Abstract
Abstract. This paper presents a study of the impact of cirrus cloud heterogeneities on the thermal infrared brightness temperatures at the top of the atmosphere (TOA). Realistic 3-D cirri are generated by a cloud generator based on simplified thermodynamic and dynamic equations and on the control of invariant scale properties. The 3-D thermal infrared radiative transfer is simulated with a Monte Carlo model for three typical spectral bands in the infrared atmospheric window. Comparisons of TOA brightness temperatures resulting from 1-D and 3-D radiative transfer show significant differences for optically thick cirrus (τ > 0.3 at 532 nm) and are mainly due to the plane-parallel approximation (PPA). At the spatial resolution of 1 km × 1 km, two principal parameters control the heterogeneity effects on brightness temperatures: i) the optical thickness standard deviation inside the observation pixel, ii) the brightness temperature contrast between the top of the cirrus~and the clear-sky atmosphere. Furthermore, we show that the difference between 1-D and 3-D brightness temperatures increases with the zenith view angle from two to ten times between 0° and 60° due to the tilted independent pixel approximation (TIPA).
Highlights
Cirrus clouds cover 15 % to 40 % of the Earth’s surface (Sassen et al, 2008)
CIRCLE II was an airborne campaign dedicated to the study of cirrus optical properties and the validation of space measurements made by the Cloud-Aerosol Lidar with Orthogonal Polarization (CALIOP; Winker et al, 2009) and the Infrared Imaging Radiometer (IIR; Garnier et al (2012, 2013) on board Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observation (CALIPSO)
– We plot the absolute value of the brightness temperatures (BTs) difference due to the horizontal transport which is computed with the calculation of |BT3Dvho100m − BT1 − D100m| in red, with BT3Dvho100m corresponding to 3-D radiative transfer with vertically homogeneous extinction
Summary
Cirrus clouds cover 15 % to 40 % of the Earth’s surface (Sassen et al, 2008). The temperature difference between the cloud top and the surface leads to a warming of the atmosphere by capturing a part of the infrared radiation emitted by the Earth’s surface and atmosphere. The first cirrus field (Fig. 1) is based on meteorological profiles to form a cirrus cloud as presented by Starr and Cox (1985), with the addition of a wind profile to form virgas From this first realization, the influence on TOA BT of the cirrus mean optical thickness τc, the cirrus heterogeneity parameter ρτ , the ice crystal effective diameter Deff and the cirrus altitude are tested. CIRCLE II was an airborne campaign dedicated to the study of cirrus optical properties and the validation of space measurements made by the Cloud-Aerosol Lidar with Orthogonal Polarization (CALIOP; Winker et al, 2009) and the Infrared Imaging Radiometer (IIR; Garnier et al (2012, 2013) on board Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observation (CALIPSO).
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