Abstract
Clouds limit the quality and availability of optical wavelength surface observations from Earth Observation (EO) satellites. This limitation is particularly relevant for the generation of systematic thematic products from EO medium spatial resolution polar orbiting sensors, such as Landsat, which have reduced temporal resolution compared to coarser resolution polar orbiting sensors such as the Moderate Resolution Imaging Spectroradiometer (MODIS). MODIS on the Terra satellite is in the same orbit as Landsat 7 with an approximately 30 minute overpass difference. In this study, one year of global Landsat 7 Enhanced Thematic Mapper Plus (ETM+) image cloud fractions over land are compared with collocated MODIS cloud fractions, generated by combining the MODIS-Terra global daily cloud mask product (MOD35) with the Landsat 7 ETM+ image footprints and acquisition calendar. The results show high correlation between the MODIS and Landsat 7 ETM+ cloud fractions (R2 = 0.83), negligible bias (median difference: <0.01) and low dispersion around the median (interquartile range: [−0.02, 0.06]). These results indicate that, globally, the cloud cover detected by MODIS-Terra data can be used as a proxy for Landsat 7 ETM+ cloud cover.
Highlights
Earth Observation (EO) data sensed in the optical portion of the electromagnetic spectrum are widely used for studying the Earth’s biosphere, and its dynamics and disturbances from a regional to global scale
The near daily global coverage 1 km cloud product derived from the Moderate Resolution Imaging Spectroradiometer (MODIS) [10] has been used in feasibility studies to assess the potential impact of cloud obscuration on other land monitoring satellites, using the MODIS time series to generate a spatially explicit climatology of cloud cover probabilities [11]
A systematic global comparison between the Landsat 7 ETM+ image cloud fraction over land, and equivalent cloud fractions derived from contemporaneous MODIS-Terra (MOD35) cloud observations was undertaken
Summary
Earth Observation (EO) data sensed in the optical portion of the electromagnetic spectrum are widely used for studying the Earth’s biosphere, and its dynamics and disturbances from a regional to global scale. The near daily global coverage 1 km cloud product derived from the Moderate Resolution Imaging Spectroradiometer (MODIS) [10] has been used in feasibility studies to assess the potential impact of cloud obscuration on other land monitoring satellites, using the MODIS time series to generate a spatially explicit climatology of cloud cover probabilities [11] Moderate resolution data, such as Landsat, cannot be directly used for generating such a climatology because of the limited number of available acquisitions, due to the low revisit frequency and to the fragmentary state of the archive outside the United States [12]. Researchers have used the MODIS global cloud mask product as a proxy to estimate the effect of cloud cover on the HyspIRI mission design [13], and to define the observation requirements for operational cropland monitoring from medium resolution systems [14]
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