INTRODUCTION

ROCINN algorithms have been employed as cloud retrieval algorithms based on measurements in and aroundthe O2 A-band at 760 nm for the GOME-family of sensors. These are all based on the Independent Pixel Approximation (IPA), which is the assumption that the "radiative properties of a single satellite “Pixel” are considered in isolation from neighbouring pixels" (definition of the American Meteorological Society). The IPA allows for the application of one-dimensional plane-parallel radiative transfer (RT) theory in the forward simulation of cloud-contaminated atmospheric scenarios. The ROCINN algorithm is also based on O2 A-band measurements, and is currently being used in the operational GOME and GOME-2 products. ROCINN 2.0 retrieves as primary quantities the cloud-top height and cloud albedo. The broad-band polarization measurements from GOME, SCIAMACHY and GOME-2 are used for computing cloud fraction, see for example OCRA which is also based on the IPA. In OCRA, optical sensor measurements are divided into two components: a cloud-free background and a remainder expressing the influence of clouds. OCRA was first developed for GOME in the late 1990s, when enough data from the three sub-pixel broad-band PMDs (Polarization Measurement Devices) had accumulated to allow for the construction of the global cloud-free composite which is the key element in the algorithm. Over the course of the 16-year GOME record, the algorithm was refined and the cloud-free composite adjusted as more data became available. OCRA has also been applied to SCIAMACHY and GOME-2. Initial cloud-free composites for these sensors were based on GOME data before dedicated measurements became available from SCIAMACHY and GOME-2. For S5p, the initial cloud-free composite will be based on GOME-2 and OMI (see [RD4], chapter 5.2).

ROCINN is based on the comparison of measured and simulated satellite sun-normalized radiances in and near the O2 A-band, and it uses a neural network algorithm to retrieve cloud-top height and cloud-top albedo. ROCINN uses the cloud fraction input from OCRA as one starting point. Early versions of ROCINN used a transmittance model to compute simulated radiances, but the latest versions are based on the use of the VLIDORT radiative transfer scattering model. For GOME and GOME-2, ROCINN Version 2.0 is the current operational algorithm in the GDP [GOME Data Processor]. This version is based on the assumption that clouds are simply Lambertian reflecting surfaces, so that the two main retrieval products are the cloud- top height and the cloud-top albedo itself. This is the “clouds-as-reflecting-boundaries” (CRB) model. Although ROCINN 2.0 is the heritage algorithm, there is an important point of departure for S5p. For TROPOMI/S5p we will use ROCINN Version 3.0, which is based on a more realistic treatment of clouds as optically uniform layers of light-scattering particles (water droplets). This is the “clouds-as-layers" (CAL) model – here, the two main retrieval products are the cloud-top height and the cloud optical thickness. Although the CAL model will be the default for S5p, it has been requested that the CRB method should also be retained as an option. CAL is the preferred method for the relatively small TROPOMI/S5p ground pixels (7x7 km2). The CRB approach works best with large pixels such as those from GOME (footprint 320 x 40 km). Studies have shown that for the smaller GOME-2 pixels, CAL retrieval produces more reliable cloud information than that from CRB, not only with regard to the accuracy of the cloud parameters themselves, but also with regard to the effect of cloud parameter uncertainties on total O3 accuracy [RD4]. In the current S5p/TROPOMI L2 Cloud both CAL and CRB models are included.

USER DOCUMENTATION: ATBD

The Algorithm Theoretical Basis Document (ATBD) can be found by clicking here.

PUM & SAMPLE DATA

The Product User Manual (PUM) and Sample Data Files are being updated based on a new release of the processing software and will be available for download by mid-May 2017.

After launch, preliminary product results will be provided during the Commissioning Phase and the Operational Phase.