Availability Of Cloud-free Images Research Articles

Study of 40-year glacier retreat in the northern region of the Cordillera Vilcanota, Peru, using satellite images: preliminary results

ABSTRACTIn this article, the possibility of using a Landsat image series to calculate the decadal changes in the area of mountain glaciers in the eastern cordillera of Peru is explored. In the pilot study site, changes in the glacier area at different elevation ranges to understand the altitudinal gradients and the directional properties of glacier retreat during the period 1975–2015 were calculated. This approach can be used in the entire tropical Andes, particularly in the outer tropics, depending on the availability of cloud-free images during the dry season. Similar to other regions in the tropical Andes, glaciers situated below 5000 m a.s.l. have lost nearly 81% of the initial area. It is observed that those glaciers directed towards north-east and east shows higher retreat and this may influence the glacier-fed rivers in the Amazon Basin. Future research will estimate the glacier variations in the entire eastern cordilleras and the corresponding changes in the water discharge volume in the Amazon Basin.

Seasonal variation of land cover classification accuracy of Landsat 8 images in Burkina Faso

Abstract. In the seasonal tropics, vegetation shows large reflectance variation because of phenology, which complicates land cover change monitoring. Ideally, multi-temporal images for change monitoring should be from the same season, but availability of cloud-free images is limited in wet season in comparison to dry season. Our aim was to investigate how land cover classification accuracy depends on the season in southern Burkina Faso by analyzing 14 Landsat 8 OLI images from April 2013 to April 2014. Because all the images were acquired within one year, we assumed that most of the observed variation between the images was due to phenology. All the images were cloud masked and atmospherically corrected. Field data was collected from 160 field plots located within a 10 km × 10 km study area between December 2013 and February 2014. The plots were classified to closed forest, open forest and cropland, and used as training and validation data. Random forest classifier was employed for classifications. According to the results, there is a tendency for higher classification accuracy towards the dry season. The highest classification accuracy was provided by an image from December, which corresponds to the dry season and minimum NDVI period. In contrast, an image from October, which corresponds to the wet season and maximum NDVI period provided the lowest accuracy. Furthermore, the multi-temporal classification based on dry and wet season images had higher accuracy than single image classifications, but the improvement was small because seasonal changes affect similarly to the different land cover classes.

CHARACTERIZING CLOUD COVER AND SATELLITE REVISIT WITH CLOUD MASKS IN NORTH WEST ENGLAND

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.

Validation of Satellite-Derived Sea Surface Temperatures for Waters around Taiwan

1 Department of Environmental Biology and Fisheries Science, National Taiwan Ocean University, Keelung, Taiwan, ROC 2 Center for Atmospheric and Oceanic Studies, Faculty of Science, Tohoku University, Sendai, Japan 3 Ocean Data Bank of National Center for Ocean Research, Taipei 106, Taiwan * Corresponding author address: Prof. Ming-An Lee , Department of Environmental Biology and Fisheries Science, National Taiwan Ocean University, Keelung, Taiwan, ROC; E-mail: malee@mail.ntou.edu.tw In order to validate the Advanced Very High Resolution Radiometer (AVHRR)-derived sea surface temperatures (SST) of the waters around Taiwan, we generated a match-up data set of 961 pairs, which included in situ SSTs and concurrent AVHRR measurements for the period of 1998 to 2002. Availability of cloud-free images, i.e., images with more than 85% of cloud-free area in their coverage, was about 2.23% of all AVHRR images during the study period. The range of in situ SSTs was from 10.5°C to 30.8°C. The satellite derived-SSTs through MCSST and NLSST algorithms were linearly related to the in situ SSTs with correlation coefficients of 0.985 and 0.98, respectively. The MCSSTs and NLSSTs had small biases of 0.009 C° and 0.256 C° with root mean square deviations of 0.64 C° and 0.801 C°, respectively, therefore the AVHRR-based MCSSTs and NLSSTs had high accuracy in the seas around Taiwan. (