Determining Cloud Liquid Water Path

1
Determining Cloud Liquid Water Path from
Radiometer measurements at Chilbolton
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Chilbolton radiometers pointing atzenith via
reflector plate angled at 45o

10 second integration
22.2, 28.8 37.5 GHz radiometers
2.5 degree beam width
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Radiometer Calibration Correction for lens in
antenna
Sky

Lens (t)
Radiometer
TB TB Tlens (1 e -t ) e t
Liquid Nitrogen Load
TB is actual brightness temperature TB is
observed brightness temperature Tlens is assumed
equal to air temperature(K) t is lens opacity
4
Radiometer Processing Steps

• Calibration of RCRU radiometers is performed
  using absorber held at temperature of liquid
  nitrogen. Calibration is not performed through
  the receiving antennas
• Additional correction has now been applied to
  measured sky brightness temperatures to account
  for the additional loss in the antenna lenses.
• Loss in Rx antennas
• 22GHz is 0.172dB / 28GHz is 0.091dB /
  37GHz is 0.24dB
• Algorithm to estimate liquid water path, and
  total integrated water vapour, from the corrected
  sky brightness temperatures is applied to 22 28
  GHz values, and to the 22 37 GHz values.
  Algorithm coefficients based on analysis of
  multiple radiosonde ascents
• Levels of integrated water vapour path are
  compared with values derived from radiosonde
  ascents made at Larkhill 35km to the West

5
Validation of IWV estimates

• Levels of integrated water vapour path are being
  compared with values of IWV derived from
• Radiosonde ascents made at Larkhill 35km to the
  West,
• GPS derived values of IWV, and
• ECMWF model data.

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• Liquid water path for 19th June 2003

LWP retrieval error estimated at /-
0.025 Kg/m2
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Cloud Cover at Chilbolton on 19th June 2003
405 UT 420 435 450
505 520 535
550 605 620 635
650 705 720
735 750 805 820
835 850 905
920 935 950 1005
1020 1035 1050
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CFARR radiometer data - status

Quality Controlled microwave radiometer data
from CFARR is now available on the BADC
at http//badc.nerc.ac.uk/cgi-bin/data_browser/da
ta_browser/badc/chilbolton/data/radiometers-microw
ave Data set includes - Brightness
Temperatures at 22.2 GHz and 28.8 GHz -
Integrated water vapour path - Liquid water
path. Corrections for absorption in lens have
been applied. Processed data already on BADC-
15th May 2003 to 29th February 2004. 1st to
31st March 2004 data should be available now
Generally 95 of data in a month is
available
9

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LWP IWV retrieval algorithms

• Temperature and Water Vapour profiles from all
  radiosonde ascents at Larkhill during period 1997
  2000 were processed.
• Cloud liquid water was estimated for each
  radiosonde ascent.
• Estimate of LWP was based on the comparison of
  a critical humidity function as defined by
  Salonen, and relative humidity.
• To ensure that precipitation did not skew the
  retrieval algorithms, all those radiosonde
  ascents for which the estimated values of LWP
  were greater than 0.5kg.m-2 were discarded.
• Using the remaining 2190 radiosonde ascents as a
  training set, values of brightness temperature at
  the three radiometer frequencies were calculated
  by applying the model for predicting atmospheric
  absorption that is described by Liebe.
• From that data set, coefficients for a transfer
  function of the form
• LWP b0 SbiBTi
• i represents one of the radiometer frequencies
• were established by multiple linear regression.
  
• By applying the established transfer functions,
  LWP and IWV were estimated by combining sky
  brightness temperatures recorded at 2, (or 3),
  frequencies.

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Example of stratoculus cloud observed on 19th
June 2003 during CWAVE03 at Chilbolton

• Subsequent slides show
• Cloud field observed at zenith during the day.
• Period from 1050 to 1505 showed total
  cloud cover at zenith
• 
  (Field of view is 60 degrees by 45 degrees)
• 94 GHz radar reflectivity
• IR ceilometer backscatter
• Two retrievals of liquid water path estimated
  from brightness temperature measurements at 22.2,
  28.8 and 37.6 GHz

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• Liquid water path for 19th June 2003

LWP retrieval error estimated at /- 0.03
kg.m-2
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• 94 GHz radar reflectivity

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• Attenuated backscatter coefficient

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• Liquid water path for 19th June 2003

LWP retrieval error estimated at /-
0.025 Kg/m2
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Multifrequency mm-wave Radiometer (MFR)
On loan at Chilbolton

• Specification
• 22.235 GHz /- 170 MHz
• 23.87 GHz /- 135 MHz
• 31.65 GHz /- 120 MHz
• 2 degree beamwidth
• 1 second integration time

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Validation of LWP estimates

• Levels of liquid water path have been compared
  with values of LWP derived from
• Multi-frequency radiometer (MFR) measurements.

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Radiometer Calibration

• Tip curve calibrations are not possible.
• Calibration of RCRU radiometers is performed
  using black-body absorbers held at ambient
  temperature, and the temperature of liquid
  nitrogen. Calibration is not performed through
  the receiving antenna
• Additional correction is applied to measured sky
  brightness temperatures to account for the
  additional loss in the antenna lens.
• Loss in radiometer antennas
• 22GHz is 0.172dB / 28GHz is 0.091dB /
  37GHz is 0.24dB

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AEROSOL measurements

Cimel sun-photometer on loan from NERC until end
of December 2003
Aerosol optical depth Aerosol particle size (0.1
3 mm)
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