Measurement%20Techniques%20in%20Meteorology

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Introduction to Measurement Techniques in
Environmental Physics University of Bremen,
summer semester 2018 Measurement Techniques in
Meteorology Andreas Richter ( richter_at_iup.physik.
uni-bremen.de )
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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

 
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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
• gt we need cheap, simple, and automated
  measurements

 
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Which quantities do we need to measure?
 

• Quantity Units Range
• air temperature K 180..310 K
• wind speed and direction m/s, 0..100 m/s
• pressure mbar, Pa 0..1023 mbar
• humidity relative 0..100
• humidity absolute mbar, Tdew 0..23 mbar,
  180..310K
• visibility km 0.1..100 km
• cloud distribution
• cloud type
• type and amount of precipitation

 
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Measurements of air temperature I
 

• liquid filled / metallic thermometers
• effect T-dependence of volume
• use volume change ?V V0(a1- a2) ?T
• ?l ?V / A
• where A area of tube
• a1 coefficient of expansion of liquid
• a2 coefficient of expansion of reservoir
• resistance thermometer
• effect T-dependence of electrical resistance of
  platinum or nickel(e.g. Pt100 with 100 O at 0
  C )
• use R R20 (1 a ?T)
• T 20 C (R/R20 - 1) / a
• the temperature coefficient a 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
 

• 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)
• gt generally overestimation of T during the day
• gt underestimation of T during night
• gt 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
• this becomes very important for balloon (sonde)
  measurements

 
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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 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 gt 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 days examples
• dry air in heated rooms
• fogging of glasses
• white plumes above chimneys

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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

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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 es wet c (Tdry - Twet)

factor of proportionality
 
water vapour saturation pressure at Twet
water vapour partial pressure
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Measurements of air pressure
 

• mercury barometer
• effect air weight is balanced by mercury weight
  in a tube which is open on one end
• use ?p p2 p1 ?gh
• aneroid barometer
• effect sealed metal box with reduced internal
  air pressure is contracting and expanding in
  response to pressure changes
• electric readout through capacity

• dependence on
• location (through g)
• temperature (through ?)
• altitude (through both)

0
density of mercury
gravitational acceleration
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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

Force Speed km/h Name
0 lt 2 Calm
1 1-5 Light air
2 6-11 Light breeze
3 12-19 Gentle breeze
4 20-29 Moderate breeze
5 30-39 Fresh breeze
6 40-50 Strong breeze
7 51-61 Near gale
8 62-74 Gale
9 76-87 Strong gale
10 88-102 Storm
11 103-118 Violent storm 
12 119 Hurricane
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Cup anemometer measurements of wind speed

• force balance for cup anemometer
• F11/2 Cd1?A(U Ux)2
• F21/2 Cd2?A(U Ux)2
• Cd1?A(U Ux)2 Cd2?A(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
• ? is the density of air
• gt 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

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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

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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 doesnt ascend with a balloon but is
  falling on a parachute after being dropped from
  an airplane

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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

http//www.wetteronline.de/regenradar/niedersachse
n
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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.u
s.conus.shtml
reflectivity
relative speed
one hour rain fall
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Weather Radar III
http//www.wetteronline.de/regenradar/niedersachse
n
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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

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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

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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

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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
• Measurements in strong absorption bands
• T-profile

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http//www.metoffice.gov.uk/satpics/latest_VIS.htm
l
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http//www.metoffice.gov.uk/satpics/latest_VIS.htm
l
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Satellite imagery
Visible image
IR image

• Large sea land contrast
• Brightness corresponds to cloud fraction and
  cloud thickness
• Cloud height not relevant
• Only daytime observations

• Contrast depends on surface temperature
• Low clouds difficult to detect
• Brightness corresponds to cloud top height
• Day and night observations

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Excursion volcanic eruptions
MERIS 15-04-2010 Eyjafjallajoekull eruption
FLEXPART Simulations
http//savaa.nilu.no/Home/tabid/2619/Default.aspx
http//www.esa.int/esaCP/SEMFYR9MT7G_index_1.html
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Excursion volcanic eruptions

• Daily GOME-2 measurements can be analysed for
  volcanic SO2
• SO2 is often but not always linked to ash
  emissions
• Strong and very specific signal

SAVAA GOME-2 SO2 alert servicehttp//www.doas-br
emen.de/gome2_so2_alert.htm
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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

• http//www.physics.uwo.ca/whocking/p103/instrum.h
  tml
• http//de.wikipedia.org
• http//www.met.wau.nl/education/fieldpract/field2
  0course20micrometeorology202005.pdf
• http//weather.noaa.gov/radar/mosaic/DS.p19r0/ar.u
  s.conus.shtml
• http//www.usatoday.com/weather/wmeasur0.htm