Measuring cloud thermodynamic phase with shortwave infrared imaging spectroscopy

Abstract

Shortwave Infrared imaging spectroscopy enables accurate remote mapping of cloud thermodynamic phase at high spatial resolution. We describe a measurement strategy to exploit signatures of liquid and ice absorption in cloud top apparent reflectance spectra from 1.4 to 1.8 μm. This signal is generally insensitive to confounding factors such as solar angles, view angles, and surface albedo. We first evaluate the approach in simulation and then apply it to airborne data acquired in the Calwater‐2/ACAPEX campaign of Winter 2015. Here NASA's “Classic” Airborne Visible Infrared Imaging Spectrometer (AVIRIS‐C) remotely observed diverse cloud formations while the U.S. Department of Energy ARM Aerial Facility G‐1 aircraft measured cloud integral and microphysical properties in situ. The coincident measurements demonstrate good separation of the thermodynamic phases for relatively homogeneous clouds.

Global spectroscopic survey of cloud thermodynamic phase at high spatial resolution, 2005–2015

“…Prior work used this spectral interval to measure cloud phase based on the optical absorption properties of liquid and ice (Pilewskie and Twomey, 1987). The fraction of the total path absorption due to liquid (the liquid thickness fraction, or LTF) had high sensitivity to pure and mixed phases (Thompson et al, 2016). Liquid and ice show highly diagnostic absorption shapes which are robust to po-D. R. Thompson et al: Global spectroscopic survey of cloud phase tential confounding effects such as surface reflectance, observation geometry, and mismatch in particle modeling assumptions.…”

Global Spectroscopic Survey of Cloud Thermodynamic Phase at High Spatial Resolution, 2005–2015

“…These bulk absorption spectra were generally independent of particle scattering and did not relate directly to particle size. However, combined with a free continuum, they fit the observed spectra of opaque clouds over short spectral intervals without precise knowledge of particle properties (Thompson et al, 2016).…”

“…We first applied the cloud phase retrieval algorithm of Thompson et al (2016), validated in the prior work by coincident remote and in-situ aircraft measurements. We defined the TOA reflectance, ρ:…”

“…Liquid and ice show highly diagnostic absorption shapes which are robust to potential confounding effects such as surface reflectance, observation geometry, and mismatch in particle modeling assumptions. Remote imaging spectrometers measure these spectra from cloud tops at millions of spatial locations, resolving cloud phase at tens of meter scales (Thompson et al, 2016). This could constrain characteristic spatial lengths of these processes.…”

“…Here as in prior work, we separated the Equivalent Water Thickness due to Ice, EWT ice from that of liquid, EWT liquid . As in Thompson et al (2016), we defined the Liquid Thickness Fraction (LTF) as:…”