UV linear stokes imaging of optically thin clouds

Abstract

Cirrus clouds are important to the radiation energy budget due to their temporal duration and >50% global coverage.¹ The variety of ice crystal shapes and sizes in a cirrus cloud create challenges differentiating radiation insulated by the Earth's atmosphere from that reflected back to space. The optical thickness of these clouds is often too thin to be sensed using any current passive satellite radiometers. Sensitivity studies in the UV have shown that the angle of linear polarization (AoLP) of solar radiation backscattered from thin cirrus clouds and thin liquid water clouds is rotated.² Pust and Shaw also demonstrated subvisual clouds detection in degree of linear polarization (DoLP) and AoLP.³ An Ultraviolet Stokes Imaging Polarimeter (ULTRASIP) was designed and developed for optically thin clouds and sky observations in the 360 nm - 450 nm range.⁴ ULTRASIP is a time modulated polarimeter rotating a wire-grid polarizer in front of a 16-bit, water-cooled, back-illuminated CCD sensor. Polarized light scattering models will be compared in the visible and the UV to motivate measurements in this waveband.⁵