Transcript meteo_1_lecture_6

REVIEW SLIDES:
We know this is the cause of decreasing air pressure with elevation
(vertical gradient):
Air pressure is a measure of the overlying air mass. (mass = vol/density)
• However, what causes air pressure changes in the horizontal
directions?
• Why does air pressure change at the surface?
The heating and
cooling of air columns
causes horizontal
pressure variations
aloft and at the surface.
These pressure
variations force the air
to move from areas of
higher pressure toward
areas of lower
pressure. In
conjunction with these
horizontal air motions,
the air slowly sinks
above the surface high
and rises above the
surface low.
The area shaded
gray in the diagram
represents a surface
of constant pressure.
Because of the
changes in air
density, a surface of
constant pressure
rises in warm, lessdense air and lowers
in cold, more-dense
air. These changes in
height of a constant
pressure (500-mb)
surface show up as
contour lines on a
constant pressure
(isobaric) 500-mb
map.
(a) The effect of surface friction is to slow down the wind so that,
near the ground, the wind crosses the isobars and blows toward
lower pressure. (b) This phenomenon at the surface produces an
inflow of air around a low and an outflow of air around a high. Aloft,
the winds blow parallel to the lines, usually in a wavy west-to-east
pattern. Both diagram (a) and (b) are in the Northern Hemisphere.
Winds and air motions associated with surface highs and lows in the
Northern Hemisphere.
Chapter 7
Atmospheric circulations
•
•
•
•
Scales of Circulation (micro-, meso-, macro-)
Thermal Circulation
Seasonal Changes (monsoon)
Global Wind and Pressure Systems
• single-cell and three-cell circulation
• Inter Tropical Convergence Zone (ITCZ)
• Polar Front
• Ocean Currents
• Regional Climate
• El Niño/La Niña
Scales of atmospheric motion. The tiny microscale motions
constitute a part of the larger mesoscale motions, which, in
turn, are part of the much larger macroscale. Notice that as
the scale becomes larger, motions observed at the smaller
scale are no longer visible.
The scales of atmospheric motion with the phenomenon’s average size and life span.
(Because the actual size of certain features may vary, some of the features fall into
more than one category.)
A thermal circulation
produced by the heating
and cooling of the
atmosphere near the
ground. The H’s and L’s
refer to atmospheric
pressure. The lines
represent surfaces of
constant pressure (isobaric
surfaces), where 1000 is
1000 millibars.
Development of a sea
breeze and a land breeze.
(a) At the surface, a sea
breeze blows from the
water onto the land,
whereas
(b) the land breeze blows
from the land out over
the water. Notice that the
pressure at the surface
changes more rapidly
with the sea breeze. This
situation indicates a
stronger pressure
gradient force and higher
winds with a sea breeze.
Diagram (a) shows the general circulation of air on the side of the
earth facing the sun on a nonrotating earth uniformly covered with
water and with the sun directly above the equator. (Vertical air
motions are highly exaggerated in the vertical.) Diagram (b) shows the
names that apply to the different regions of the world and their
approximate latitudes.
http://www.youtube.com/watch?v=Ye45DGkqUkE
The
idealized
wind and
surfacepressure
distribution
over a
uniformly
watercovered
rotating
earth.
Average position of the polar jet stream and the subtropical jet
stream, with respect to a model of the general circulation in winter.
Both jet streams are flowing from west to east.
Average sea-level pressure distribution and surface wind-flow patterns
for January. The solid red line represents the position of the ITCZ.
Average sea-level pressure distribution and surface wind-flow patterns
for July. The solid red line represents the position of the ITCZ.
http://www.youtube.com/watch?v=qh011eAYjAA
Average position and extent of the major surface ocean currents. Cold currents are shown in
blue; warm currents are shown in red.
Coastal Upwelling
• Ekman transport moves
surface seawater
offshore.
• Cool, nutrient-rich deep
water comes
up to replace displaced
surface waters.
• Example: U.S.
West Coast
Coastal Downwelling
• Ekman transport moves
surface seawater
toward shore.
