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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 ). Overview . basic measurement quantities in meteorology

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slide1

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)

slide2

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
slide3

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
slide4

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
slide5

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
slide6

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
slide7

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
slide8

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

slide9

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

slide10

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
slide11

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
slide12

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
slide13

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
slide14

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
slide15

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

slide16

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
slide17

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
slide18

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
slide19

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
slide20

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