Estimating cloud optical thickness and associated surface UV irradiance from SEVIRI by implementing a semi-analytical cloud retrieval algorithm

Abstract

Abstract. In this paper, we describe the implementation of the Semi-Analytical Cloud Retrieval Algorithm (SACURA), to obtain scaled cloud optical thickness (SCOT) from satellite imagery acquired with the SEVIRI instrument and surface UV irradiance levels. In estimation of SCOT particular care is given to the proper specification of the background (i.e. cloud-free) spectral albedo and the retrieval of the cloud water phase from reflectance ratios in SEVIRI's 0.6 µm and 1.6 µm spectral bands. The SACURA scheme is then applied to daytime SEVIRI imagery over Europe, for the month of June 2006, at 15-min time increments. The resulting SCOT fields are compared with values obtained by the CloudSat experimental satellite mission, yielding a negligible bias, correlation coefficients ranging from 0.51 to 0.78, and a root mean square difference of 1 to 2 SCOT increments. These findings compare favourably to results from similar intercomparison exercises reported in the literature. Based on the retrieved SCOT from SEVIRI and radiative transfer modelling approach, simple parameterisations are proposed to estimate the surface UV-A and UV-B irradiance. The validation of the modelled UV-A and UV-B irradiance against the measurements over two Belgian stations, Redu and Ostend, indicate good agreement with the high correlation, index of agreement and low bias. The SCOT fields estimated by implementing SACURA on imagery from geostationary satellite are reliable and its impact on surface UV irradiance levels is well produced.

Cloud-top pressure retrieval with DSCOVR EPIC oxygen A- and B-band observations

“…At this point the CTP and cloud pressure thickness are the only unknown variables. The cloud pressure thickness or the cloud vertical distribution has a substantial impact on the accuracy of the CTP retrievals (Carbajal Henken et al, 2015;Fischer and Grassl, 1991;Rozanov and Kokhanovsky, 2004;Preusker and Lindstrot, 2009). In this study, the cloud pressure thickness is used as an input parameter to retrieve the CTP.…”

“…For a long time, various efforts have been devoted to the study of radiative transfer in the atmosphere, including scattering, absorption, and emission (Chandrasekhar, 1960;Irvine, 1964;Ivanov and Gutshabash, 1974;van de Hulst, 1980van de Hulst, , 2012Ishimaru, 1999;Thomas and Stamnes, 2002;Davis and Marshak, 2002;Kokhanovsky et al, 2003;Marshak and Davis, 2005;Pandey et al, 2012). In this study, we develop an analytic radiative transfer equation to analyze the radiative transfer at the oxygen A and B bands.…”

“…Based on asymptotic approximation (Kokhanovsky et al, 2003;Pandey et al, 2012), the reflection of a cloud without considering below-cloud interactions is given by Eq. 6.…”

“…Below cloud. The equivalence theorem used for below cloud is similar to within cloud (Kokhanovsky et al, 2003;Pandey et al, 2012).…”

Variability of aerosol and cloud optical properties and their effect on the transfer of solar irradiance in the atmosphere

“…As a result, it was found that for high solar zenith angles (sza) the attenuation is larger on total UV than UVer. However UVer is more attenuated in situations with small sza and cloudy or overcast conditions Pandey et al (2012). used a cloud retrieval algorithm to obtain the scaled cloud optical thickness from the SEVIRI images and than 45° (north and south) Fotiadi et al (2006).…”