Introduction to Measurement Techniques in Environmental Physics University of Bremen, summer term 2006 Measurement Tech

1. Introduction to Measurement Techniques in Environmental Physics University of Bremen, summer term 2006 Measurement Techniques in Meteorology Andreas Richter (richter@iup.physik.uni-bremen.de)

2. Overview • basic measurement quantities in meteorology • different instruments used to take the measurements • physical principles behind the measurements • some problems related to the measurements • outlook to satellite meteorology

3. Which quantities do we need to measure? • air temperature • wind speed and direction • pressure • humidity • visibility • cloud distribution • cloud type • type and amount of precipitation How do we want to measure them? • in as many places as possible • as continuously as possible • as reproducible as possible • => we need cheap, simple, and automated measurements

4. Measurements of air temperature I • liquid filled / metallic thermometers • effect: T-dependence of volume • use: volume change ΔV = V0(α1- α2) ΔT • Δl = ΔV / A • where A = area of tube • α1 = coefficient of expansion of liquid • α2 = coefficient of expansion of reservoir • resistance thermometer • effect: T-dependence of electrical resistance of platinum or nickel(e.g.: Pt100 with 100 Ω at 0 °C ) • use: R = R20 (1 + α · ΔT) • T = 20 °C + (R/R20-1) / α • the temperature coefficient α is constant in first approximation but tabulated for higher accuracy • thermistor thermometer • effect: (negative) T-dependence of semiconductor resistance

5. Measurements of air temperature II • energy budget of thermometer • sensible heat transfer • radiative heat transfer: • short wave (gain) • long wave (loss or gain, depending on surroundings) • (latent heat transfer if wet) • => generally overestimation of T during the day • => underestimation of T during night • => underestimation of T if wet • response time of thermometer • finite time lag between temperature change and change in measured value • depends on thermal mass of thermometer • depends strongly on wind speed

6. Reminder: water vapour in the atmosphere The amount of water in a given air volume is crucial for its ability to transfer energy. Common moisture parameters are: mass mixing ratio: where mv is the mass of water vapour and md the mass of dry air saturation vapour pressure:the vapour pressure that is reached in equilibrium above a plane surface of pure water es or over ice esi. Note that es and esi depend only on temperature and that es > esi at all temperatures. relative humidity: dew point: Temperature at which water vapour in a given air volume would start to condensate frost point: Temperature at which water vapour in a given volume would start to freeze • water saturation pressure is an exponential function of temperature • small changes in temperature have a large effect on the amount of water that can be present as water vapour • Every day’s examples: • dry air in heated rooms • “fogging” of glasses • white plumes above chimneys

7. Measurements of air humidity I • hair hygrometer • effect: detection of change of length of a human (or horse) hair in response to relative humidity changes • hair length changes as in keratin hydrogen bonds are broken in the presence of water vapour • slow response • capacity hygrometer • effect: hygroscopic polymer is placed between two electrodes. In the presence of water vapour, the volume of the polymer increases, decreasing the capacity of the device • are easily contaminated • absorption hygrometer • absorption spectroscopy on H2O can also be used to measure water vapour concentration

8. Measurements of air humidity II • dew point hygrometer • effect: detection of dew on temperature controlled mirror by observation of change in reflectance • very accurate • psychrometer • effect: T-difference between two ventilated thermometers, one of which is covered by a wet wick (wet bulb temperature). T-difference is proportional to relative humidity • use: • e = esat wet – c (Tdry - Twet) water vapour saturation pressure at Twet water vapour partial pressure

9. Measurements of air pressure • mercury barometer • effect: air weight is balanced by mercury weight in a tube which is open on one end • use: Δp = p2 – p1 = ρgh • aneroid barometer • effect: sealed metal box with reduced internal air pressure is contracting and expanding in response to pressure changes = 0 density of mercury gravitational acceleration

10. Measurements of wind speed and direction • wind vane • effect: vane aligns in air flow • windsock • effect: sock aligns in wind flow and changes shape depending on wind speed (qualitatively) • cup anemometer • effect: pressure differences produce force on cups which rotate proportional to wind speed • problems: only wind speed in one plane, slow response, overshooting • (ultra)sonic anemometer • effect: measurement of sound velocity • all 3 wind components, fast, no inertia, simultaneous virtual temperature measurement • hot wire anemometer • effect: energy loss of a heated wire • very fast but fragile

11. Cup anemometer measurements of wind speed • force balance for cup anemometer: • F1=1/2 Cd1ρA(U – Ux)2 • F2=1/2 Cd2ρA(U + Ux)2 • Cd1ρA(U – Ux)2 = Cd2ρA(U + Ux)2 • where • Cd1 and Cd2 are the drag coefficients for the concave and convex side of the cup • A is the area of the cup • U is the wind speed • Ux is the tangential speed of the cups • ρ is the density of air • => angular velocity of the cup anemometer is proportional to the wind speed • 3 cup anemometers have larger torque and react faster to changes in wind speed • conical cups are better • rings for turbulence suppression help

