The Sentinel-5p TROPOMI L2_CLOUD data product (Loyola et al., 2018) contains several cloud-related quantities, retrieved with the Optical Cloud Recognition Algorithm (OCRA) and with the Retrieval of Cloud Information using Neural Networks (ROCINN) either with the Cloud-As-Layers approach (CAL) or the Cloud-as-Reflecting Boundaries approach (CRB) (Lutz et al., 2016, Loyola et al., 2010):
- OCRA radiometric Cloud Fraction (CF);
- ROCINN_CAL Cloud Top Height (CTH) and Cloud Optical Thickness (COT);
- ROCINN_CRB Cloud Height (CH) and Cloud Albedo (CA).
The Sentinel-5p MPC performs in routine the following validation checks:
- ROCINN_CAL CTH and ROCINN_CRB CH are compared to correlative ground-based measurements from the CLOUDNET and ARM networks;
- ROCINN_CAL CTH and COT are compared to those measured by Suomi-NPP VIIRS;
- OCRA CF, ROCINN_CAL CTH and ROCINN_CRB CH are compared to Sentinel-5p radiometric cloud fraction and cloud height data retrieved with the FRESCO algorithm;
- OCRA CF, ROCINN_CAL CTH and ROCINN_CRB CH are compared to the Aura OMI O2-O2 measurements.
Given their different nature, we look at low (cloud top height below 4km) and high clouds separately. The upgrade of CLOUD v1 to v2 in July 2020 (at orbits 14238 and 14285 for OFFL and NRTI respectively) has an important impact, e.g., with a reduction of deviations at high-latitude and coastal sites. For low clouds, the bias of ROCINN_CAL cloud top height vs. CLOUDNET cloud top height is within mission requirements, but not for high clouds. (see table below). However, given the large difference in sensitivity between the TROPOMI near-infrared observations and the ground-based lidar/radar observations, the agreement is fair.
|Product ID||Stream||Product||Bias||Dispersion||Special features|
|CALv1 CTH (high)||-30%||2 km||Low clouds: CLOUDNET CTH<4km; high clouds: CLOUDNET CTH>4km.
Snow/ice albedo degrades retrievals, improved with version 02.01.03.
Occurrence of C(T)H equal to surface height at low cloud fraction.
Across track CTH and CF pattern and North-South cloud albedo pattern, improved with version 02.01.03.
|CALv2 CTH (high)||-40%||2 km|
|CALv1 CTH (low)||-15%||0.8 km|
|CALv2 CTH (low)||-15%||0.4 km|
|CRB CH (high)||-20%||1.5 km|
|CRB CH (low)||-40%||0.5 km|
A detailed description of the method and a comprehensive discussion of validation results can be found in the Sentinel-5p Quarterly Validation Report #12: April 2018 - September 2021
|Latest Quarterly Validation Report||Product info||Current Processor Version|
|#12: April 2018 - September 2021||See sentinel.esa.int||
Loyola, D. G. et al., The operational cloud retrieval algorithms from TROPOMI on board Sentinel-5 Precursor, Atmos. Meas. Tech. , 11, 409-427, 2018, doi:10.5194/amt-11-409-2018
Lutz, R. et al., OCRA radiometric cloud fractions for GOME-2 on MetOp-A/B, Atmos. Meas. Tech., 9, 2357-2379,2016, doi:10.5194/amt-9-2357-2016
Loyola, D. G.et al., Global patterns in daytime cloud properties derived from GOME backscatter UV-VIS measurements, Int. J. Remote. Sens., 31, 2010, 4295-4318, doi:10.1080/01431160903246741