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CSU AT 540 - The Primary Variables

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The Primary Variables• Atmospheric Pressure• Temperature• Moisture– Water vapor in the air– Precipitation•Wind– Direction– SpeedWhat is Atmospheric Pressure?• Atmospheric Pressure is the force per unit area of a column of air above you (extending all the way to the top of the atmosphere)• In other words, pressure is the weight of the column of air above you - a measure of how hard this column of air is pushing down• More fundamentally - atmospheric pressure arises from gravity acting on a column of airHow do we measure atmospheric pressure?• Barometer– fluid column• Why mercury ?– aneroid • evacuated, expandable chamberAneroid BarographPressure vs Height Sea Level Value Units of Pressure: (average) 1 atmosphere 14.7 lbs./sq.in. 760 mm. of mercury 29.92 in. of mercury 33.9 ft. of water 1013.25 millibars Why does pressure decrease with altitude? Change in weight of air column above.∂P/ ∂z = -ρg (Hydrostatic Equation)-change in pressure over some small height is given by the air density times the gravitational constantTemperature scalesmeasure of the motion (kinetic energy) of air molecules. Degrees Celsius (Centigrade) deg C = 5/9 (deg F -32) Degrees Fahrenheit deg F = 9/5 (deg C) +32 Kelvin K = deg C + 273.16 1 deg Celsius change = 1.8 deg Fahrenheit change 10 deg Celsius change = 18 deg Fahrenheit change deg C -40 -17.8 0 10 20 30 100deg F -40 0 32 50 68 86 212K 233.16 290.94 273.16 283.16 293.16 303.16 373.16How do we measure Temperature?• Conventional thermometry– mercury thermometers, bimetallic strips• Electronic thermometers– resistance thermometers (thermistors), thermocouples • Remote sensing using radiation emitted by the air and surface (particularly, though not exclusively, from satellites) - radiometersThe atmosphereis layered accordingto temperaturestructureIn some layersthe temperatureincreases with heightIn others it decreaseswith height or is constant…pause is a level… sphere is a layerWind ProfilersHow do we measure water vapor?• Psychrometer• Hygrometers–hair– Electronic – carbon-coated plate changes conductivity with humidity• Remote sensing from radiation emitted by water vapor moleculesThe Sling PsychrometerMeasures water vapor content of airHow do we measure wind?• Anemometers–cup/vane– vane/propeller• Remote Sensing–radar • Doppler radars• profiler• scatterometer• Tracer techniques– tracking balloons– chemical tracers (SF6)– cloudsWind MeasurementsVane & Cup AnemometerAerovaneWIND - direction and speed - movement of air Wind is the air's response to pressure differences. Units of measure: Speed: Miles per hour 1 mi/hr = .87 knots Knot (nautical miles per hour) 1 knot = 1.15 miles per hour Meters/second 1 m/s = 2.24 miles per hour Feet/second 1 ft/s = 0.68 miles per hourWind DirectionN 0oor 360o Wind from the north 0°Wind from the east 90°Wind from the south 180°Wind from the west 270°Wind toward the north 180°Wind toward the west 90°Wind from the northeast 45°E 90oW 270o45o135o315o225oS 180oHow do we measure Precipitation?• Conventional rain gauge• Tipping bucket rain gauge• Remote Sensing– radars– passive satellite retrievals• Snow is difficult to measure because wind– creates non-uniform depth– affects collectionRain GagesStandard Rain GageTipping Bucket GageAutomated Surface Observing System(ASOS)Surface NetworkSurface Station ModelUpper air observations -The RawinsondeMeasures:TemperatureRelative HumidityPressureWind - direction & SpeedUpper Air Sounding NetworkHow do we measure clouds?• Visual observations• Aircraft• Remote Sensing– ceilometers - lasers bouncing light off the clouds from below– satellites - measuring radiation reflected and emitted by cloudsRemote Sensors• Obtain information about properties of an object or volume without coming into physical contact with that object – can be passive or active• Advantages– Fully automated, thus require only an occasional technician– Excellent coverage (horizontally) even over oceans• Disadvantages– Does not measure state variables directly – must be inferred or retrieved– Satellites have poor vertical resolution– ExpensiveElectromagnetic SpectrumPlanck’s Law• Expression for the blackbody radiation emitted by an object at a given wavelength.[]1516 2 2 *12exp( / 1)3.74 10 ; 1.44 10CECTcxWmCxmλλλ−=−==Wien’s Displacement Lawλmax= 2987µmK / TProvides wavelength of maximum emissionStefan-Boltzmann LawE = σT4where σ = 5.67x10-8Wm-2K-4Provides blackbody irradiance (flux)Kirchoff’s Lawaλ= ελMaterials that are strong absorbers at a given λ are also good emitters at that λSolar Irradiation SpectrumTerrestrial Emission SpectrumSatellites• 850 km orbit above earth’s surface• 35,786 km orbit above earth’s surface• 102 min period• ~24 hr periodexamplesRadars• Radar Reflectivity (Z)Z = ∫ n(D)D6dD• If the drop-size distribution is assumed or estimated, then Z can be related to the rain rate.Doppler Radars• Measures phase shift to determine radial velocity• Weather radars (3 – 10 cm wavelengths) observe the motion of hydrometeors (e.g., rain, snow, hail)examplesScanning Airborne Doppler RadarScan geometry for the NCAR ELDORA systemVelocity-Azimuth-Display (VAD)• Can estimate the 3-D wind field from a single Doppler radar• Must make major assumption about the scan volume (e.g., assume that the wind field and distribution of scatterers are uniform in the area)Multiple Doppler Radars• Can determine the 3-D wind field when 3 Doppler radars scan the same volumeMulti-parameter Radars - CHILL• Based on dual polarization (vertical and horizontal)• Differential reflectivity (Zdr) is the ratio of returned horizontally-polarized radiation to vertically-polarized radiation• This ratio is about 1 (i.e., 0 Zdr) for hailstones and large for large for large raindrops (oblate)Wind Profilers• Retrieve winds by sending and receiving radiation in at least 3 fixed directions (vertical, 15° from zenith N, 15° from zenith E)• Differ from weather radars in that they operate at longer λ and can sense variation in the refractive index• Radio Acoustic Sounding System (RASS) is often used in conjunction with the profiler to obtain Tvsoundings


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CSU AT 540 - The Primary Variables

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