Comparing airborne and satellite retrievals of cloud optical thickness and particle effective radius using a spectral radiance ratio technique: two case studies for cirrus and deep convective clouds

Abstract

Abstract. Solar radiation reflected by cirrus and deep convective clouds (DCCs) was measured by the Spectral Modular Airborne Radiation Measurement System (SMART) installed on the German High Altitude and Long Range Research Aircraft (HALO) during the Mid-Latitude Cirrus (ML-CIRRUS) and the Aerosol, Cloud, Precipitation, and Radiation Interaction and Dynamic of Convective Clouds System -Cloud Processes of the Main Precipitation Systems in Brazil: A Contribution to Cloud Resolving Modelling and to the Global Precipitation Measurement (ACRIDICON-CHUVA) campaigns. On particular flights, HALO performed measurements closely collocated with overpasses of the Moderate Resolution Imaging Spectroradiometer (MODIS) aboard the Aqua satellite. A cirrus cloud located above liquid water clouds and a DCC topped by an anvil cirrus are analyzed in this paper. Based on the nadir spectral upward radiance measured above the two clouds, the optical thickness τ and particle effective radius r eff of the cirrus and DCC are retrieved using a radiance ratio technique, which considers the cloud thermodynamic phase, the vertical profile of cloud microphysical properties, the presence of multilayer clouds, and the heterogeneity of the surface albedo. For the cirrus case, the comparison of τ and r eff retrieved on the basis of SMART and MODIS measurements yields a normalized mean absolute deviation of up to 1.2 % for τ and 2.1 % for r eff . For the DCC case, deviations of up to 3.6 % for τ and 6.2 % for r eff are obtained. The larger deviations in the DCC case are mainly attributed to the fast cloud evolution and three-dimensional (3-D) radiative effects. Measurements of spectral upward radiance at near-infrared wavelengths are employed to investigate the vertical profile of r eff in the cirrus. The retrieved values of r eff are compared with corresponding in situ measurements using a vertical weighting method. Compared to the MODIS observations, measurements of SMART provide more information on the vertical distribution of particle sizes, which allow reconstructing the profile of r eff close to the cloud top. The comparison between retrieved and in situ r eff yields a normalized mean absolute deviation, which ranges between 1.5 and 10.3 %, and a robust correlation coefficient of 0.82.

Characterizing Variability of Solar Irradiance in San Antonio, Texas Using Satellite Observations of Cloudiness

“…The GOES dataset used here, if fact, uses the cloud optical thickness to parameterize the clouds. Some examples of studying the impact of clouds on solar irradiance using optical thickness include References [44][45][46].…”

Estimation of cloud optical thickness, single scattering albedo and effective droplet radius using a shortwave radiative closure study in Payerne

“…However, assuming a homogeneous and plane-parallel water cloud layer, the SSAc and the phase function of the cloud droplets play a minor role in the determination of the transmission of the cloud layer, in contrast to COT (Rawlins and Foot, 1990). Under this consideration, the shortwave radiative effect of a water cloud can be either characterised by the COT alone or by a combination of the r eff and the LWC (Leontyeva and Stamnes, 1994). For the shortwave radiation range, the extinction coefficient in clouds, and thus also COT, has a weak dependence on the wavelength (Slingo and Schrecker, 1982).…”

“…Cloud optical properties can also be estimated from airborne measurements (e.g. Finger et al, 2016;Krisna et al, 2018). Flying directly below or above clouds allows both accurate measurements and direct comparisons and validations of the COT values retrieved from satellite sensors.…”

“…A number of studies have presented methods for the retrieval of COT using data from ground-based instruments, for example, from lidars (Gouveia et al, 2017), broadband pyranometers (Leontyeva and Stamnes, 1994; Barnard and Long, 2004;Qiu, 2006), sunphotometers (Min and Harrison, 1996;Chiu et al, 2010) or UV radiometers (Serrano et al, 2014). With ground-based microwave instruments the liquid water path (LWP) is determined (Dupont et al, 2018), which can be used to calculate the cloud optical thickness, knowing or assuming r eff (Stephens, 1994).…”

Comparison of Aircraft Measurements during GoAmazon2014/5 and ACRIDICON-CHUVA

“…The SMART-Albedometer has been utilized to measure the spectral upward and downward irradiances; thereby it is called as an albedometer, as well as to measure the spectral upward radiance. The SMART-Albedometer is designed initially to cover measurements in the solar spectral range between 300 and 2,200 nm (Krisna et al, 2018;Wendisch et al, 2001;Wendisch et al, 2016). However, due to decreasing sensitivity of the spectrometers at large wavelengths, the use of the wavelengths was restricted to 300 -1,800 nm.…”