Cloud fractions estimated from shipboard whole-sky camera and ceilometer observations

Abstract:

Shipboard observations on cloud fraction were carried out along the round research cruise between Japan and Antarctica from November 2013 to Aril 2014 using a whole-sky camera and a ceilometer onboard Research Vessel (R/V) Shirase.
We retrieve cloud fraction from the whole-sky camera based on the brightness and color of the images, while we estimate cloud fraction from the ceilometer as an occurrence frequency.
As a result of the comparison of the cloud fractions from the whole-sky camera and the ceilometer, it is found that the correlation coefficient was 0.86 for the 2024 match-up dataset between Australia and Antarctica from 27 November to 9 December 2013. The results are consistent between the two observations as a whole, but there exists some underestimation with the whole-sky camera compared to the ceilometer.
One of the reasons is possibly that the imager is apt to dismiss an optically thinner cloud that can be detected by the ceilometer. On the other hand, the difference of their view angles between the imager and the ceilometer possibly affects the estimation. Therefore, it is necessary to elucidate the cloud properties with detailed match-up analyses in future. Another future task is to compare the cloud fractions with satellite observation such as MODIS cloud products.
Shipboard observations in themselves are very valuable for the validation of products from satellite observation, because we do not necessarily have many validation sites over ocean in particular.