Level-2 Algorithms - Cloud Parameters

Sentinel-5P Technical Guide - Level-2 Processing Overview - Cloud Parameters

There are two main algorithms used for TROPOMI cloud retrieval:

  • S5P_CLOUD_OCRA is the default algorithm for obtaining the fractional cloud cover.
  • S5P_CLOUD_ROCINN is the default algorithm for the cloud optical thickness and cloud top-height products.

 

Fractional Cloud Cover

Algorithm S5P_CLOUD_OCRA is based closely on OCRA - the Optical Cloud Recognition Algorithm [Loyola and Ruppert, 1998]. The main product parameter for S5P_CLOUD_OCRA is the fractional cloud cover fc (between 0 and 1), obtained through comparisons of TROPOMI broad-band reflectance measurements in the UV/VIS/NIR spectral regions with those from an offline-created global cloud-free composite data set made up of minimum reflectances.

 

Cloud Optical Thickness and Cloud Top-Height

Algorithm S5P_CLOUD_ROCINN is based on ROCINN – Retrieval Of Cloud Information Through Neural Networks [Loyola et al, 2007]. The main product parameters for S5P_CLOUD_ROCINN are cloud optical thickness and cloud-top height. These are determined through a Tikhonov regularization method based on a comparison between measurement and simulated reflectances in and around the O2 A-band. Earlier versions of ROCINN were based on the treatment of Clouds as Reflecting Boundaries (ROCCIN-CRB). The new algorithm version for TROPOMI is able to treat Clouds as optically uniform layers of light-scattering particles (water droplets). This is the Clouds-As-Layers (ROCCIN-CAL) model. Although the CAL model will be the default algorithm, providing better results for small pixels such as TROPOMI's (7x7 km2), the CRB method will also be implemented as an option. Note that when using the CRB method the retrieved cloud optical thickness is instead a retrieved cloud albedo.

 

Spectral Window

For the S5P_CLOUD_OCRA algorithm, the reflectances measured by the instrument are integrated from the detector's spectral bands 3 and 4 (310-405 nm and 405-500 nm), and spectral bands 5 and 6 (675-725 nm and 725-775 nm). For the S5P_CLOUD_ROCINN algorithm (CAL and CRB), the spectral window used is in and around the oxygen A-band (spectral band 6), typically 758-772 nm.

The TROPOMI cloud product is designed for optimal cloud correction in the S5P trace gas algorithms.