• Water piles up, moves
downward in water
column
• Lack of marine life
BONUS SLIDES: Regional Weather
In diagram (a), under ordinary conditions higher pressure over the southeastern
Pacific and lower pressure near Indonesia produce easterly trade winds along the
equator. These winds promote upwelling and cooler ocean water in the eastern
Pacific, while warmer water prevails in the western Pacific. The trades are part of a
circulation (called the Walker circulation) that typically finds rising air and heavy rain
over the western Pacific and sinking air and generally dry weather over the eastern
Pacific. When the trades are exceptionally strong, water along the equator in the
eastern Pacific becomes quite cool. This cool event is called La Niña.
In diagram (b)—atmospheric pressure decreases over the eastern Pacific and rises
over the western Pacific. This change in pressure causes the trades to weaken or
reverse direction. This situation enhances the countercurrent that carries warm water
from the west over a vast region of the eastern tropical Pacific. The thermocline, which
separates the warm water of the upper ocean from the cold water below, changes as
the ocean conditions change from non-El Niño to El Niño.
Chapter 8
Air Masses, Fronts, and Midlatitude cyclones
• Air Masses
• Fronts
• look at example from 1976
• Polar Front
• Cyclone Development
• Clouds, Weather, Vertical Motion, & Upper Level
Support
Here, a large, extremely cold winter air mass is dominating the weather over much of the
United States. At almost all cities, the air is cold and dry. Upper number is air
temperature (°F); bottom number is dew point (°F).
A weather map showing surface pressure systems, air masses, fronts, and isobars (in
millibars) as solid gray lines. Large arrows in color show air flow. (Green-shaded area
represents rain; pink-shaded area represents freezing rain and sleet; white-shaded area
represents snow.)
A closer look at
the surface
weather
associated with
the cold front
situated in the
southern United
States. (Gray lines
are isobars.
Green-shaded
area represents
rain; white-shaded
area represents
snow.)
A Doppler radar image showing precipitation patterns along a cold front similar to the
cold front in the previous slide. Green represents light-to-moderate precipitation; yellow
represents heavier precipitation; and red the most likely areas for thunderstorms. (The
cold front is superimposed on the radar image.)
A vertical view of the weather across the cold front in the previous
slides along the line X–X´. (active figure!)
Surface weather
associated with a
typical warm front. A
vertical view along
the dashed line P-P′
is shown in the next
slide. (Green-shaded
area represents rain;
pink-shaded area
represents freezing
rain and sleet; whiteshaded area
represents snow.)
Vertical view of clouds, precipitation, and winds across
the warm front in the previous slide along the line P–P′.
http://www.youtube.com/watch?v=huKYKykjcm0&feature=related
The idealized life cycle of a mid-latitude cyclonic storm
(a through f) in the Northern Hemisphere based on the
polar front theory. As the life cycle progresses, the
system moves northeastward in a dynamic fashion. The
small arrow next to each L shows the direction of storm
movement.
The idealized life cycle of a mid-latitude cyclonic storm
(a through f) in the Northern Hemisphere based on the
polar front theory. As the life cycle progresses, the
system moves northeastward in a dynamic fashion. The
small arrow next to each L shows the direction of storm
movement.
http://www.youtube.com/watch?v=pm457OdgdnQ
A series of wave cyclones (a “family” of cyclones) forming along
the polar front.
BONUS SLIDES:
Convergence, Divergence, and Weather
If lows and highs aloft were always directly above lows and highs
at the surface, the surface systems would quickly dissipate.
Convergence,
divergence, and
vertical motions
associated with
surface pressure
systems. Notice
that for the
surface storm to
intensify, the
upper trough of
low pressure must
be located to the
left (or west) of
the surface low.
The formation of convergence (CON) and divergence (DIV) of air with a constant wind speed
(indicated by flags) in the upper troposphere. Circles represent air parcels that are moving
parallel to the contour lines on a constant pressure chart. Below the area of convergence the
air is sinking, and we find the surface high (H). Below the area of divergence the air is rising,
and we find the surface low (L).
(a) As the polar jet stream and its area of maximum winds (the jet
streak, or core) swings over a developing mid-latitude cyclone, an area
of divergence (D) draws warm surface air upward, and an area of
convergence (C) allows cold air to sink. The jet stream removes air
above the surface storm, which causes surface pressures to drop and
the storm to intensify. (b) When the surface storm moves northeastward
and occludes, it no longer has the upper-level support of diverging air,
and the surface storm gradually dies out.
Summary of clouds, weather, vertical motions, and upper-air
support associated with a developing mid-latitude cyclone.