A new way to see the clouds: the hyper-angular rainbow polarimeter (HARP2) on the NASA PACE satellite mission

The Hyper-Angular Rainbow Polarimeter 2 (HARP2) on the NASA Plankton Aerosol Cloud ocean Ecosystem (PACE) mission is a wide field of view imaging polarimeter instrument designed for highly accurate and resolved cloud observations. HARP2 is uniquely sensitive to the polarized cloudbow, a ring-like structure in polarized light that appears above liquid water clouds. The structure of the cloudbow encodes information about the droplet size distribution, which is a critical link between cloud microphysical and radiative properties. Matching a multi-angle measurement of the cloudbow to Mie scattering predictions allows for a retrieval of important cloud properties: droplet effective radius and variance. HARP2 is the first instrument of its kind suitable for this retrieval at 5 km spatial resolution. Its wide swath facilitates global coverage of polarimetric measurements in 2 days, making it a uniquely powerful tool for studying cloud microphysics. This paper briefly presents the HARP2 instrument, demonstrates its retrieval capabilities, and discusses future science that it makes possible.

1 Introduction

Clouds play a crucial role in regulating Earth’s energy balance, but the complex interactions between negative and positive cloud radiative forcing present a challenge due to the inherent spatiotemporal variability of clouds (Intergovernmental Panel on Climate Change, 2023). Understanding cloud microphysics is essential for accurately modeling weather and climate systems (Stephens, 2005). Remote sensing allows for passive and active observation of clouds across broader spatial and temporal scales, capturing both macro- and microphysical properties. A major breakthrough in cloud remote sensing came with the development of the bispectral algorithm for retrieving liquid cloud droplet effective radius and cloud optical depth, pioneered by Twomey and Seton (1980) and Nakajima and King (1990). The bispectral retrieval algorithm, which leverages measurements from both water-absorbing and non-absorbing spectral bands, was initially applied to NOAA’s Advanced very-high-resolution (AVHRR) radiometer, and was later refined and adapted for other instruments (Platnick and Twomey, 1994; Platnick et al., 2020; Minnis et al., 2021).

A further leap in cloud retrievals came with the launch of the POLarization and Directionality of the Earth’s Reflectances (POLDER) instrument on ADEOS I in 1996. POLDER provided the first observation of a cloudbow, an optical phenomenon caused by the single scattering of light by spherical liquid cloud droplets. This observaiton was quickly followed by the first polarimetric retrieval of liquid cloud droplet size distribution (DSD) (Breon and Goloub, 1998). Unlike bispectral algorithms, polarimetric retrievals are sensitive to both liquid cloud droplet effective radius $(r_{\text{eff}})$ and effective variance $(v_{\text{eff}})$, the latter describing the width of the DSD. This is possible because the cloudbow appears in the polarized radiance as oscillations in intensity, which vary with the scattering angle between 135° and 170°, with an absolute maximum near 140° (Hansen, 1971b). The polarized cloudbow signal encodes microphysical information: the angular position of the cloudbow peaks shifts with $r_{\text{eff}}$, while the amplitude of the supernumerary cloudbow peaks is modulated by $v_{\text{eff}}$ (Hansen, 1971b). The sensitivity of the polarized phase function to cloud microphysics is demonstrated in Figure 1, which shows the simulated polarized phase function $(P_{12})$ at 670 nm for a variety of $r_{\text{eff}}$ and $v_{\text{eff}}$ given a two parameter gamma DSD as described in Hansen (1971b). Cloudbows are observable wherever liquid water droplets are present, typically in low-to mid-level clouds such as stratocumulus, cumulus, and altocumulus. Given that approximately 67$\%$ of Earth’s surface is covered by clouds at any given time (King et al., 2013), there are abundant opportunities for retrieving cloud microphysical properties from cloudbow observations.

Figure 1

Figure 1. Sensitivity of the polarized phase function to $r_{\text{eff}}$ and $v_{\text{eff}}$. Simulated gamma distribution as in Hansen (1971a) for 670 nm: (top-left) normalized droplet size distribution for $v_{\text{eff}}=0.01$ and multiple $r_{\text{eff}}$; (top-right) polarized phase function for $v_{\text{eff}}=0.01$ and multiple $r_{\text{eff}}$; (bottom-left) normalized droplet size distribution for $r_{\text{eff}}=10\mu m$ and multiple $v_{\text{eff}}$; (bottom-right) polarized phase function for $r_{\text{eff}}=10\mu m$ and multiple $v_{\text{eff}}$.

