Spatial distribution of cloud droplet size properties from Airborne Hyper-Angular Rainbow Polarimeter (AirHARP) measurements

Abstract

The global variability of clouds and their interactions with aerosol and radiation make them one of our largest sources of uncertainty related to global radiative forcing. The droplet size distribution (DSD) of clouds is an excellent proxy that connects cloud microphysical properties with radiative impacts on our climate. However, traditional radiometric instruments are information-limited in their DSD retrievals. Radiometric sensors can infer droplet effective radius directly but not the distribution width, which is an important parameter tied to the growth of a cloud field and to the onset of precipitation. DSD heterogeneity hidden inside large pixels, a lack of angular information, and the absence of polarization limit the amount of information these retrievals can provide. Next-generation instruments that can measure at narrow resolutions with multiple view angles on the same pixel, a broad swath, and sensitivity to the intensity and polarization of light are best situated to retrieve DSDs at the pixel level and over a wide spatial field. The Airborne Hyper-Angular Rainbow Polarimeter (HARP) is a wide-field-ofview imaging polarimeter instrument designed by the University of Maryland, Baltimore County (UMBC), for retrievals of cloud droplet size distribution properties over a wide swath, at narrow resolution, and at up to 60 unique, colocated view zenith angles in the 670 nm channel. The cloud droplet effective radius (CDR) and variance (CDV) of a unimodal gamma size distribution are inferred simultaneously by matching measurement to Mie polarized phase functions. For all targets with appropriate geometry, a retrieval is possible, and unprecedented spatial maps of CDR and CDV are made for cloud fields that stretch both across the swath and along the entirety of a flight observation. During the NASA Lake Michigan Ozone Study (LMOS) aircraft campaign in May-June 2017, the Airborne HARP (AirHARP) instrument observed a heterogeneous stratocumulus cloud field along the solar principal plane. Our retrievals from this dataset show that cloud DSD heterogeneity can occur at the 200 m scale, much smaller than the 1-2 km resolution of most spaceborne sensors. This heterogeneity at the sub-pixel level can create artificial broadening of the DSD in retrievals made at resolutions on the order of 0.5 to 1 km. This study, which uses the AirHARP instrument and its data as a proxy for upcoming HARP CubeSat and HARP2 spaceborne instruments, demonstrates the viability of the HARP concept to make cloud measurements at scales of individual clouds, with global coverage, and in a low-cost, compact CubeSatsized payload.

Inversion of multiangular polarimetric measurements from the ACEPOL campaign: an application of improving aerosol property and hyperspectral ocean color retrievals

“…In addition, aerosol deposition into ocean waters contributes to the availability of nutrients that modulate phytoplankton growth and ultimately influence the trophic state of ocean ecosystems (Mahowald et al, 2005;Westberry et al, 2019). Furthermore, the ocean itself and the biological activity it supports may also be a source of aerosol (O'Dowd et al, 2002;McCoy et al, 2015;Croft et al, 2019). Better characterization of aerosol microphysical and optical properties is expected to improve the retrieved ocean color signal and, therefore, the derived geophysical products that describe ocean ecosystems (PACE, 2018;Werdell et al, 2019).…”

Retrieval of aerosol properties from Airborne Hyper-Angular Rainbow Polarimeter (AirHARP) observations during ACEPOL 2017

“…Multiple polarimeters and lidars were mounted on the NASA ER-2 aircraft. These included AirHARP as well as AirM-SPI (Diner et al, 2013), RSP (Cairns et al, 1999(Cairns et al, , 2003, SPEX airborne (Smit et al, 2019), HSRL2 (Burton et al, 2018;Hair et al, 2008) and Cloud Physics Lidar (CPL) (McGill et al, 2002). ACEPOL also made use of groundbased instruments such as AERONET and the Micro-Pulse Lidar Network (MPLNET) for validation of aircraft measurements (Holben et al, 2001;Welton et al, 2001).…”

The Aerosol Characterization from Polarimeter and Lidar (ACEPOL) airborne field campaign

“…The figure also shows an example of data collected during the ACEPOL campaign, its second deployment. The instrument's maiden field campaign was the Lake Michigan Ozone Study (LMOS) in 2017 (McBride et al, 2020). For ACEPOL, AirHARP collected data over defined targets, as the processing speed of the onboard data acquisition system precluded continuous data collection.…”

“…This study (2008)(2009)(2010)(2011)(2012)(2013)(2014)(2015)(2016)(2017)(2018) was devoted to technological and scientific developments to address aerosol-cloud climate forcing uncertainty along with improved observations of ocean color to better characterize ocean biology. Designs for an ocean color instrument were enhancements of previous ocean color satellite instruments, such as the Sea-viewing Wide Field-of-view Sensor (SeaWiFS;McClain et al, 2004). These prototypes were, in part, developed and tested by deploying airborne instruments on high-altitude aircraft.…”