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Radiometer retrievals of LWP

This study combines statistical and physical methods to estimate key parameters for radiometer retrievals of Liquid Water Path (LWP). The analysis involves measuring Tb at specific frequencies, using ECMWF model data and radiosonde observations. A bi-linear regression approach is employed to derive a22, a28, and b, with a focus on maintaining accuracy in both clear sky and cloudy conditions.

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Radiometer retrievals of LWP

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  1. Radiometer retrievals of LWP Nicolas GAUSSIAT

  2. Statistical/physical retrieval Physical retrieval : kl, kv,d are estimated using GCM model vertical information, then with Statistical retrieval : a22, a28 and b are assumed to be constant and directly deduced from observations of Tb and LWP SOME BASICS: Radiometers measure Tb at 22.2 GHz end 28.8 GHz with

  3. Model data /radiosondes 2 years of model data LWP 4 years of radiosondes VWP

  4. Physical retrieval • (Kl22, kv22, d22),(Kl28, kv28, d28) are estimated using GCM model (ECMWF) • for each profiles, and a22, a28 and b are deduced. a22 Kl22, Kl28 Kv22, Kv28 a28 d22, d28 b

  5. Statistical retrieval Use of a bi-linear regression to work out a22, a28 and b directly We assume that LWPi depends linearly on the optical depths 22i and 28i Ideally, we want to find a22, a28, b for all the N measurements : a2222i+ a2828i+ b =LWPi But the best we can do is to minimize the sum : R2=(a2222i+ a2828i+ b - LWPi)2 In matrices notations A=[…(22i28i1) ;…(22N28N1) ] X=[a22 ;a28 ; b] Y=[…LWPi ;… LWPN] X = (AtA)-1 AtY

  6. Results can look good but … statistical physical clear sky • Lidar identify liquid-cloud-free regions (LWP should be 0)

  7. when it comes to the details… • Fluctuations must be due to a calibration problem… minutes

  8. Cunning technique… • In clear sky conditions LWP=0 so we have : • then (1) • A second constrain is found by minimizing the cost fct J, • then (2) • So, from (1) and (2), we work out C22 , C28 in the clear sky regions and interpolate values in between

  9. …cunning results(1) • Chilbolton offset (lens ?) … solved

  10. …cunning results(2) • Palaiseau calibration problem… not a problem.

  11. Further improvement • Lidar measure cloud-base and using model temperature a better estimate of Kv is obtained.

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