METADATA TO DOCUMENT SURFACE OBSERVATION

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METADATA TO DOCUMENT SURFACE OBSERVATION

• Michel Leroy, Météo-France

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METADATA

• Metadata is necessary to use efficiently observed
  data.
• Latitude, longitude, altitude, station Id., date
  and time are obvious metadata.
• A detailed description of the site and the
  instruments used, their characteristics, the
  historic of any instrument and site change, etc.
  is highly recommended and wished by
  climatologists. But the way to document this
  information is not yet standardized, this
  information is often missing and when available,
  the information is not easy to use by automatic
  means, due to its complexity.
• The site environment is one of the important
  factor affecting a field measurement and its
  representativeness for various applications.
• Though quite well known by the meteorological
  services, WMO siting recommendations are not
  always followed in the real world (or cannot be
  followed).
• It is the same thing for the measurement
  uncertainty when compared to recommended and
  achievable measurement uncertainty stated in WMO
  doc n8 (CIMO Guide),

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Some condensed metadata

• In order to document the site environment and the
  sustained characteristics of the measurement
  system in an easy to handle way, Météo-France has
  defined two classifications
• A siting classification, ranging from 1 to 5, for
  each basic parameter.
• A maintained performance classification,
  ranging from A to E, for each basic measurement.
• Reducing the site characteristics and the
  equipments performances to single numbers or
  letters hide many interesting details, but a
  major advantage is to let the results easy to
  use. And these single numbers dont restrict an
  additional detailed documentation (such as
  photos).
• The definition of these classifications is coming
  from an initial analysis of quality factors
  influencing a measurement

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Drawbacks

• These classifications dont allow any corrections
  of the data. They are not developed for that.
• Especially for wind, may be for precipitation,
  some correction methods exist and could be
  applied. These methods need a detailed knowledge
  of the site environment and sometimes additional
  parameters. There would be a great interest in
  applying standardized methods to correct raw
  measurements using the available metadata of a
  site. But the set of metadata needed to apply
  corrections is not clearly defined or
  standardized (except for wind for the reduction
  of the measured wind to a standard wind at 10
  meters with a roughness length of 0.03 m). It
  would be ideal to have them, but this approach
  may be impracticable in the real world.
• The advantage of the proposed classification is
  its practicability in the real world, therefore
  adding a practicable value to the information.

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Quality factors of a measurement

• The intrinsic characteristics of sensors or
  measurement methods
• The maintenance and calibration needed to
  maintain the system in nominal conditions.
• The site representativeness

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Site representativeness

• Exposure rules from CIMO recommendations.
• But not always followed and not always possible
  to follow, depending on the geographical
  situation.
• In 1997, Météo-France defined a site
  classification for some basic surface variables.
• Class 1 is for a site following WMO
  recommendations
• Class 5 is for a site which should be absolutely
  avoided for large scale or meso-scale
  applications.
• Class 2, 3 and 4 are intermediate
• This classification has been presented during
  TECO98 in Casablanca.

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Classification for wind measurements

•  Roughness classification Davenport, see CIMO
  Guide, WMO Doc n8
• Siting classification
• The existence of obstacles nearly always lead to
  a decrease of the mean wind speed. Extreme values
  are generally also decreased, but not always.
  Obstacles increase turbulence and may lead to
  (random) temporary increase of instantaneous wind
  speed.
• The following classes are considering a
  conventional 10 m measurement.

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• Class 1
• -      The wind tower must be erected at a
  distance of at least 10 times the height of the
  nearby obstacles (therefore seen under an
  elevation angle below 5.7)
• -      An object is considered as an obstacle if
  it is seen under an angular width greater than
  10.
• -      The obstacles must be below 5.5 m within a
  150 m distance around the tower (and if possible
  be below 7 m within a 300 m distance).
• -      The wind sensors must be located at a
  minimum distance of 15 times the width of thin
  nearby obstacles (mast, thin tree with angular
  width lt 10).
• -      The surrounding country must not present
  any relief change within a 300 m radius. A relief
  change is a 5 m height change.

