27 Oct 2022
27 Oct 2022
Status: this preprint is currently under review for the journal AMT.

The CHROMA cloud top pressure retrieval algorithm for the Plankton, Aerosol, Cloud, ocean Ecosytem (PACE) satellite mission

Andrew Mark Sayer1,2, Luca Lelli3,4, Brian Cairns5, Bastiaan van Diedenhoven6, Amir Ibrahim2, Kirk Knobelspiesse2, Sergey Korkin1,2, and P. Jeremy Werdell2 Andrew Mark Sayer et al.
  • 1Goddard Earth Sciences Technology And Research (GESTAR) II, University of Maryland Baltimore County, Baltimore, MD, USA
  • 2NASA Goddard Space Flight Center, Greenbelt, MD, USA
  • 3Institute of Remote Sensing Methods, German Aerospace Agency (DLR), Wessling, Germany
  • 4Institute of Environmental Physics and Remote Sensing, University of Bremen, Bremen, Germany
  • 5NASA Goddard Institute for Space Studies, New York, NY, USA
  • 6SRON Netherlands Institute for Space Research, Leiden, Netherlands

Abstract. This paper provides the theoretical basis and simulated retrievals for the Cloud Height Retrieval from O2 Molecular Absorption (CHROMA) algorithm. Simulations are performed for the Ocean Color Instrument (OCI), which is the primary payload on the forthcoming NASA Plankton, Aerosol, Cloud, ocean Ecosystem (PACE) mission, and the Ocean Land Colour Instrument (OLCI) currently flying on the Sentinel 3 satellites. CHROMA is a Bayesian approach which simultaneously retrieves cloud optical thickness (COT), cloud top pressure/height (CTP/CTH), and (with a significant prior constraint) surface albedo. Simulated retrievals suggest that the sensor and algorithm should be able to meet the PACE mission goal for CTP error, which is ±60 mb for 65 % of opaque (COT≥ 3) single-layer clouds on global average. CHROMA will provide pixel-level uncertainty estimates, which are demonstrated to have skill at telling low-error situations from high-error ones. CTP uncertainty estimates are well-calibrated in magnitude, although COT uncertainty is overestimated relative to observed errors. OLCI performance is found to be slightly better than OCI overall, demonstrating that it is a suitable proxy for the latter in advance of PACE’s launch. CTP error is only weakly sensitive to correct cloud phase identification or assumed ice crystal habit/roughness. As with other similar algorithms, for simulated retrievals of multi-layer systems consisting of optically thin cirrus clouds above liquid clouds, retrieved height tends to be underestimated because the satellite signal is dominated by the optically-thicker lower layer. Total (liquid plus ice) COT also becomes underestimated in these situations. However, retrieved CTP becomes closer to that of the upper ice layer for ice COT≈3 or higher.

Andrew Mark Sayer et al.

Status: open (until 17 Dec 2022)

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Andrew Mark Sayer et al.

Andrew Mark Sayer et al.


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Short summary
This paper presents a method to estimate the height of the top of clouds above Earth's surface using satellite measurements. It is based on light absorption by oxygen in Earth's atmosphere, which darkens the signal that a satellite will see at certain wavelengths of light. Clouds "shield" the satellite from some of this darkening, dependent on cloud height (and other factors), because clouds scatter light at these wavelengths. The method will be applied to the future NASA PACE mission.