12. Measurements of precipitation • rain gauge • effect: precipitation is collected and the amount measured e.g. by a tipping bucket. Precipitation collector is heated to convert hail and snow to water • optical rain gauge • effect: particles passing through a light beam cause scintillations http://www.usatoday.com/weather/wtipgage.htm Problems in measurements of precipitation • gauge may alter air flow and thus precipitation locally • wind shields are necessary • optical measurement relies on assumptions on droplet size

13. Measurements of upper air weather • radio sonde • small instrument package (temperature, pressure, relative humidity) connected to a balloon filled e.g. with helium. The balloons usually burst at about 30 km. Data is sent to ground via radio transmission • ozone sonde • radio sonde which also contains an ozone monitor • rawinsonde • radiosonde that tracks its position in space and time allowing determination of wind speed and direction • dropsonde • sonde that doesn’t ascend with a balloon but is falling on a parachute after being dropped from an airplane

14. Weather Radar I • RAdio Detection And Ranging • effect: radio wave pulses are emitted and scattered back by precipitation particles. From the time between emission and detection, the distance can be computed; the signal intensity depends on the concentration of scatterers, the size of the particles and their type (snow, hail, rain). Radar data is usually shown as reflectivity in decibels. • use: • distance d = (c t) / 2 • maximum distance dmax = c / (2 PRF) (PRF = pulse repetition frequency) • problems: large dependence on particle radius, dependence on type of scatterer, other echoes

15. Weather Radar II • Doppler Radar (Doppler mode, velocity mode) • effect: using the Doppler effect, the direction and speed of precipitation can be determined • Wind profiler • effect: using the Doppler effect, Radar can provide vertical wind speed in the absence of precipitation by using the echoes from aerosols, insects or turbulence eddies http://weather.noaa.gov/radar/mosaic/DS.p19r0/ar.us.conus.shtml reflectivity relative speed one hour rain fall

16. Reminder: radiation in the atmosphere • Short wave radiation: • comes from the sun • about half reaches the ground • about 30% is reflected / scattered back • rest is absorbed • Long wave radiation: • is absorbed and re-emitted in the atmosphere • emitted from the surface • counterradiation from the atmosphere

17. Radiation measurements I • Pyrheliometer: direct sunshine • Angstrom compensation pyrheliometer • effect: two manganin strips, one heated by the sun, the other electrically until they have the same temperature. The current needed is proportional to the incoming short wave radiation • Pyranometer: short wave radiation on a plane • Kipp solarimeter • effect: thermopile under two domes (0.3 – 3 μm transmission + radiation shield + aspiration to establish radiance balance) measures temperature difference between housing and detector • Eppley pyranometer • effect: as Kipp solarimeter, but temperature difference between black and white sectors of the detector are measured

18. Radiation measurements II • Pyrgeometer: long wave radiation • effect: as for pyranometers, only that dome is transparent for 3 – 50 μm radiation • Net radiometer: total net long and short wave radiation • either two instruments or one combined instrument with ventilated polyethylene dome and carefully balanced detector response • energy balance radiation measurements: • shortwave and longwave incoming radiation • longwave radiation from the dome(s) • heat conduction to the housing • convective heat losses • temperature of housing and dome (for pyrgeometer) is measured • good ventilation crucial • good radiation shields needed

19. Satellite imagery • visible images • show thick clouds as bright white areas. Brightness is determined by cloud droplet size • IR images (10 – 12 μm) • show high (cold) clouds as bright areas, low (warm) clouds as grey areas. Together with vertical profiles of temperature and assumptions on emissivity, cloud top altitude can be determined • H2O images (6.5 – 6.9 μm) • provide information on the water vapour content of the atmosphere, mainly between 500 and 200 mbar. • measurements at different IR wavelengths • can also provide indication on the phase (liquid vs. ice) of cloud particles • image sequences • show movement of clouds which can be converted to wind velocities at different altitudes

20. Summary • meteorology depends on frequent and accurate measurements of the basic quantities air temperature, wind speed and direction, pressure, humidity, cloud distribution, cloud type, type and amount of precipitation and radiation • standard instruments are available for most of the quantities on the surface using different techniques • sonding and remote sensing is used for upper air weather measurements • satellite meteorology gets more and more important but can not replace surface measurements Some References to sources used • http://www.physics.uwo.ca/~whocking/p103/instrum.html • http://de.wikipedia.org • http://www.met.wau.nl/education/fieldpract/field%20course%20micrometeorology%202005.pdf • http://weather.noaa.gov/radar/mosaic/DS.p19r0/ar.us.conus.shtml • http://www.usatoday.com/weather/wmeasur0.htm