Transcript Measurement Techniques in Meteorology

Introduction to Measurement Techniques in
Environmental Physics
University of Bremen, summer term 2006
Measurement Techniques in Meteorology
Andreas Richter ( [email protected] )
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 exchange (Savigny)
Measurement Techniques
in Soil physics (Fischer)
Introduction to Measurement Techniques in Environmental Physics, A. Richter, Summer Term 2006
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Overview
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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
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Which quantities do we need to measure?
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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?
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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
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Measurements of air temperature I
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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
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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
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Measurements of air temperature II
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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
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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:
w
mv
md
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:
RH  100
w
ws
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
Introduction to Measurement Techniques in Environmental Physics, A. Richter, Summer Term 2006
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Measurements of air humidity I
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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
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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
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Introduction to Measurement Techniques in Environmental Physics, A. Richter, Summer Term 2006
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Measurements of air humidity II
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dew point hygrometer
• effect: detection of dew on temperature
controlled mirror by observation of
change in reflectance
• very accurate
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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
Introduction to Measurement Techniques in Environmental Physics, A. Richter, Summer Term 2006
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Measurements of air pressure
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mercury barometer
• effect: air weight is
balanced by mercury weight
in a tube which is open on
one end
• use:
Δp = p2 – p1 = ρgh
density of mercury
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gravitational acceleration
aneroid barometer
• effect: sealed metal box
with reduced internal air
pressure is contracting and
expanding in response to
pressure changes
Introduction to Measurement Techniques in Environmental Physics, A. Richter, Summer Term 2006
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Measurements of wind speed and direction
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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
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Cup anemometer measurements of wind speed
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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
Ux  U
1  Cd 1Cd 2
 3U
Cd 1  Cd 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
Introduction to Measurement Techniques in Environmental Physics, A. Richter, Summer Term 2006
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Measurements of precipitation
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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
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gauge may alter air flow and thus precipitation locally
wind shields are necessary
optical measurement relies on assumptions on droplet
size
Introduction to Measurement Techniques in Environmental Physics, A. Richter, Summer Term 2006
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Measurements of upper air weather
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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
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Weather Radar I
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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
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Weather Radar II
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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
Introduction to Measurement Techniques in Environmental Physics, A. Richter, Summer Term 2006
one hour rain fall
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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
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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
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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
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Satellite imagery
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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)
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• 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
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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
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http://www.physics.uwo.ca/~whocking/p103/instrum.html
http://de.wikipedia.org
http://www.met.wau.nl/education/fieldpract/field%20course%2
0micrometeorology%202005.pdf
http://weather.noaa.gov/radar/mosaic/DS.p19r0/ar.us.conus.shtml
http://www.usatoday.com/weather/wmeasur0.htm
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