资源环境科技发展态势分析平台

Surface-based and satellite-based observations remain the fundamental source of cloud amount data for climatologists. However, both data sets show inconsistency related to the interpretation of instantaneous cloud detection, whether measured using the okta scale for surface-based (SYNOP) observations or cloud mask classes (for satellite-based observations). This study compared mean monthly SYNOP cloud amount with those reported for the moderate resolution imaging spectroradiometer (MODIS) cloud imager onboard Aqua and Terra satellites. Data were collected simultaneously (+/- 10min) over Poland between 2003 and 2013 (a total of 3240 MODIS passes). Results showed that in daytime conditions the average range of inconsistency for SYNOP (10.8%) is similar to MODIS (10.1%). However, for MODIS it doubles (20.9%) at nighttime. This is attributed to the change in frequency of MODIS cloud mask intermediate classes. Furthermore, in particular months' range of inconsistency for MODIS exceeded 20% in daytime and 50% at nighttime, while maximum values for SYNOP were always lower than 20% (a consequence of the okta scale). The findings also confirmed that the standard procedure used to interpret SYNOP okta values (i.e. the assumption that 1 okta equals 12.5%) is a relatively accurate (+/- 2%) estimate of monthly mean cloud amount, although this assumes that surface-based observers always correctly assign the correct okta value to the cloud fraction. Finally, the error in the SYNOP cloud amount estimation was correlated with the average cloud amount for a given location. Those findings will improve the reliability of cloud amount validation efforts that use SYNOP data as a reference.