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• Class 2 (error 10 ?)
• -      The wind tower must be erected at a
  distance of at least 10 times the height of the
  nearby obstacles (elevation angle lt 5.7)
• -      An object is considered as an obstacle if
  it is seen under an angular width greater than
  10.
• -      A relief change within a 100 m radius is
  also considered as an obstacle.
• -      The wind sensors must be located at a
  minimum distance of 15 times the width of thin
  nearby obstacles (mast, thin tree with angular
  width lt 10).
•  Class 3 (error 20 ?)
• -      The wind tower must be erected at a
  distance of at least 5 times the height of the
  nearby obstacles (elevation angle lt 11.3)
• -      A relief change within a 50 m radius is
  also considered as an obstacle.
• -      The wind sensors must be located at a
  minimum distance of 10 times the width of thin
  nearby obstacles.

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• Class 4 (error 30 ?)
• -      The wind tower must be erected at a
  distance of at least 2.5 times the height of the
  nearby obstacles (elevation angle lt 21.8)
•  
• Class 5 (error gt 40 ?)
• - Obstacles are existing at a distance less than
  2.5 times their height.
• - Obstacles with a height greater than 8 m, at a
  distance less than 25 m.

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St-SulpiceNord
Est
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St-SulpiceSud
Ouest
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St-Sulpice. Relevé de masques

• Class 4 for wind.
• New Radome AWS settled at a distance of 60 m,
  away from the woods ? class 3

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Saint Sulpice, DIRCERatio of mean wind speed (10
min.) between Patac et XariaSouth winds
North winds
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Classification of stations

• Between 2000 and 2006, 400 AWS have been
  installed for the Radome network.
• The objective was class 1 for each parameter
  (Temp, RH, wind, precip., solar radiation).
• But class 2 or class 3 were accepted when class 1
  not possible.
• Météo-France is now classifying al the surface
  observing stations, including the climatological
  cooperative network 4300 sites, before the end
  of 2008.
• Update at least every 5 years.

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Where are we ?
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Other quality factors

• Intrinsic performances
• Maintenance and calibration
• Within a homogeneous network, these factors are
  known and generally the same. But Météo-France is
  using data from various networks
• Radome (554)
• Non-proprietary AWS (800)
• Climatological cooperative network (gt 3000)
• The intrinsic performances, maintenance and
  calibration procedures are not the same.

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Several reasons

• The objectives may be different.
• But some uncertainty objectives are sometimes
  (often) unknown !
• To get cheap measurements ?
• The maintenance and/or the calibration are not
  always organized !
• Within the ISO 9001-2000 certification process,
  Météo-France was forced to increase his knowledge
  of the various networks characteristics.

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Another classification !

• After site classification (1 to 5), definition of
  an additional classification, to cover the two
  quality factors
• Intrinsic performances
• Maintenance and calibration
• 5 levels were defined
• Class A WMO/CIMO recommendations (Annex 1B of
  CIMO guide)
• Class B Lower specs, but more realistic or
  affordable good performances and good
  maintenance and calibration. RADOME specs.
• Class C Lower performances and maintenance, but
  maintenance/calibration organized.
• Class D No maintenance/calibration organized.
• Class E Unknown performances and/or maintenance
• This classification is called Maintained
  performance classification

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Air temperature

• Class A Overall uncertainty of 0.1C. Therefore,
  the uncertainty of the temperature probe lower
  than 0.1C and use of a perfect artificially
  ventilated screen. Achievable measurement
  uncertainty is 0.2C.
• Class B Pt100 (or Pt1000) temperature probe of
  class A (? 0.25C). Acquisition uncertainty lt
  0.15C. Radiation screen with known
  characteristics and over-estimation of Tx (daily
  max. temperature) lt 0.15C in 95 of cases.
  Laboratory calibration of the temperature probe
  every 5 years.
• Class C Temperature probe with uncertainty lt
  0.4C. Acquisition uncertainty lt 0.3C. Radiation
  screen with known characteristics and
  over-estimation of Tx lt 0.3C in 95 of cases.
• Class D Temperature probe and/or acquisition
  system uncertainty lower than for class C.
  Radiation screen or with unacceptable
  characteristics (for example, over-estimation of
  Tx gt 0.7C in 5 of cases).

