Climatology of estimated liquid water content and scaling factor for warm clouds using radar–microwave radiometer synergy

• Published in: Atmospheric measurement techniques 2023-03, Vol.16 (5), p.1211-1237
• Main Authors: Vishwakarma, Pragya, Delanoë, Julien, Jorquera, Susana, Martinet, Pauline, Burnet, Frederic, Bell, Alistair, Dupont, Jean-Charles
• Format: Article
• Language: English
• Subjects: Algorithms; Analysis; Atmospheric and Oceanic Physics; Climate; Climatology; Cloud droplets; Cloud systems; Cloud types; Clouds; Constants; Continuous radiation; Earth Sciences; Estimation; Fog; Frequency dependence; Frequency modulation; Heterogeneity; In situ measurement; Laws, regulations and rules; Liquid water content; Meteorology; Microwave radiometers; Moisture content; Parameter sensitivity; Parameters; Perturbation; Physics; Power law; Radar; Radar reflectivity; Radiometers; Reflectance; Remote sensing; Retrieval; Scaling; Scaling factors; Sciences of the Universe; Sea level; Sensitivity analysis; Sensors; Statistical analysis; Statistical methods; Statistics; Uncertainty; Water; Water content
• ISSN: 1867-8548; 1867-1381; 1867-8548
• DOI: 10.5194/amt-16-1211-2023

Cloud radars are capable of providing continuous high-resolution observations of clouds and now offer new capabilities within fog layers thanks to the development of frequency-modulated continuous-wave 95 GHz cloud radars. These observations are related to the microphysical properties of clouds. Power law relations in the form of Z=aâLWCb are generally used to estimate liquid water content (LWC) profiles. The constants a and b from the power law relation vary with the cloud type and cloud characteristics. Due to the variety of such parameterizations, selecting the most appropriate Z-LWC relation for a continuous cloud system is complicated. Additional information such as liquid water path (LWP) from a co-located microwave radiometer (MWR) is used to scale the LWC of the cloud profile. An algorithm for estimating the LWC of fog and warm clouds using 95 GHz cloud radar-microwave radiometer synergy in a variational framework is presented. This paper also aims to propose an algorithm configuration that retrieves the LWC of clouds and fog using radar reflectivity and a climatology of the power law parameters. To do so, variations in the scaling factor ln a (the logarithm of pre-factor a from power law relation) when MWR observations are available are allowed in each cloud profile to build a climatology of the scaling factor ln a that can be used when MWR observations are not available. The algorithm also accounts for attenuation due to cloud droplets. In this algorithm formulation, the measure of uncertainty in the observations, the forward model, and the a priori information of desired variables acts as weights in the retrieved quantities. These uncertainties in the retrieval are analyzed in the sensitivity analysis of the algorithm. The retrieval algorithm is first tested on a synthetic profile for different perturbations in sensitivity parameters. The sensitivity study has shown that this method is susceptible to LWP information because LWP is point information for the whole cloud column. By further investigating the sensitivity analysis of various biases in LWP information, it was found that it is beneficial to incorporate LWP, even if it is biased, rather than not assimilate any LWP.