View from above the clouds

Cloud Top Pressure development from Sentinel-3 OLCI

View from above the clouds
View from above the clouds

This study will develop a Cloud Top Pressure (CTP) product from the Copernicus Sentinel-3 Ocean and Land Colour Imager (OLCI) instrument.

Last Updated

03, November 2020

The main purpose of the study Cloud Top Pressure Development from Sentinel-3 OLCI (OCTPO2) is to develop an advanced, high quality, Level-2 Cloud Top Pressure (CTP) product based on Copernicus Sentinel-3 Ocean and Land Colour Imager (OLCI) measurements. The algorithm will use the measurements within the Oxygen A-band around a wavelength of 765 nm to infer the height of the cloud detected in the instrument field of view over all surface types.

Figure 1: Preliminary example of CTP retrieval from OLCI, 10 November 2016
Figure 1: Preliminary example of CTP retrieval from OLCI, 10 November 2016

Objectives

The main objectives of OCTPO2 are to:

  • Develop an algorithm to retrieve cloud properties from OLCI Sentinel-3 measurements, with a key focus on Cloud Top Pressure.
  • Take advantage of all OLCI spectral channels within the O2 A-band absorption region.
  • Generate CTP over all types of surfaces lying underneath the detected cloud layer.
  • Demonstrate the quality of the retrieval based on analysis of the OLCI O2 A-band measurements and a comprehensive assessment of the performance of the derived L2 CTP.

Overview

The challenge

The use of the O2 A-band absorption region to estimate CTP started in the sixties, but due to uncertainties in the spectral measurements and insufficient radiative transfer modelling, the retrievals often failed. Although instruments and retrieval algorithms have improved significantly over the last two decades shortcomings still remain, such as the challenge for the O2 A-band CTP algorithms to account for the vertical structure of the cloud optical thickness and to detect high-level thin clouds.

The challenge for the next generation of O2 A-band based CTP retrievals is the efficient use of the three OLCI O2 A-band channels to account more realistically for the penetration depth of the photons into the clouds, by the introduction of the natural variability of cloud extinction profiles.

CTP algorithm improvements

Analysis of the former absorption within the O2 A-band pointed to a shortage in the simulation of the O2 absorption lines, whereby an appropriate description of the continuum absorption has been identified [Drouin et al., 2017]. The improved OCTPO2 CTP algorithm benefits from a revision of the line-by-line model [Doppler et al., 2014], which now considers both the foreign- and self-continuum absorption covering the entire absorption O2 A-band, while the foreign-continuum absorption is dominating [Mlawer et al., 2012]. All the necessary look-up tables used in the OCTPO2 CTP algorithm are generated with the extended line-by-line model.

The OCTPO2 cloud algorithm also takes advantage of a new detailed temporal model for the spectral characterisation for all five OLCI cameras.

Furthermore, the CTP algorithm has been improved by the inclusion of a more realistic description of the vertical cloud profiles, which are now approximated by geometrical thickness and weighting parameters in the retrieval scheme. This leads to significantly improved CTP retrievals [Preusker and Fischer, 2020].

The algorithm is based on an Optimal Estimation technique, including an uncertainty estimate of the retrieved parameter.

Applications and validation

OLCI CTP products derived from the new OCTPO2 algorithm will be compared against MODIS and lidar Calipso CTP products.

Figure 2: Simulated Top-of-Atmosphere radiance around 760 nm and instrument response functions of the O2 A-band OLCI channels 13, 14, 15 and the reference window channels 12 and 16
Figure 2: Simulated Top-of-Atmosphere radiance around 760 nm and instrument response functions of the O2 A-band OLCI channels 13, 14, 15 and the reference window channels 12 and 16

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