with time of the minimum
pressure and maximum sustained wind speeds for
Andrew. Unfortunately, Andrew was close to its greatest
intensity when it made landfall on the 24th.
The minimum pressure of 922 mb was the third lowest
central pressure this century for a hurricane making
landfall in the United States.
The sustained winds of 125 knots created a storm tide
along the coast ranging up to nearly 17 feet.
Life Cycle of a Typical Cumulonimbus Cell
Life Cycle of a Typical Cumulonimbus Cell
The initial cumulus stages usually lasts for about 15
minutes. During this period, the cell grows laterally from 2
or 4 km in diameter to 10 or 15 km, and vertically to 8 or
10 km.
The mature stage begins when rain reaches the ground
and usually lasts for 15 to 30 minutes. During this stage,
the drops and ice crystals in the clouds grow so large that
the updrafts can no longer support them, and they begin
to fall as large drops or hail.
The mature stage is the most intense part of the
thunderstorm.
The final or dissipating stage begins when the
downdraft has spread over the entire cell. With the
updraft cut off, the rate of precipitation eventually
diminishes and so the downdrafts are also gradually
subdued.
Finally, the last flashes of lightning fade and the cloud
begins to dissolve, perhaps persisting for a while in a
stratified form.
Flow in Severe Thunderstorm
In Figure 5.24 a storm is moving toward the right and
is being continually supplied with warm, moist, lowlevel air at its leading edge. In the updraft fed by this
inflow, condensation produces rain below the freezing
level and ice at higher levels. To the rear of the storm,
dry middle-level air is incorporated into the storm. As
evaporation of rain cools this air, it becomes
negatively buoyant and sinks. When the resulting
downdraft reaches the ground, it spread out and forms
the gust front.
Nature’s Wonders