atmo 336 weather, climate and society surface and upper-air maps
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ATMO 336
Weather, Climate and SocietySurface and Upper-Air Maps
N. Pacific Pressure Analysis (isobars every 4 mb)
Pressure varies by 1 mb per 100 km horizontally or 0.0001 mb per 10 m
2000 km
Review: Pressure-HeightRemember…Pressure falls very rapidly with height near sea-level 3,000 m 701 mb2,500 m 747 mb2,000 m 795 mb1,500 m 846 mb1,000 m 899 mb500 m 955 mb0 m 1013 mb
1 mb per 10 m height
Consequently………. Vertical pressure changes from differences in station elevation dominate horizontal changes
Station Pressure
Pressure is recorded at stations with different altitudesStation pressure differences reflect altitude differences Wind is forced by horizontal pressure differences Since horizontal pressure variations are 1 mb per 100 km We must adjust station pressures to one standard level:
Mean Sea Level
Ahrens, Fig. 6.7
Reduction to Sea-Level-Pressure
Station pressures are adjusted to Sea Level PressureSea Level Pressure Make altitude correction of 1 mb per 10 m elevation
Ahrens, Fig. 6.7
Summary
• Because horizontal pressure differences are the force that drives the wind
Station pressures are adjusted to one standard level…Mean Sea Level…to remove the dominating impact of different elevations on pressure change
Correction for TucsonElevation of Tucson AZ is ~800 m
Station pressure at Tucson runs ~930 mb
So SLP for Tucson would be
SLP = 930 mb + (1 mb / 10 m) x 800 m
SLP = 930 mb + 80 mb = 1010 mb
Correction for DenverElevation of Denver CO is ~1600 m
Station pressure at Denver runs ~850 mb
So SLP for Denver would be
SLP = 850 mb + (1 mb / 10 m) x 1600 m
SLP = 850 mb + 160 mb = 1010 mb
Actual pressure corrections take into account temperature and pressure-height variations, but 1 mb / 10 m is a good approximation
Lets Try for PhoenixElevation of Phoenix AZ is ~340 m
Assume station pressure at PHX is ~977 mb
What would the SLP for PHX be?
Correction for PhoenixElevation of PHX Airport is ~340 m
Station pressure at PHX is ~977 mb
So, SLP for PHX would be
SLP = 977 mb + (1 mb / 10 m) x 340 m
SLP = 977 mb + 34 mb = 1011 mb
Local ExampleStation pressure at PHX is ~977 mb.
Station pressure at TUS is ~932 mb.
Which station has that higher SLP?
Correction for TucsonElevation of TUS Airport is ~800 m
Station pressure at TUS was ~932 mb
So, SLP for TUS would be
SLP = 932 mb + (1 mb / 10 m) x 800 m
SLP = 932 mb + 80 mb = 1012 mb
PHX (prior slide) has SLP = 1011 mb
Thus, the SLP was higher in TUS than PHX
Sea Level Pressure Values
Ahrens, Fig. 6.3
(October, 2005)Wilma
882 mb (26.04 in.)
Summary
• Because horizontal pressure differences are the force that drives the wind
Station pressures are adjusted to one standard level…Mean Sea Level…to mitigate the impact of different elevations on pressure
Ahrens, Fig. 6.7
PGF
Surface Maps
• Pressure reduced to Mean Sea Level is plotted and analyzed for surface maps.Estimated from station pressures
• Actual surface observations for other weather elements (e.g. temperatures, dew points, winds, etc.) are plotted on surface maps.
NCEP/HPC Daily Weather MapUIUC 2010 Surface Maps
Station Plot Explanation
Winds blow from high to low pressure.
Force of Friction 1
Pressure Gradient Force
Coriolis Force
Geostrophic Wind
1004 mb
1008 mb
As the wind speed becomes slower, the Coriolis Force would also decrease.
Friction
Frictional Force is directed opposite to velocity. It acts to slow down (decelerate) the wind.
Force of Friction 1
Pressure Gradient Force
Coriolis Force
Geostrophic Wind
1004 mb
1008 mb
Geostrophic balance is no longer possible!
Friction
Coriolis force no longer can balance the larger Pressure Gradient Force, so the parcel will accelerate since the net force is not zero.
Force of Friction 2
Pressure Gradient Force
Coriolis Force
Wind
1004 mb
1008 mb
Because PGF is larger than CF, air parcel will turn toward lower pressure.
Friction Turns Wind Toward Lower Pressure.
Friction
Force of Friction 3
PGFCF
Wind1004 mb
1008 mb
Eventually, a balance among the PGF, Coriolis and Frictional Force is achieved.
PGF + CF + Friction = 0
Net force is zero, so parcel does not accelerate.
Fr
Force of Friction 4
1004 mb
1008 mb
The decrease in wind speed and deviation to lower pressure depends on surface roughness. Smooth surfaces (water) show the least slowing and turning (typically 20o-30o from geostrophic).Rough surfaces (mtns) show the most slowing and turning (typically 30o-40o from geostrophic).
MtnsWater
20o-30o
30o-40o
Force of Friction 5
1004 mb
1008 mb
Friction is important in the lowest km above sfc.Its impact gradually decreases with height. By 1-2 km, the wind is close to geostrophic or gradient wind balance.
SFC
~1 km0.6 km
0.3 km
Gedzelman, p250
Force of Friction: Ekman Spiral
Speed and direction change with height.
Wind direction turns clockwise with height in the NH.
Wind speeds increase with height.
Wind goes to the geostrophic/gradient value at ~1-2 km
Gedzelman, p249
Flow at Surface LowsLows and HighsHighs
Spirals OutwardDivergence
Spirals InwardConvergence
www.met.tamu.edu
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