Although two POLDER missions were cut short for various reasons, these platforms laid the groundwork for subsequent advances in passive cloud retrieval techniques and the development of the next-generation of Earth-observing satellites (Werdell et al., 2019). The decommissioning of the Polarization and Anisotropy of Reflectances for Atmospheric Science coupled with Observations from LIDAR (PARASOL) platform in 2013 marked the end of orbital polarimeters with publicly available data which restricted the availability of $v_{\text{eff}}$ from space, as radiometric methods alone cannot fully characterize DSD. Polarimetric measurements, particularly from orbital hyper-angular polarimeters, are crucial for simultaneously retrieving the two parameters of liquid cloud DSDs — $r_{\text{eff}}$ and $v_{\text{eff}}$ — over large spatial and temporal scales (Dubovik et al., 2021). However, not all polarimetric instruments are capable of high-quality DSD retrievals. Accurate retrievals require hyper-angular sampling with high angular and spatial resolution to effectively resolve the cloudbow signal from space (Miller et al., 2018). For instance, POLDER’s 14 viewing angles, each separated by $10°$, are insufficient for distinguishing between narrow and broad DSDs at some observing geometries (Deschamps et al., 1994; Shang et al., 2015; Miller et al., 2018).

There is a new generation of polarimeters being developed for aerosol and cloud retrievals. The Multi-viewing Multi-channel Multi-polarization Imager (3MI), based on POLDER’s heritage, offers improvements in spectral range, spatial resolution, and swath width, but the angular resolution is still insufficient for single wavelength retrievals of DSD (Fougnie et al., 2018; Miller et al., 2018). Similarly, the Directional Polarimetric Camera (DPC), also lacks the necessary angular resolution to differentiate narrow and broad DSDs at some geometries (Li et al., 2018; Wang et al., 2023). The SPEXone spectro-polarimeter aboard PACE provides continuous spectral coverage in the range 385–770 nm for 5 viewing angles (0°, $\pm$ 22° and $\pm$ 58°) (Hasekamp et al., 2019). Although SPEXone does not have the angular resolution necessary for polarimetric cloud retrievals as described above, recent advancements in neural network retrieval methods show promising results that could allow SPEXone (and other instruments) to contribute to cloud studies that exploit their high spectral resolution (Segal-Rozenhaimer et al., 2018; Di Noia et al., 2019).

The next-generation of polarimeters also includes a fleet of airborne instruments. While they cannot provide the continuous long-term observations that satellites offer, airborne polarimeters are valuable tools for understanding cloud microphysical processes, as well as developing and validating instruments and algorithms. Many orbital instruments have airborne proxies for this reason (e.g., SPEXone and AirSPEX, HARP and AirHARP, etc.). The Research Scanning Polarimeter (RSP) (Alexandrov et al., 2012a) and Airborne Multiangle SpectroPolarimetric Imager (AirMSPI) (Diner et al., 2013) expanded the legacy of POLDER. RSP’s high angular resolution and polarization accuracy make it attractive for polarimetric cloud retrievals, but its restrictive single-pixel cross-track swath limits its ability to characterize a full cloud field (Alexandrov et al., 2012b; Alexandrov et al., 2015; Cairns et al., 1999).

The launch of the Hyper-Angular Rainbow Polarimeter (HARP) Cubesat in 2020 marked the beginning of the new era of orbital polarimetric observations (Martins et al., 2018) and laid the groundwork for the 2024 launch of HARP2 aboard NASA’s Plankton Aerosol Cloud ocean Ecosystem (PACE) mission. Before HARP, no orbital instrument could simultaneously achieve both high spatial resolution (less than 40 km per pixel from POLDER) and high native angular density for polarimetric cloud DSD retrievals. The HARP2 instrument, launched on NASA’s PACE mission, is a groundbreaking advancement in this field. With its ability to retrieve cloud microphysics at unprecedented spatial resolution and angular sampling at a global scale, HARP2 addresses a crucial gap in our understanding of cloud processes. Its capacity to retrieve two parameters of cloud DSDs in various cloud types and across different environments makes it a key asset in studying the spatial distribution of cloud microphysics on a global scale. With hyper-angular capabilities and global coverage every 2 days, HARP2 offers unparalleled detail in cloud and aerosol measurements from space, addressing the high variability of clouds and their impact on climate. This paper explores HARP2’s ability to retrieve liquid cloud DSD, focusing on its innovative approach to polarimetric cloudbow observations which will help further our understanding of cloud microphysics and their role in climate and weather systems.

