Simultaneous retrieval of water vapour, temperature and cirrus clouds properties from measurements of far infrared spectral radiance over the Antarctic Plateau

Abstract

Abstract. The possibility separating the contributions of the atmospheric state and ice clouds by using spectral infrared measurements is a fundamental step to quantifying the cloud effect in climate models. A simultaneous retrieval of cloud and atmospheric parameters from infrared wideband spectra will allow the disentanglement of the spectral interference between these variables. In this paper, we describe the development of a code for the simultaneous retrieval of atmospheric state and ice cloud parameters, and its application to the analysis of the spectral measurements acquired by the Radiation Explorer in the Far Infrared -Prototype for Applications and Development (REFIR-PAD) spectroradiometer, which has been in operation at Concordia Station on the Antarctic Plateau since 2012. The code performs the retrieval with a computational time that is comparable with the instrument acquisition time. Water vapour and temperature profiles and the cloud optical and microphysical properties, such as the generalised effective diameter and the ice water path, are retrieved by exploiting the 230-980 cm −1 spectral band. To simulate atmospheric radiative transfer, the LineBy-Line Radiative Transfer Model (LBLRTM) has been integrated with a specifically developed subroutine based on the δ-Eddington two-stream approximation, whereas the singlescattering properties of cirrus clouds have been derived from a database for hexagonal column habits. In order to detect ice clouds, a backscattering and depolarisation lidar, co-located with REFIR-PAD has been used, allowing us to infer the position and the cloud thickness to be used in the retrieval. A climatology of the vertical profiles of water vapour and temperature has been performed by using the daily radiosounding available at the station at 12:00 UTC. The climatology has been used to build an a priori profile correlation to constrain the fitting procedure. An optimal estimation method with the Levenberg-Marquardt approach has been used to perform the retrieval. In most cases, the retrieved humidity and temperature profiles show a good agreement with the radiosoundings, demonstrating that the simultaneous retrieval of the atmospheric state is not biased by the presence of cirrus clouds. Finally, the retrieved cloud parameters allow us to study the relationships between cloud temperature and optical depth and between effective particle diameter and ice water content. These relationships are similar to the statistical correlations measured on the Antarctic coast at Dumont d'Urville and in the Arctic region.

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

“…Temperature and water vapour concentrations for each layer were obtained by interpolating the corresponding dropsonde measurements onto the 0.1 km vertical grid, with the exception of the layer closest to the aircraft which was prescribed by the onboard measurements. In the absence of direct measurements, concentrations of CO 2 and minor trace gases were obtained from a standard midlatitude winter (MLW) atmospheric profile (Anderson et al, 1986) and scaled to present-day concentrations using data from Mace Head (Dlugokencky et al, 2019). Ozone concentrations were obtained from co-located ERA-I data and interpolated to the required vertical resolution.…”

“…Unsurprisingly, these showed excellent agreement with the observations in the MIR window from 820 to 960 cm −1 but large residuals were apparent in the FIR from 330 to 600 cm −1 . More recent work by Palchetti et al (2016) and di Natale et al (2017) has suggested that simulated and observed downwelling radiances at the surface can be reconciled within uncertainties across much of the FIR and MIR range if temperature, water vapour and cirrus properties are simultaneously retrieved from the observations. Maestri et al (2019) reported the first tentative retrievals of ice particle habit from downwelling FIR spectra obtained by REFIR-PAD over Antarctica in 2013, although they noted that large residuals in their retrievals did not permit clear identification of ice particle habit in most cases.…”

Antarctic Ice Cloud Identification and Properties Using Downwelling Spectral Radiance From 100 to 1,400 cm−1

“…Data analyzed in the present work are collected at the Italian‐French research station of Concordia, located on the Antarctic Plateau at Dome C (75° 06 ′ S, 123° 23 ′ E) at 3.233 m a.s.l. Several scientific objectives are pursued by this experiment (Liuzzi et al, ; Rizzi et al, ; Di Natale et al, ). Among them, of our interest, is the study of clouds and the improvement of radiative transfer forward models and retrieval methodologies with particular attention devoted to the underexplored FIR region.…”

Spaceborne Middle‐ and Far‐Infrared Observations Improving Nighttime Ice Cloud Property Retrievals

“…Between 400 and 600 cm −1 , several semitransparent FIR “dirty window” bands where water vapor absorption is moderate (Rathke et al, 2002) are useful for ice cloud property retrievals as they have sufficient sensitivity to ice cloud properties (Yang et al, 2003). Most previous studies focus on the sensitivity of ground‐based FIR measurements to ice cloud properties (Di Natale et al, 2017; Maestri et al, 2014; Mlynczak et al, 2016), and only a few studies investigate the corresponding sensitivity of upwelling FIR signals (Bantges et al, 2020; Cox et al, 2010). Libois and Blanchet (2017) demonstrate that synergistic measurements of upwelling MIR‐FIR radiation significantly reduce the uncertainty in ice cloud property retrievals.…”