Abstract
The aim of this study is to determine the availability of cloud-free images in relation to satellite revisit periods for the UK and in particular for the North West of England. Cloud cover was analysed with cloud masks from AVHRR/APOLLO and TERRA/MODIS cloud products. Availability of cloud-free images was determined from revisit frequency and the numbers of monthly images from Landsat ETM+, ASTER, Quickbird and SPOT satellite sensor series. The average number of cloud-free days at Risley Moss and Charter's Moss is five days per month with a minimum of one cloud-free day. The results show that satellite revisit periods and cloud cover are major determinants of satellite data availability and in this study it was also found that contrary to popular opinion, satellite overpass does not necessarily mean imaging opportunity and additional cloud-free images could be obtained when there are no satellite overpasses. The numbers of cloud-free images at a given site may be increased by combining data from different satellite sensors.
Highlights
Cloud cover information is vital in optical remote sensing because it determines the availability of data sets for many operational applications and may be used to determine whether a satellite overpass has an unobstructed field-of-view (FOV) of a specific pixel in a satellite image (Ackerman et al 1998)
This method is suitable for high temporal resolution imagery, like TERRA/Moderate Resolution Imaging Spectroradiometer (MODIS), but it reduces the available data so that short-term surface dynamics may be undetected; clouds may still be a major source of residual contamination in such composites
Cloud cover probability was modelled over the UK and in particular for the two study sites in the North West of England using cloud masks from the AVHRR/APOLLO and TERRA/MODIS cloud products
Summary
Cloud cover information is vital in optical remote sensing because it determines the availability of data sets for many operational applications and may be used to determine whether a satellite overpass has an unobstructed field-of-view (FOV) of a specific pixel in a satellite image (Ackerman et al 1998). Efforts made in reducing the effects of cloud cover on satellite data use compositing of selected cloud-free images in collocated pixels that are geometrically registered with the maximum Normalized Difference Vegetation Index (NDVI), or by compositing temporally overlapping satellites images of different overpass dates (Cihlar et al 1997; Roy 2000). This method is suitable for high temporal resolution imagery, like TERRA/MODIS, but it reduces the available data so that short-term surface dynamics may be undetected; clouds may still be a major source of residual contamination in such composites. Characterizing cloud cover and satellite revisit with cloud masks in North West England of predicting the quality and availability of satellite data, with a high level of confidence, for continuous monitoring and retrieval of cloud-free pixels, at both temporal and spatial scales can be provided by the use of satellite cloud mask
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