2 Materials and methods

2.1 The hyper-angular rainbow polarimeter (HARP) instrument

The HARP instruments are a series of wide field of view (FOV) polarimeters capable of measuring polarized radiance with fine angular $(\lesssim 2°)$ resolution and high polarization accuracy (better than the required 1$\%$ and achieving $\lesssim 0.5\%$ in most of the FOVs) (Sienkiewicz et al., 2025). AirHARP was the first aircraft-mounted version of HARP which was flown during the NASA Lake Michigan Ozone Study (LMOS) air campaign in May-June 2017 (McBride et al., 2020). HARP CubeSat, the first spaceborne version of HARP, had its first light in April 2020 and was active through May 2022, successfully demonstrating the mission concept and collecting valuable science data. The next-generation airborne and orbital HARP polarimeters, AirHARP2 and HARP2, contain the same fundamental optics as the previous HARP instruments with modifications made to improve its calibration accuracy (Fernandez-Borda et al., 2009; Sienkiewicz et al., 2024).

Light incident on the HARP2 wide-FOV telecentric lens is split into three unique polarization states (0$°$, 45$°$, and 90$°$) by a combination of a modified Phillips prism and linear polarizers. The orientation of the polarizers is such that the linear Stokes parameters (I, Q, and U) of the scene can be retrieved in a single co-aligned pixel from a linear combination of sufficiently separate polarization states. The polarized light leaving each prism port passes through a custom stripe filter which divides the along-track view into 120 sectors with four spectral channels (440, 550, 670, and 870 nm). Each view sector corresponds to a given viewing angle. The hyper-angular (670 nm) channel has 60 view sectors ($\approx 2°$ angular resolution), and the rest of the spectral channels have up to 20 view sectors, from which 10 sectors are selected for downlink. The light, now divided into three polarization states and 90 view sectors, is then incident on three charged coupled device (CCD) detectors. The wide FOV and angular resolution in the red channel provide the scattering angle coverage necessary for polarimetric DSD retrievals.

2.2 HARP data

The HARP instruments are push-broom imagers, meaning that as the instrument moves in the along-track direction, successive measurements from a single view angle can be combined to form an image of a given scene from that specific viewing angle. Figure 2 shows pushbroom images for five view angles of a HARP2 cloud case from 27 July 2024. The same physical cloud features are visible in each panel, but the bright cloudbow stripe appears in a different location in each image. This shift is not because the cloudbow is moving, but because each view angle samples a different part of the cloudbow. The scattering angle changes with each view angle and this determines the portion of the cloudbow we observe. When a target (e.g., one of these cloud features) is imaged at all view angles, its polarized structure can be reconstructed and used to retrieve the DSD.

Figure 2

Figure 2. Pushbroom images of the product of Degree of Linear Polarization and total radiance at 670 nm for the 27 July 2024, 12:56Z case at 5 different HARP2 view angles. Each pushbroom shows the same 2,059 × 1,534 km (along-track x cross-track) area and the nominal view angles are indicated at the top of the respective pushbroom. This illustrates how the part of the cloudbow being observed depends on the viewing angle. Measurements at each view angle are combined to fully characterize the cloud phase function.

All HARP2 data used in this work is from the publicly available HARP2 L1C data products that are on a grid common to all three PACE instruments for ease of intercomparison and synthesis. All PACE instruments are binned to 5.2 × 5.2 km horizontal spatial resolution at the surface; for HARP2, this corresponds to 457 bins in the across-track direction. L1C products incorporate data from the full swath of all instruments, including portions of the HARP2 swath that extend beyond nadir views due to broadening at larger viewing angles. L1C files are created as granules with a default file size of 5 min swaths. HARP2’s multiangular data are projected to the ground surface altitude; retrievals of cloud optical properties require a projection to a cloud top height. Detailed specifications for PACE L1C products can be found in Volume 12 of the PACE Technical Report Series (Knobelspiesse et al., 2024).

