Transcript Section5Slides

Figure 7.2.1 (p. 202)
Precipitation formation. Water droplets in clouds are formed by
nucleation of vapor on aerosols, then go through many
condensation-evaporation cycles as they circulate in the cloud,
until they aggregate into large enough drops to fall through the
cloud base (from Marsh (1987)).
Horizontal Convergence
• Large-scale air mass convergence (typically occurs in Tropics
[Inter Tropical Convergence Zone (ITCZ)]-- see atmos. circ.
schematic
• Also occur in tropical cyclones (low pressure systems):
Plan:
Cross-section:
L
• High pressure systems have the opposite circulation (anticyclones) and downwelling motion
• Subtropical cyclones also form at higher latitudes
Frontal Convergence
• Convergence of air masses with different temperatures
(temperature fronts); generally occur in midlatitudes
• “Front” is region of strong thermal contrast
• Cold front: colder air approaching warmer; labeled with pointed
arrow indicating flow direction
• Warm front: warmer air approaching colder; labeled with
rounded arrow indicating flow direction
• Fronts collide and deflect warmer air rises
Figure 7.2.2 (p. 203)
Cyclonic storms in mid-latitude (from Masch (1984)).
How to Read a Surface Map
Surface maps depict the large-scale elements of the weather. These elements include high and
low pressure systems, cold and warm fronts, and precipitation areas.A high pressure system is
an area of relative pressure maximum that has diverging winds and a rotation opposite to the
earth's rotation. Fair weather is typically associated with high pressure. A low pressure system is
an area of relative pressure minimum that has converging winds and rotates in the same
direction as the earth. This is counterclockwise in the Northern Hemisphere and clockwise in the
Southern Hemisphere. Stormy weather is often associated with low pressure systems. A cold
front is the leading edge of an advancing cold air mass that is under running and displacing the
warmer air in its path. Generally, with the passage of a cold front, the temperature and humidity
decrease, the pressure rises, and the wind shifts (usually from the southwest to the northwest in
the Northern Hemisphere). Precipitation is generally at and/or behind the front, and with a fastmoving system, a squall line may develop ahead of the front. A warm front is the leading edge of
an advancing warm air mass that is replacing a retreating relatively colder air mass. Generally,
with the passage of a warm front, the temperature and humidity increase, the pressure rises,
and although the wind shifts (usually from the southwest to the northwest in the Northern
Hemisphere), it is not as pronounced as with a cold frontal passage. Precipitation, in the form of
rain, snow, or drizzle, is generally found ahead of the surface front, as well as convective
showers and thunderstorms. Fog is common in the cold air ahead of the front. Although clearing
usually occurs after passage, some conditions may produce fog in the warm air. Current surface
maps are updated every hour. Forecast surface maps are updated once each day
Orographic Lifting
• Air mass forced to move over obstacle (topography)
• If air is moist enough and/or topography high enough can
cause cloud formation and potentially precipitation
• For large mountain ranges (Hawaii, Rockies, Andes) causes
sharp contrast:
 humid on windward side
 dry “rain shadow” on leeward side
Figure 7.2.3 (p. 203)
Orographic storm (from Masch (1984)).
Hawaii: Average Annual Precip. (mm)
Tradewinds
Windward side
Leeward side
Hawaii
Thermal Convection
• Caused by differential surface heating (land surface absorbs
more radiation than overlying atmosphere)
• Warm air beneath cooler air causes instability
• Mechanism principally responsible for convective/
thunderstorms of short duration and high intensity
 have diurnal cycle (late afternoon)
 uplift velocities high due to instability
 causes tall (cumulus) clouds, sometimes to the top of the
troposphere
Figure 7.2.4 (p. 204)
Convective storm (from Masch (1984)).
Precipitation Climatology (annual)
Precipitation Climatology (annual)
Figure 7.2.7 (p. 206)
Average annual precipitation for the world's land areas,
excepting Antarctica (from Marsh (1987)).
Figure 7.2.5 (p. 205)
Mean annual precipitation for the United States in
centimeters and inches (from Marsh (1987)).
Precipitation Extremes
0.35
0.40
0.45
0.50
0.55
Figure 7.2.14a (p. 215)
(a) 2-year 5-minute precipitation (in) (from Frederick, Meyers,
and Auciello (1977)).
0.80
0.75
0.70
0.80
0.85
0.90
Figure 7.2.14b (p. 216)
(b) 100-year 5-minute precipitation (in) (from Frederick, Meyers,
and Auciello (1977)).
Mean Areal Precipitation Estimation: Thiessen
Polygon Method
Thiessen Polygon Method for Mean
Areal Precip. Estimation
G
P̂   wg pg
g1
Polygons can be constructed
graphically to determine areal
weights…
Or for larger problems determined via automated
method (i.e. nearest-neighbor algorithm in Matlab)…
Figure 7.2.11b (p. 211)
(b) Computation of areal average rainfall by the Thiessen method
for 24-hour storm. This method assumes that at any point in the
watershed the rainfall is the same as that at the nearest gauge, so
the depth recorded at a given gauge is applied out to a distance
walkway to the next station in any direction. The relative weights
for each gauge are determined from the corresponding areas of
application in a Thiessen polygon network., the boundaries of the
polygons being formed by the perpendicular bisectors of the
lines joining adjacent gauges for J gauges, the area within the
watershed assigned to each is AJ and Pj is the rainfall recorded at
the jth gauge, the areal average precipitation for the watershed is
where the watershed area (after Roberson et al.(1998). (Detail on
next slide.)
NEXRAD Stations
Minot AFB Base Reflectivity
Satellite-based Estimates