Comparison of measured and modeled stratus cloud infrared spectral signatures

Abstract

Abstract. The purpose of the paper is to evaluate our capability to model upwelling radiances at high resolution over broad spectral ranges in clear and cloudy conditions. Spectroradiometric measurements are taken from aircraft flying at low altitude over a clear scene and over a low-level stratus cloud. It is the ideal case to capture the change in signal due to the presence of a cloud without the interfering effect of the atmosphere above, unavoidable in higher-altitude measurements. The knowledge of the variability of the radiometric signal over the measurement periods and the availability of in situ measurement from the same aircraft of the meteorological parameters (temperature and humidity profile) and cloud parameters (depth and microphysical parameters of the cloud layer) allow a nearly complete control of the experiment. The results for the clear case show deviations between simulation and measurement generally smaller than 0.5%, while in the cloudy case, differences over most of the spectral range are less than the variability seen during the recording of the measured spectra. The cloud result is not significantly changed when tested with realistic changes in the measured cloud properties. Over cloud an absorbing medium alone is insufficient, and full scattering is required to achieve good agreement. This shows that care is required when assimilating infrared radiances from above such clouds, which can often extend over large areas. The adopted simulation methodology generates spectral radiances in very good agreement with the measurements both in clear air and above the stratus cloud layers.

The Role of Emissivity in the Detection of Arctic Night Clouds

“…Cloudy sky radiance spectra are simulated by using the Radiative Transfer model (RTX) [46][47][48][49]. RTX solves the radiative transfer equation with the adding and doubling method taking into account multiple scattering by randomly-oriented particles.…”

Combining visible and infrared radiometry and lidar data to test simulations in clear and ice cloud conditions

“…Post-processing of high resolution radiances to produce un-apodised spectra at S-HIS resolution is done as described in Rizzi et al (2001). Simulated MAS radiances are obtained by convolving the spectral values at the indicated resolution with the MAS response functions (NASA, cited 2008).…”

“…The present study was initiated to evaluate the quality of a forward modeling methodology, called Line-By-Line Multiple Scattering (LBLMS), that is an extension to the shortwave of a state-of-the-art methodology already used to simulate high resolution spectral data in the infrared spectral range, from 3000 to 50 cm −1 (Rizzi et al, 2001;Amorati and Rizzi, 2002;Maestri and Rizzi, 2003;Tjemkes et al, 2003;Rizzi and Maestri, 2003;Maestri et al, 2005).…”

Atmospheric composition and thermodynamic retrievals from the ARIES airborne FTS system – Part 1: Technical aspects and simulated capability

“…Data from ARIES have been used operationally to remotely retrieve surface parameters such as T s and ε s (e.g. Newman et al, 2005;Thelen et al, 2009); and to evaluate the infrared signatures of cirrus clouds (Baran and Francis, 2004), stratus clouds (Rizzi et al, 2001), and Saharan dust aerosol (Highwood et al, 2003). The high spectral resolution and excellent radiometric calibration and noise characteristics of the instrument have also allowed ARIES to be used to validate and characterise improvements in reference spectroscopic data sets such as the HIgh-resolution TRANsmission molecular absorption (HITRAN) database (e.g.…”