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Determining Cloud Liquid Water Path from Radiometer measurements at Chilbolton

Determining Cloud Liquid Water Path from Radiometer measurements at Chilbolton. Chilbolton radiometers pointing at zenith via reflector plate angled at 45 o. 10 second integration. 22.2, 28.8 & 37.5 GHz radiometers. ~2.5 degree beam width.

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Determining Cloud Liquid Water Path from Radiometer measurements at Chilbolton

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  1. Determining Cloud Liquid Water Path from Radiometer measurements at Chilbolton

  2. 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

  3. Radiometer Calibration & Correction for lens in antenna Sky Lens (t) Radiometer TB = [ T’B – Tlens (1 – e -t )] e t Liquid Nitrogen Load TB is actual brightness temperature T’B is observed brightness temperature Tlens is assumed equal to air temperature(K) t is lens opacity

  4. 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] Radiometer Processing Steps

  5. 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. Validation of IWV estimates

  6. Liquid water path for 19th June 2003 LWP retrieval error estimated at: +/- 0.025 Kg/m**2

  7. Cloud Cover at Chilbolton on 19th June 2003 4:05 UT 4:20 4:35 4:50 5:05 5:20 5:35 5:50 6:05 6:20 6:35 6:50 7:05 7:20 7:35 7:50 8:05 8:20 8:35 8:50 9:05 9:20 9:35 9:50 10:05 10:20 10:35 10:50

  8. 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/data_browser/badc/chilbolton/data/radiometers-microwave 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. 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. LWP & IWV retrieval algorithms

  10. Subsequent slides show: Cloud field observed at zenith during the day. [Period from 10:50 to 15:05 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 Example of stratoculus cloud observed on 19th June 2003 during CWAVE’03 at Chilbolton

  11. Liquid water path for 19th June 2003 LWP retrieval error estimated at: +/- 0.03 kg.m-2

  12. 94 GHz radar reflectivity

  13. Attenuated backscatter coefficient

  14. Liquid water path for 19th June 2003 LWP retrieval error estimated at: +/- 0.025 Kg/m**2

  15. 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

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

  17. 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] Radiometer Calibration

  18. AEROSOLmeasurements 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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