Harmonized MODIS-VIIRS Global Ocean Cloud Droplet Number Concentration Long-Term Dataset

This dataset is a long-term MODIS-VIIRS global ocean cloud droplet number concentration ($N_d$) product harmonized and calibrated via machine learning, covering the period from 2003 to 2023. Late orbital drift of the Aqua-MODIS satellite induces spurious anomalies in retrieved $N_d$. Meanwhile, inherent systematic biases exist between the MODIS and VIIRS sensors, making the raw datasets impossible to be directly spliced into a continuous time series. Based on overlapping observations of the two satellites from 2012 to 2021, this study constructs an MLP correction model to uniformly calibrate the MODIS-retrieved $N_d$ values from 2003 to 2021 against the VIIRS reference standard, and concatenates VIIRS-derived $N_d$ for 2022 onward. The input variables of the model we adopted include cloud droplet effective radius, cloud optical thickness, solar zenith angle, satellite zenith angle, longitude and latitude, and cloud droplet number concentration retrieved from MODIS observations. The model exhibits excellent overall performance with an RMSE of approximately 53 cm⁻³. The merged dataset largely preserves the long-term spatial and temporal variability of MODIS oceanic cloud droplet number concentration and features seamless temporal transition. It can provide a reliable observational basis for research on aerosol-cloud interactions and performance evaluations of climate model simulations. Technical info Product information File 'Harmonized_nd_xxxx.nc': Harmonized MODIS-VIIRS Global Ocean $N_d$ Dataset for the year xxxx, with a spatial resolution of 1°×1° Time: Observation time (UTC) expressed as HHMM lat: Geographical latitude of cloud pixel center, valid range [-90, 90] lon: Geographical longitude of cloud pixel center, valid range [-180, 180] Year: Calendar year of the satellite observation Day: Day of the year for the satellite retrieval, ranging from 1 to 366 harmonized_nd: MLP harmonized cloud droplet number concentration consistent across MODIS-Aqua and VIIRS sensors, unit: cm⁻³, valid range [0, 9999] File 'VIIRS_filtered_nd_xxxx.nc': $N_d$ dataset retrieved and filtered from VIIRS observational data for the year xxxx, with a spatial resolution of 1°×1° Time: Observation time (UTC) expressed as HHMM lat: Geographical latitude of cloud pixel center, valid range [-90, 90] lon: Geographical longitude of cloud pixel center, valid range [-180, 180] Year: Calendar year of the satellite observation Day: Day of the year for the satellite retrieval, ranging from 1 to 366 viirs_nd: Cloud droplet number concentration retrieved and filtered from VIIRS observations, unit: cm⁻³, valid range [0, 9999]