Airborne validation of radiative transfer modelling of ice clouds at millimetre and sub-millimetre wavelengths

Abstract

Abstract. The next generation of European polar orbiting weather satellites will carry a novel instrument, the Ice Cloud Imager (ICI), which uses passive observations between 183 and 664 GHz to make daily global observations of cloud ice. Successful use of these observations requires accurate modelling of cloud ice scattering, and this study uses airborne observations from two flights of the Facility for Airborne Atmospheric Measurements (FAAM) BAe 146 research aircraft to validate radiative transfer simulations of cirrus clouds at frequencies between 325 and 664 GHz using the Atmospheric Radiative Transfer Simulator (ARTS) and a state-of-the-art database of cloud ice optical properties. Particular care is taken to ensure that the inputs to the radiative transfer model are representative of the true atmospheric state by combining both remote-sensing and in situ observations of the same clouds to create realistic vertical profiles of cloud properties that are consistent with both observed particle size distributions and bulk ice mass. The simulations are compared to measurements from the International Submillimetre Airborne Radiometer (ISMAR), which is an airborne demonstrator for ICI. It is shown that whilst they are generally able to reproduce the observed cloud signals, for a given ice water path (IWP) there is considerable sensitivity to the cloud microphysics, including the distribution of ice mass within the cloud and the ice particle habit. Accurate retrievals from ICI will therefore require realistic representations of cloud microphysical properties.

Synergistic radar and radiometer retrievals of ice hydrometeors

“…In general, the passive-only and the combined retrievals display stronger sensitivity to the assumed particle shape than the radar-only retrieval. This is plausible since the increased sensitivity especially of the sub-millimeter radiometer channels has been highlighted in several studies Fox et al, 2019).…”

Revisiting particle sizing using greyscale optical array probes: evaluation using laboratory experiments and synthetic data

“…This paper has described tests on a greyscale OAP using a droplet generator, the results of which have been compared to synthetic data. Despite recent advances in holographic instruments for cloud microphysical measurements (Fugal and Shaw, 2009) work is still needed to better characterise the uncertainties associated with this technique. Additionally holographic probes require high-performance computers to postprocess the significant amounts of data they generate (e.g.…”

“…Therefore a minimum size threshold of 35 µm is applied, above which it is estimated that the probe's detection rate is greater than 90 % (Schlenczek, 2017). Shattered particles were minimised by removing all particles with inter-particle distances of less than 10 mm (Fugal and Shaw, 2009;O'Shea et al, 2016).…”

“…These measurements provide insights into key cloud microphysical processes such as ice nucleation, particle growth and precipitation (Field, 1999;Lawson et al, 2015). In situ measurements are an important means to constrain remote sensing retrievals, which are routinely used to initialise operational weather forecast models (Fox et al, 2019;Mace and Benson, 2017).…”

A test of the ability of current bulk optical models to represent the radiative properties of cirrus cloud across the mid-and far-infrared

“…The lidar installed on the aircraft was a Leosphere ALS450 355nm elastic backscatter lidar [Marenco, 2010] which provided cloud vertical profile information and ice volume extinction profiles at 355 nm from the range-corrected backscatter profiles following the two-stage process of Marenco et al [2011]. Vertical profiles of particle extinction coefficient were estimated [Fox et al, 2019] from which the cloud optical depth at 355 nm, (τ355), was derived. Finally, along with the 100 standard aircraft positioning sensors, temperature and humidity sensors, the aircraft was also equipped with the Airborne Vertical Atmospheric Profiling System (AVAPS) [Vaisala, 1999] which launched RD94 dropsondes [Vaisala, 2010] at various stages in the flight to enable characterisation of the atmospheric column below the aircraft.…”