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Introduction to Measurement Techniques in Environmental Physics, A. Richter, Summer Term 2006 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 )
Date 9 – 11 11 – 13 14 – 16
April 19 Atmospheric Remote Sensing I (Savigny)
Oceanography (Mertens)
Atmospheric Remote Sensing II (Savigny)
April 26 DOAS (Richter) Radioactivity (Fischer)
Measurement techniques in Meteorology (Richter)
May 3 Chemical measurement
techniques (Richter)
Soil gas ex- change (Savigny)
Measurement Techniques in Soil physics (Fischer)
Introduction to Measurement Techniques in Environmental Physics, A. Richter, Summer Term 2006 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
Introduction to Measurement Techniques in Environmental Physics, A. Richter, Summer Term 2006 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
Introduction to Measurement Techniques in Environmental Physics, A. Richter, Summer Term 2006 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 / Awhere 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
Introduction to Measurement Techniques in Environmental Physics, A. Richter, Summer Term 2006 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
Introduction to Measurement Techniques in Environmental Physics, A. Richter, Summer Term 2006 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
sw
wRH 100
d
v
m
mw
Introduction to Measurement Techniques in Environmental Physics, A. Richter, Summer Term 2006 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
Introduction to Measurement Techniques in Environmental Physics, A. Richter, Summer Term 2006 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 partial pressure
water vapour saturation pressure at Twet
Introduction to Measurement Techniques in Environmental Physics, A. Richter, Summer Term 2006 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
Introduction to Measurement Techniques in Environmental Physics, A. Richter, Summer Term 2006 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
Introduction to Measurement Techniques in Environmental Physics, A. Richter, Summer Term 2006 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
UCC
CCUU
dd
ddx 3
1
21
21
• 3 cup anemometers have larger torque and react faster to changes in wind speed
• conical cups are better
• rings for turbulence suppression help
Introduction to Measurement Techniques in Environmental Physics, A. Richter, Summer Term 2006 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
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
http://www.usatoday.com/weather/wtipgage.htm
Introduction to Measurement Techniques in Environmental Physics, A. Richter, Summer Term 2006 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
Introduction to Measurement Techniques in Environmental Physics, A. Richter, Summer Term 2006 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
Introduction to Measurement Techniques in Environmental Physics, A. Richter, Summer Term 2006 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
reflectivity relative speed one hour rain fall
http://weather.noaa.gov/radar/mosaic/DS.p19r0/ar.us.conus.shtml
Introduction to Measurement Techniques in Environmental Physics, A. Richter, Summer Term 2006 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
Introduction to Measurement Techniques in Environmental Physics, A. Richter, Summer Term 2006 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
Introduction to Measurement Techniques in Environmental Physics, A. Richter, Summer Term 2006 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
Introduction to Measurement Techniques in Environmental Physics, A. Richter, Summer Term 2006 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
Introduction to Measurement Techniques in Environmental Physics, A. Richter, Summer Term 2006 20
Summary
• 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
Some References to sources used
• 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
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