Scale dependence of cirrus horizontal heterogeneity effects on TOA measurements – Part I: MODIS brightness temperatures in the thermal infrared

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, cloudtop and base altitudes at 10 and 12 km, respectively, consisting of aggregate column crystals of D eff = 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-spatialresolution results (above ∼ 250 m) with averaged values of up to 5-7 K, while the IPAE mainly impacts the high-spatialresolution 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.

Cirrus Horizontal Heterogeneity and 3‐D Radiative Effects on Cloud Optical Property Retrievals From MODIS Near to Thermal Infrared Channels as a Function of Spatial Resolution

“…The heterogeneity of the cloud is often characterized by the ρ inhomogeneous parameter (Davis et al, ; Szczap et al, ) or the fractional standard deviation (FSD), both defined as the standard deviation divided by the mean (Shonk et al, ). Fauchez, Platnick, Meyer et al () present a short review about heterogeneity of macroscale and optical properties of cirrus and summarize the range and mean values from scientific literature (0.1 to 1.5 for the value of the inhomogeneous parameter) that we used in this study. These mean values are also corroborated by the work of Alkasem et al ().…”

“…The cirrus cloud structure used in this study is generated by the 3DCLOUD model (Szczap et al, 2014) used in Fauchez, Platnick, Sourdeval et al (), Fauchez, Platnick, Meyer et al (), and Fauchez et al (). It consists of a fallstreak structure with a mean COT of 1.4 at 12.03 μm wavelength, an inhomogeneous parameter , and cloud top and base altitudes at 10 and 12 km, respectively.…”

“…Note that the negative Δ CER is associated with a high cost function (~40) illustrating the difficulty that the OE has to converge on a single solution from NIR/SWIR reflectances and TIR radiances. Below 500 m, horizontal heterogeneity effects are negligible for both the TIR and NIR/SWIR ranges (Fauchez, Platnick, Meyer et al, ); thus, retrievals from both are close to the real mean optical property from a 1‐D homogenous field leading to a small cost function. For RT computations at 50‐m spatial resolution there is no heterogeneity, and we can see that the retrieved optical properties and the cost function are nearly zero for every channel combination showing that under the 1‐D independent pixel approximation assumption the algorithm uncertainty is negligible.…”

“…Zhou et al () found similar results for solar reflectance retrievals with COT underestimated by 1 to 24% and CER overestimated by 0.6–15 μm, depending on cirrus characteristics, sensor settings, and solar zenith angles. Fauchez, Platnick, Meyer et al () and Fauchez et al () studied the impact of horizontal heterogeneity and 3‐D effects on the TOA radiation in the TIR and NIR/SWIR, from 50‐m to 10‐km spatial resolution. The effects are at a minimum between 100‐ and 250‐m spatial resolution in the TIR, but NIR/SWIR solar geometries make estimating the scale of minimum effects difficult to determine.…”

An Assessment of the Impacts of Cloud Vertical Heterogeneity on Global Ice Cloud Data Records From Passive Satellite Retrievals

“…The IPA assumption prohibits radiation transfer from one column (pixel) to contiguous columns (pixels), allowing the use of a 1D radiative transfer model in the cloud retrieval algorithm. Studies showing that pixel‐level cloud retrievals may be biased using the PPH and/or IPA assumptions have been reported for liquid water clouds (e.g., Marshak et al, ; Platnick, ; Várnai & Davies, ; Zhang et al, , ) and for ice clouds (Fauchez, Davis, et al, ; Fauchez, Platnick, et al, ). These pixel‐level retrieval biases may impact studies relying on satellite climate records (e.g., Level‐3 aggregated cloud products).…”