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ATMO 336 Weather, Climate and Society Surface and Upper-Air Maps

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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

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Review: Pressure-Height Remember… Pressure falls very rapidly with height near sea-level 3,000 m 701 mb 2,500 m747 mb 2,000 m 795 mb 1,500 m846 mb 1,000 m899 mb 500 m955 mb 0 m1013 mb 1 mb per 10 m height Consequently………. Vertical pressure changes from differences in station elevation dominate horizontal changes

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Station Pressure Pressure is recorded at stations with different altitudes Station 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

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Reduction to Sea-Level-Pressure Sea Level Pressure Station pressures are adjusted to Sea Level Pressure Make altitude correction of 1 mb per 10 m elevation Ahrens, Fig. 6.7

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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

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Correction for Tucson Elevation 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

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Correction for Denver Elevation 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

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Lets Try for Phoenix Elevation of Phoenix AZ is ~340 m Assume station pressure at PHX is ~977 mb What would the SLP for PHX be?

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Correction for Phoenix Elevation 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

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Local Example Station pressure at PHX is ~977 mb. Station pressure at TUS is ~932 mb. Which station has that higher SLP?

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Correction for Tucson Elevation 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

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Sea Level Pressure Values Ahrens, Fig. 6.3 (October, 2005) Wilma 882 mb (26.04 in.)

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Take Home Points Ideal Gas Law Relates Temperature, Density and Pressure Pressure Changes with Height Decreases More Rapidly in Cold air than Warm Station Pressure Reduced to Mean-Sea-Level to Mitigate the Dominate Impact of Altitude on Pressure Change

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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

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Ahrens, Fig. 6.7 PGF

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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 Map UIUC 2010 Surface Maps

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Station Plot Explanation Winds blow from high to low pressure.

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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.

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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.

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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

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Force of Friction 3 PGF CF Wind 1004 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

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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 20 o -30 o from geostrophic). Rough surfaces (mtns) show the most slowing and turning (typically 30 o -40 o from geostrophic). Mtns Water 20 o -30 o 30 o -40 o

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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 km 0.6 km 0.3 km

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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

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Gedzelman, p249 LowsHighs Flow at Surface Lows and Highs Spirals Outward Divergence Spirals Inward Convergence

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www.met.tamu.edu

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