Download Supergeostrophic

Lecture 11
March 15, 2010, Monday
Atmospheric Pressure & Wind: Part 2
 Winds in the upper free (frictionless) atmosphere
(1) geostrophic wind (straight isobars)
(2) gradient wind (curved isobars)
supergeostrophic wind & subgeostrophic wind
 Cyclones, anticyclones, troughs, ridges
 Winds near the surface (subject to friction)
 Measuring wind
Geostrophic Wind
Flow aloft : no friction, balance between PGF & Coriolis force
 Upper air moving from high to low pressure with Coriolis deflection.
 Flow parallel to straight isobars as PGF balances the Coriolis force.
 This geostrophic flow (wind) may only occur in the free atmosphere.
Gradient Wind
Gradient
Wind
 Flow aloft: air flow
remains parallel to
curved isobars or
height contours.
 Wind direction
changes, thus there
is an acceleration
caused by a net
force or an
imbalance between
PGF and the
Coriolis force.
Northern
Hemisphere
Supergeostrophic flow
Subgeostrophic flow
Gradient
Wind
 Gradient wind also
only occurs in the
upper free
atmosphere.
 Supergeostrophic
flow is around
center of high
pressure.
 Subgeostrophic
flow is around
center of low
pressure.
Southern
Hemisphere
Supergeostrophic flow
Subgeostrophic flow
Cyclone and Anticyclone
in Upper Free Atmosphere
Cyclone is around center of low pressure in both NH & SH.
Cyclone and Anticyclone
in Upper Free Atmosphere
Airflow
(Clockwise)
Airflow
(Counterclockwise)
Cyclonic flow
(Southern hemisphere)
Anticyclonic flow
(Southern hemisphere)
Anticyclone is around center of high pressure in both NH & SH.
The apparent
turn of flow to
the left (NH) or
right (SH) when
airflow
converges to
center of low
pressure is due
to the change in
the direction of
the PGF.
Coriolis force
always points
to the right in
NH and to the
left in SH.
Open but curved isobars: trough (low, L) & ridge (high, H)
Ridge
Axis
Trough
Axis
Surface cyclones (anticyclones) tend to gradually become
troughs (ridges) in the upper atmosphere (note the offsets in
horizontal locations of the centers of low and high pressures.
 Enclosed area of
low pressure,
roughly circular.
 Upper free air:
counter-clockwise
(clockwise) in the NH
(SH), inward PGF is
greater than outward
Coriolis force which
is to the right (left) in
NH (SH),
subgeostrophic
 Near the surface:
air converges to the
center of low
pressure, crossing
isobars at the left
(right) in NH (SH).
Cyclones
 Enclosed area of
high pressure,
roughly circular.
 Upper free air:
clockwise (counterclockwise) in the NH
(SH), outward PGF is
smaller than inward
Coriolis force which
is to the right (left) in
NH (SH),
Supergeostrophic
 Near the surface:
air diverges from the
center of high
pressure, crossing
isobars at the right
(left) in NH (SH).
Anti-cyclones
Cyclones (low, L) and anticyclones (high, H) defined by
closed isobars in a surface weather map.
Vertical
motions
above
surface
cyclones
anticyclones
 Air moves into the center of surface cyclone (low, L) has to
rise upward (clouds, precipitation, storms)
 Air moves out the center of surface anticyclone (high, H) is
compensated by sinking air from above (clear and fair weather)
An azimuth
 In meteorology,
wind direction
is defined as
the direction
from which
wind comes.
N
 An azimuth
W
indicates the
degree of angle
from due north
moving
clockwise from
0o to 360o.
E
S
Wind Vane
(wind direction)
Cup
Anemometer
(wind speed)
Three-Dimensional
Propeller Anemometer
(3-D velocity)
(air pollution application)
 Right: 3-D sonic anemometer is
based on Doppler principle.
 Below: an aerovane (1-piece)
measures both wind speed and
direction.
ECU Geography
Weather Station