Validation of estimated cloud fraction from MERRA-2 and AIRS data using ground based observation over IAO, Hanle

Abstract

We report for validation of cloud fraction estimated from high-resolution (hourly) data from MERRA-2 and AIRS instrument aboard NASA’s Aqua satellite during its ascending and descending orbits with hourly ground-based visual observation taken at Indian Astronomical Observatory (IAO), Hanle during 1997–2018. The study reveals 70% useful nights, equivalent to 254 spectroscopic nights in a year. A comparative study of cloud fraction was performed using homogeneous and continuous data spanning six years (2013–2018) from MERRA-2 and AIRS over IAO, Hanle and Merak sites in the Himalayan region, and several astronomical observatories located in Atacama desert in Chile, Canary islands in Spain, National Astronomical Observatory (NAO), Mexico, and Mauna Loa in Hawaii. The study reveals cloud fractions of 0.07–0.11 at 25% (1st Quartile) and 0.32–0.40 at 50% (median) at the Himalayan sites. On the other hand, the median cloud fractions are 0.06–0.13 at the Chilean sites, 0.16 at NAO, and 0.29 at Canary islands. The observed 25% of the cloud fraction at the Himalayan sites during the lowest three cloud-fractional months (October-December) are comparable with other sites located in Chile and Canary islands. The median cloud fraction at Hanle is slightly lower than Mauna Loa. The study further concluded that the percentage of useful nights obtained from the visual observation (70%) at IAO-Hanle is very close to the percentage of clear sky (68%) estimated from the MERRA-2 data.