Abstract
Abstract. This paper presents a study on the impact of cirrus cloud heterogeneities on MODIS simulated thermal infrared (TIR) brightness temperatures (BTs) at the top of the atmosphere (TOA) as a function of spatial resolution from 50 m to 10 km. A realistic 3-D cirrus field is generated by the 3DCLOUD model (average optical thickness of 1.4, cloud-top and base altitudes at 10 and 12 km, respectively, consisting of aggregate column crystals of Deff = 20 µm), and 3-D thermal infrared radiative transfer (RT) is simulated with the 3DMCPOL code. According to previous studies, differences between 3-D BT computed from a heterogenous pixel and 1-D RT computed from a homogeneous pixel are considered dependent at nadir on two effects: (i) the optical thickness horizontal heterogeneity leading to the plane-parallel homogeneous bias (PPHB) and the (ii) horizontal radiative transport (HRT) leading to the independent pixel approximation error (IPAE). A single but realistic cirrus case is simulated and, as expected, the PPHB mainly impacts the low-spatial-resolution results (above ∼ 250 m) with averaged values of up to 5–7 K, while the IPAE mainly impacts the high-spatial-resolution results (below ∼ 250 m) with average values of up to 1–2 K. A sensitivity study has been performed in order to extend these results to various cirrus optical thicknesses and heterogeneities by sampling the cirrus in several ranges of parameters. For four optical thickness classes and four optical heterogeneity classes, we have found that, for nadir observations, the spatial resolution at which the combination of PPHB and HRT effects is the smallest, falls between 100 and 250 m. These spatial resolutions thus appear to be the best choice to retrieve cirrus optical properties with the smallest cloud heterogeneity-related total bias in the thermal infrared. For off-nadir observations, the average total effect is increased and the minimum is shifted to coarser spatial resolutions.
Highlights
In the context of global climate change, the representation and role of clouds are still uncertain
We model the impact of cirrus cloud heterogeneities on top-of-the-atmosphere brightness temperatures as a function of the spatial resolution from 50 m to 10 km and at four Moderate Resolution Imaging Spectroradiometer (MODIS) thermal infrared channels centered at 8.52, 11.01, 12.03 and 13.36 μm
We assume that TOA brightness temperature differences between brightness temperatures (BTs) computation assuming 1-D radiative transfer (RT) inside a homogeneous pixel and 3-D RT inside a heterogeneous pixel depend on two effects: (i) the optical thickness horizontal inhomogeneity leading to the planeparallel approximation bias and the (ii) horizontal radiative transport effect due to the independent pixel approximation error
Summary
In the context of global climate change, the representation and role of clouds are still uncertain. Cirrus optical thickness (COT) and ice crystal effective diameter (CED) can be retrieved from radiometric measurements using dedicated operational algorithms Many of these operational algorithms are developed for solar-reflectance channels, like that of the Moderate Resolution Imaging Spectroradiometer (MODIS), for the MOD06 product (Platnick et al, 2003; Yang et al, 2007) or the Clouds and the Earth’s Radiant Energy System (CERES) product (Minnis et al, 2011) or the Visible Infrared Imager Radiometer Suite (VIIRS; Platnick et al, 2013). Based on the same spectral information, an optimal estimation method (OEM; Rodgers, 2000) is used for the Atmospheric Infrared Sounder V6 (AIRS, Kahn et al, 2014, 2015) and in the research-level code of Wang et al (2016b, a) for MODIS Another advantage of the TIR is that measurements can be obtained in nighttime conditions, which gives a distinct benefit compared to solarreflectance channels for developing ice cloud climatologies.
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