Astronomical site survey report on dust measurement, wind profile, optical turbulence, and their correlation with seeing over IAO-Hanle

Abstract

The present work discusses astronomical site survey reports on dust content, vertical distribution of atmospheric turbulence, precipitable water vapor (PWV), surface and upper-air data, and their effects on seeing over the Indian Astronomical Observatory (IAO) Hanle. Using Laser Particulate Counter, ambient dust measurements at various sizes (0.3 μm to 25 μm) were performed at various locations at the site during November 2015. Estimated volume concentration for the particle size at 0.5 μm was around 10,000 per cubic foot, which is equivalent to ten thousand class of clean room standard protocol. During the measurement, surface wind speed varied from 0-20 m s −1, while estimated aerosol optical depth (AOD) using Sky radiometer (Prede) varied from 0.02-0.04 at 500 nm, which indicates the site is fairly clean. The two independent measurements of dust content and aerosol concentrations at the site agreed well. The turbulence or wind gust at the site was studied with wind profiles at three different heights above the ground. The strength of the wind gust varies with time and altitude. Nocturnal temperature across seasons varied with a moderate at summer (6−8 ∘C) and lower in winter (4−5 ∘C). However, the contrast between the two is significantly small due to cold and extremely dry typical climatic conditions of the site. The present study also examined the effects of surface and upper-air data along with Planetary Boundary Layer (PBL) dynamics with seeing measurement over the site. Further, a comparative study of such observed parameters was conducted with other high altitude astronomical observatories across the globe.