Abstract
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-of-view 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, co-located 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 CubeSat-sized payload.
Highlights
Clouds are one of the most uncertain aspects of our climate system
We used the Airborne Hyper-Angular Rainbow Polarimeter (AirHARP) hyper-angular measurements at a single wavelength (670 nm) in a traditional parameterization scheme to demonstrate the ability of the HyperAngular Rainbow Polarimeter (HARP) concept to characterize cloud microphysical parameters across a cloud field at sub-kilometer spatial resolution
HARP measurements can be applied to the four polarized wavelengths, akin to Breon and Goloub (1998) and Shang et al (2019), but this type of retrieval is more sensitive to resolution and calibration than the hyper-angular technique presented here and most importantly requires homogeneity at cross-track scales (3 km)
Summary
Clouds are one of the most uncertain aspects of our climate system. Clouds are highly variable, yet well-dispersed across the globe and play a dual role in distributing energy: they trap infrared radiation in our atmosphere and reflect shortwave radiation back to space (Rossow and Zhang, 1995). A study by Xu et al (2018) extended the AirMSPI line-cut retrieval to the entire continuous sweep image of the cloudbow with assistance from image-specific empirical correlations between COT, CDR, and CDV This line-cut polarimetric technique requires a droplet size homogeneity assumption over the full line cut of the cloudbow, which may blur heterogeneity that exists at the pixel level and steer the retrieval towards wider DSDs. There is a strong interest in the Earth science community in a multi-angle polarimeter concept for aerosol and cloud retrievals with a wide swath for spatial context, high accuracy in polarization, high angular density for cloudbow retrieval, and narrow ground resolution (Remer et al, 2019; Dubovik et al, 2019).
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