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Relative humidity

• Class A Overall uncertainty of 1! Achievable
  2.
• Class B Sensor specified for ? 6, over a
  temperature range of 20C to 40C. Acquisition
  uncertainty lt 1. Calibration every year, in an
  accredited laboratory.
• Class C Sensor specified for ? 10, over a
  temperature range of 20C to 40C. Acquisition
  uncertainty lt 1. Calibration every two years in
  an accredited laboratory, or calibration every
  year in a non-accredited laboratory.
• Class D Sensor with specifications worst than ?
  10 over the common temperature conditions.
  Calibration not organized.

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Global solar radiation

• Class A Pyranometer of ISO class 1. Uncertainty
  of 5 for daily total. Ventilated sensor.
  Calibration every two years. Regular cleaning of
  the sensor (at least weekly).
• Class B Pyranometer of ISO class 1. No
  ventilation. Calibration every two years. No
  regular cleaning of the sensor.
• Class C Pyranometer of ISO class 2. No
  ventilation. Calibration every five years. No
  regular cleaning of the sensor.
• Class D Sensor not using a thermopile.
  Calibration not organized.

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Other parameters

• Pressure
• Amount of precipitation
• Wind
• Visibility
• Temperature above ground
• Soil temperature

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Status of the RADOME network

• Air temperature Class B
• RH Class B
• Amount of precipitation Class B or Class C,
  depending on the rain gauge used.
• Wind Class A
• Global solar radiation Class A for manned
  station, class B for isolated sites.
• Ground temperatures Class B
• Pressure Class B
• Visibility (automatic) Class B

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Status of the cooperative network

• Air temperature (liquid in glass thermometers)
  Class C
• Amount of precipitation Class B

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Status of non-Météo-France additional networks

• Air temperature Class B to D
• RH Class B to D
• Amount of precipitation Class B to C
• Wind Class B to D
• Global solar radiation Class B to D
• Ground temperature Class B to C
• Pressure Class B to D

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Metadata

• These classification for each site are meta data,
  part of the climatological database.
• Site classification is on going.
• Maintained performance classification has been
  defined this year and is being applied is it
  possible to easily classify the additional
  networks.
• With these two classifications, a measurement on
  a site can be given a short description.
• Example C3 for global solar radiation is for a
  class 2 pyranometer without ventilation,
  calibrated every 2 years, installed on a site
  with direct obstructions, but below 7.

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An image of a network
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An image of the RADOME network
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Conclusion

• These classifications are intended to describe
  the real world of measuring networks, which is
  sometimes far form the WMO/CIMO recommendations.
• WMO (CIMO, CBS) has decided to develop a site
  classification, on the example of this
  classification. Such a standard would be further
  recognized by ISO.
• This topic has been recently discussed by the
  CIMO Expert Team on Surface Technology and
  Measurement Techniques.
• Any suggestions or comments are welcomed. To be
  addressed to Michel Leroy

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Proposed change for precipitation

• Change class 1 for having in class 1 a well
  protected site homogeneous obstacles around the
  rain gauge which can reduce the wind speed at the
  gauge level.
• Class 2 unchanged no obstacles closer than 2
  times their height.

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Proposed change for temperature/humidity

• To use the climatology of wind for temperature
  classification.
• of low wind speed ( lt 1.5 or 2 m/s) ?
• 
  Trappes
• 
  St Denis, La Réunion

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Proposed change for temperature/humidity

• The perturbation from artificial surface is
  greatly reduce with wind. With a 1 m/s wind, the
  air moves by 60 m in one minute. The frequency of
  mean wind speed (at 10 m) below 1.5 m/s could be
  used to reduce the influence of artificial
  surface in the classification.
• The shading conditions currently used are a big
  constraint. It could be partly replaced by the
  global angle of view of obstacles
• No obstacles, angle of view is 0
• Obstacles everywhere angle of view is 2P
  (100).
• Screen along a wall angle of view is P (50).
• Angle of view thresholds could be 5, 10, 20
• But more difficult to evaluate.