Abstract
Abstract. A threshold-based cloud mask for the high-resolution visible (HRV) channel (1 × 1 km2) of the Meteosat SEVIRI (Spinning Enhanced Visible and Infrared Imager) instrument is introduced and evaluated. It is based on operational EUMETSAT cloud mask for the low-resolution channels of SEVIRI (3 × 3 km2), which is used for the selection of suitable thresholds to ensure consistency with its results. The aim of using the HRV channel is to resolve small-scale cloud structures that cannot be detected by the low-resolution channels. We find that it is of advantage to apply thresholds relative to clear-sky reflectance composites, and to adapt the threshold regionally. Furthermore, the accuracy of the different spectral channels for thresholding and the suitability of the HRV channel are investigated for cloud detection. The case studies show different situations to demonstrate the behavior for various surface and cloud conditions. Overall, between 4 and 24% of cloudy low-resolution SEVIRI pixels are found to contain broken clouds in our test data set depending on considered region. Most of these broken pixels are classified as cloudy by EUMETSAT's cloud mask, which will likely result in an overestimate if the mask is used as an estimate of cloud fraction. The HRV cloud mask aims for small-scale convective sub-pixel clouds that are missed by the EUMETSAT cloud mask. The major limit of the HRV cloud mask is the minimum cloud optical thickness (COT) that can be detected. This threshold COT was found to be about 0.8 over ocean and 2 over land and is highly related to the albedo of the underlying surface.
Highlights
The effect of clouds on radiative fluxes depends on cloud type and can vary strongly both in space and time
A threshold-based cloud mask for the highresolution visible (HRV) channel (1 × 1 km2) of the Meteosat SEVIRI (Spinning Enhanced Visible and Infrared Imager) instrument is introduced and evaluated. It is based on operational EUMETSAT cloud mask for the low-resolution channels of SEVIRI (3 × 3 km2), which is used for the selection of suitable thresholds to ensure consistency with its results
The goal of the present paper is to develop a cloud mask based on the HRV channel, which exploits its high spatial resolution, and is suitable to study small-scale features of clouds, including, for example, their horizontal dimensions
Summary
The effect of clouds on radiative fluxes depends on cloud type and can vary strongly both in space and time. Accurate information about the physical and radiative properties of clouds is necessary to determine the role of clouds in the climate system including their response to anthropogenic forcings (e.g., Forster et al, 2007). Geostationary satellite imagers such as Meteosat SEVIRI (Spinning Enhanced Visible and Infrared Imager) are wellsuited to monitor the temporal development of clouds, and to resolve their diurnal cycle fully over land and ocean (Roebeling and van Meijgaard, 2009). The study of Derrien et al (2010a) improves the detection of small-scale low clouds by use of the HRV channel. Few operational products based on the HRV channel are available, which is a significant hurdle for use of its finer spatial resolution for scientific studies and applications
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