NATS 101. Lecture 15 Surface and Upper-Air Maps

Transcription

1 NATS 101 Lecture 15 Surface and Upper-Air Maps

2 Due Today Turn in 4 by 6 card, include Comments on Lecture PIN (Optional) Signature (Mandatory and Legible)

3 Supplemental References for Today s s Lecture Gedzelman, S. D., 1980: The Science and Wonders of the Atmosphere. 535 pp. John-Wiley & Sons. (ISBN )

4 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

5 PGF Ahrens, Fig. 6.7

6 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

7 Isobaric Maps Weather maps at upper levels are analyzed on isobaric (constant pressure) surfaces. (Isobaric surfaces are used for mathematical reasons that are too complex to explain in this course!) Isobaric maps provide the same information as constant height maps, such as: Low heights on isobaric surfaces correspond to low pressures on constant height surfaces! Cold temps on isobaric surfaces correspond to cold temperatures on constant height surfaces!

8 Isobaric Maps Downhill (Constant height) 504 mb PGF 496 mb Some generalities: 1) 2) High/Low Warm/Cold heights temps on an isobaric surface correspond to High/Low Warm/Cold pressures temps on a on constant a constant height height surface surface Ahrens, Fig. 2, p141

9 Contour Maps Display undulations of 3D surface on 2D map A familiar example is a USGS Topographic Map It s a useful way to display atmospheric quantities such as temperatures, dew points, pressures, wind speeds, etc. Gedlezman, p15

10 Contour Maps To successfully isopleth the 50- degree isotherm, imagine that you're a competitor in a rollerblading contest and that you're wearing number "50". You can win the contest only if you roller-blade through gates marked by a flag numbered slightly less than than 50 and a flag numbered slightly greater than From Click interactive exercise Click interactive isotherm map

11 Rules of Contouring (Gedzelman, p15-16) Every point on a given contour line has the same value of height above sea level. Every contour line separates regions with greater values than on the line itself from regions with smaller values than on the line itself. The closer the contour lines, the steeper the slope or larger the gradient. The shape of the contours indicates the shape of the map features.

12 570 dam contour

13 576 dam contour

14 570 and 576 dam contours

15 All contours at 6 dam spacing

16 All contours at 6 dam spacing

17 -20 C and 15 C Temp contours

18 -20 C, 15 C, -10 C Temp contours

19 All contours at 5 o C spacing

20 Height contours Temp shading

21 PGF Wind

22 Do Rocks Always Roll Downhill? PGF Upper-Level Winds Gedzelman, p 247

23 Key Concepts for Today Station Pressure and Surface Analyses Reduced to Mean Sea Level Pressure (SLP) PGF Corresponds to Pressure Differences Upper-Air Maps On Isobaric (Constant Pressure) Surfaces PGF Corresponds to Height Sloping Downhill Contour Analysis Surface Maps-Analyze Isobars of SLP Upper Air Maps-Analyze Height Contours

24 Key Concepts for Today Wind Direction and PGF Winds more than 1 to 2 km above the ground are perpendicular to PGF! Analogous a marble rolling not downhill, but at a constant elevation with lower altitudes to the left of the marble s s direction

25 Assignment Topic Newton s s Laws Reading - Ahrens pg Problems , 6.13, 6.17, 6.19, 6.22

26 Review Ideal Gas Law By Far

27 Example: Ideal Gas Law Relation between pressure, temperature and density is quantified by the Ideal Gas Law, which can be qualitatively written as Pressure = Constant x Density x Temperature It can be expressed in equation form as P(mb) = 2.87 ρ(kg/m 3 ) T(K)

28 Ideal Gas Law P = constant ρ T With T constant, Ideal Gas Law reduces to P varies with ρ Dense air has a higher pressure than less dense air at the same temperature

29 Ideal Gas Law P = constant ρ T With ρ constant, Ideal Gas Law reduces to P varies with T Warm air has a higher pressure than cold air at the same density

30 Ideal Gas Law P = constant ρ T With P constant, Ideal Gas Law reduces to T varies with 1/ρ Cold air is more dense (ρ bigger, 1/ρ smaller) than warmer air at the same pressure

31 Example: Ideal Gas Law If Pressure at Sea Level averages 1013 mb and Temperature at Sea Level averages 288 K, what is the average Density at Sea Level? Answer can be found using the Ideal Gas Law P(mb) = 2.87 ρ(kg/m 3 ) T(K) ρ(kg/m 3 ) = P(mb) / {2.87 T(K)} ρ(kg/m 3 ) = (1013 mb) / ( K) ρ(kg/m 3 ) = 1.23 kg/m lbs/yard 3

32 Summary Ideal Gas Law Relates Pressure = Density Temperature P varies with ρ P varies with T T varies with 1/ρ

33 Pressure-Temperature-Density 8.5 km COLD Minneapolis Same pressure 100 mb 200 mb 300 mb 400 mb 500 mb 600 mb 700 mb 800 mb 900 mb 1000 mb 9.5 km WARM Houston Same pressure Pressure (vertical scale highly distorted) Decreases more rapidly with height in cold air than in warm air Isobaric surfaces will slope downward toward cold air Slope increases with height to tropopause, near 300 mb in winter

34 Pressure-Temperature-Density 8.5 km COLD L H 100 mb 200 mb 300 mb 400 mb 500 mb PGF 600 mb 700 mb 800 mb 900 mb 1000 mb H L 9.5 km WARM Pressure Higher along horizontal red line in warm air than in cold air Pressure difference is a non-zero force Pressure Gradient Force or PGF (red arrow) Air will accelerate from column 2 towards 1 Pressure falls at bottom of column 2, rises at 1

35 Pressure-Height Consequently. Vertical pressure changes from differences in station elevation dominate horizontal changes Remember Pressure falls very rapidly with height close to sea level 3,000 m 701 mb 2,500 m 747 mb 2,000 m 795 mb 1,500 m 846 mb 1,000 m 899 mb 500 m 955 mb 0 m 1013 mb

36 Ahrens, Fig. 6.4 Measuring Air Pressure Mercury Barometer Air pressure at sea level can support nearly 30 inches of Hg Hg level responds to changes in pressure Pressure can support nearly 30 feet of water

37 Recording Aneroid Barometer Ahrens, Fig. 6.6 Aneroid cell is partially evacuated Contracts as pressure rises Expands as pressure falls Changes recorded by revolving drum

38 Pressure-Height Consequently. Vertical pressure changes from differences in station elevation dominate horizontal changes Remember Pressure falls very rapidly with height near sea-level 3,000 m 701 mb 2,500 m 747 mb 2,000 m 795 mb 1,500 m 846 mb 1,000 m 899 mb 500 m 955 mb 0 m 1013 mb

39 Station Pressure Ahrens, Fig. 6.7 Pressure is recorded at stations with different altitudes Station pressure differences reflect altitude differences Wind is forced by horizontal pressure differences Adjust station pressures to standard level: Mean Sea Level

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

41 Correction for Tucson Elevation of Tucson Airport is ~800 m Station pressure at Tucson runs ~930 mb So, SLP for Tucson would be SLP = 930 mb + (1 mb / 10 m) 800 m SLP = 930 mb + 80 mb = 1010 mb

42 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) 1600 m SLP = 850 mb mb = 1010 mb Actual pressure corrections take into account temperature and pressure-height variations, but 1 mb / 10 m is a good approximation

43 Sea Level Pressure Values Ahrens, Fig. 6.3