3 Results

3.1 HARP2 global measurements

HARP2 has been in orbit since February 2024, collecting global data every 2 days (Martins et al., 2024). Figure 3 provides an example of L1C data collected by HARP2 on 27 July 2024. The top map in Figure 3 shows a false-color Red, Green and Blue (RGB) composite (670, 870, and 440 nm) which depicts Earth as we might expect to see it—with distinct features like widespread cloud cover, dark blue oceans, and diverse land surfaces. A bright glint signal is visible in the middle of the swath over the southern hemisphere, caused by sunlight reflecting off the ocean surface. While this image provides a wealth of multispectral information, the polarization images at different viewing angles reveal entirely different features. The center image in Figure 3 shows an RGB representation of the Degree of Linear Polarization (DOLP) at an along-track viewing angle of −23.1°. In this image, land surfaces disappear, and bright arcs appear in the mid-latitudes in both hemispheres where clouds were visible in the RGB image. These highly polarized areas are cloudbows, as described above (Breon and Goloub, 1998; Alexandrov et al., 2012b; McBride et al., 2020). Notably, the glint signal is absent at this angle, which simplifies cloud retrievals. The bottom map in Figure 3 also shows an RGB representation of DOLP but at a forward-viewing direction (42.5°). At this viewing geometry, the glint signal reappears. This glint measurement is valuable for retrieving parameters like wind speed over the ocean but can complicate other retrievals due to its relative intensity. Comparing the three panels in Figure 3 highlights the stark differences between intensity and polarization signals, as well as the additional information gained through multiangular polarimetric measurements.

Figure 3

Figure 3. Composites of HARP2 L1C data collected in 2 days. The top image is a false color RGB (670, 870, and 440 nm), while the middle and bottom images are RGB representations of DOLP at two different along-track viewing geometries: −23.1° and 42.5°, respectively.

HARP2’s hyper-angular polarimetric measurements reveal hidden information from clouds. In the top global map in Figure 3 the cloudbow signal is not visible because the intensity of unpolarized light dominates over the weaker polarized signal. The DOLP at −23.1°(middle image in Figure 3) reveals abundant cloudbows in the mid-latitudes of both the Northern and Southern Hemispheres. In contrast, the cloudbow is less visible in the forward along-track viewing angle, where the scattering geometry is less favorable. These cloudbows appear wherever liquid water clouds are present, provided the appropriate solar and viewing geometry, offering numerous opportunities for retrieving liquid cloud droplet microphysical properties. Because the cloudbow is exclusively generated by liquid cloud droplets, high-altitude cirrus clouds have low DOLP and are not prominent in these images.

3.2 Liquid cloud properties from HARP2

Figure 4 shows the RGB of intensity and DOLP for a single HARP2 granule from 27 July 2024 12:56Z. In the intensity RGB, we see a marine stratocumulus cloud deck that has both closed-cell and open-cell forms with varying optical thicknesses. The cloudbow is only faintly visible across this image but, as expected, is extremely prominent in DOLP. The polarized signal from this scene was used to retrieve the liquid cloud $r_{\text{eff}}$ and $v_{\text{eff}}$ using a parametric retrieval algorithm as described in McBride et al. (2020). The retrieved $r_{\text{eff}}$ along the transition between open and closed cell structure between $18°$S and $21°$S is relatively large while the $v_{\text{eff}}$ in the entire region above $21°$S is highly variable, even in the region where $r_{\text{eff}}$ is much smaller. The $v_{\text{eff}}$ in the center of the granule is smaller while the $r_{\text{eff}}$ is larger until the transition between closed and open cell structure near $27°$S. This demonstration shows that HARP2 can retrieve cloud properties across a wide swath, enabling characterization of large, complex cloud fields and offering valuable insight into broader cloud processes.

Figure 4

Figure 4. Cloud scene from HARP2 (27 July 2024, 12:56Z) off the coast of West Africa. The coastline is shown in white: (top-left) RGB (670, 550, and 440 nm) of reflectance; (top-right) RGB of DOLP; (bottom-left) spatial map of retrieved $r_{\text{eff}}$; (bottom-right) spatial map of retrieved $v_{\text{eff}}$.

