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