4 Discussion

Multiangular polarimeters are the best tools we have for characterizing cloud microphysics on a global scale, but the limited availability of polarimetric cloud data has slowed the progression of this field. HARP2 is one of the most advanced orbital polarimeters to date, meeting a crucial need in the remote sensing and Earth science community with its global coverage, hyper-angular capability, and high polarization accuracy. Building on the legacy of instruments like POLDER, MODIS, and airborne polarimeters, HARP2 is the first instrument capable of retrieving liquid cloud DSD at high spatial resolution, providing detailed measurements over a large spatial field. Its detailed polarization data across a wide angular range will enable the retrieval of cloud DSD for any pixel containing liquid water clouds with suitable observation geometry. This ability to perform high-resolution cloud DSD retrievals on a global scale with high temporal resolution enables HARP2 to address long-standing gaps in climate modeling and atmospheric science that no previous instrument could.

As part of NASA’s PACE mission, HARP2 represents a significant leap forward in cloud microphysical property retrievals, but its potential extends far beyond the applications demonstrated in this work. HARP2’s sensitivity to cloud microphysics enhances our ability to study cloud evolution, including the transition from cloud to drizzle and precipitation onset (Sinclair et al., 2021; Miller et al., 2016). Its multiangular observations will improve our ability understand and constrain the volumetric extent of clouds, supporting applications in cloud tomography (Levis et al., 2020; Ronen et al., 2025). Additionally, the application of the Rainbow Fourier Transform algorithm to HARP2 data enables the retrieval of liquid cloud DSD in complex cloud structures (Alexandrov et al., 2012b; Alexandrov et al., 2016). HARP2’s capability to independently detect clouds over highly reflective surfaces such as ice and snow expands our ability to study clouds in polar regions, where traditional cloud-detection techniques rely on multiple spectral bands or complex algorithms to distinguish clouds from bright surfaces (van Diedenhoven et al., 2012; Zhou et al., 2020; Frey et al., 2008). With frequent polar observations, HARP2 provides new opportunities to improve cloud detection and analysis in these challenging environments.

Although this work focuses on cloud science applications, multiangular polarimeters like HARP2 are also highly effective for aerosol characterization (Dubovik et al., 2019; Dubovik et al., 2021). HARP2’s ability to retrieve aerosol properties such as particle size distributions, refractive index, and sphericity (Espinosa et al., 2019) will enhance our understanding of cloud-aerosol interactions, a major source of uncertainty in climate science (Sinclair et al., 2020). By resolving both cloud and aerosol properties with greater precision, HARP2 enables new insights into their interactions and impacts on climate.

HARP2’s multiangular capabilities provide high information density, enabling the development of novel algorithms to address the complexities of cloud-aerosol interactions and their climate implications. With its unprecedented spatiotemporal resolution and broad angular swath, HARP2 facilitates the first retrievals of liquid cloud microphysics with near-global coverage, allowing us to monitor cloud and aerosol properties and accumulate climatological statistics. HARP2 measurements, when applied to existing retrieval algorithms, will expand our understanding of cloud processes. While the HARP2 mission is still in its early stages, initial results demonstrate promising advancements in cloud research.

Data availability statement

All data used in this work is publicly available and can be download using the NASA EARTHDATA search tool at https://search.earthdata.nasa.gov/search.

Author contributions

RS: Writing – original draft, Writing – review and editing. BM: Writing – review and editing. XX: Software, Writing – review and editing. AP: Software, Writing – review and editing. NS: Writing – review and editing. JC: Writing – review and editing. LR: Writing – review and editing. RF-B: Writing – review and editing. JM: Funding acquisition, Project administration, Supervision, Writing – review and editing.

Funding

The author(s) declared that financial support was received for this work and/or its publication. This work was funded as part of the PACE project, GESTAR-II cooperative agreement 80NSSC22M0001.

Acknowledgements

The authors thank the engineers, scientists, and support staff at the UMBC Earth and Space Institute for their dedication to the HARP2 mission, as well as the previous HARP missions, without which HARP2 would not have been possible.

Conflict of interest

The author(s) declared that this work was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

Generative AI statement

The author(s) declared that generative AI was not used in the creation of this manuscript.

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