Retrieval and validation of cloud condensation nuclei from satellite and airborne measurements over the Indian Monsoon region

The estimation of cloud condensation nuclei (CCN) concentration at the cloud base is an essential constraint to aerosol-cloud interaction (ACI) studies. The long-term CCN measurements are sparse and difficult to obtain, which often led to the use of aerosol optical properties as proxies of CCN in ACI studies and remains the most significant uncertainty in numerical models and observational studies. In the present study, we estimate the height-resolved CCN from Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observations (CALIPSO) using Optical Modelling of CALIPSO Aerosol Microphysics (OMCAM) algorithm and validated the results with airborne in-situ CCN measurements collected during the Cloud Aerosol Interaction and Precipitation Enhanced Experiment (CAIPEEX) at six locations over the Indian Summer Monsoon (ISM) region. Out of 40 CAIPEEX flight measurements collected in the year of 2009, 15 profiles were selected for the validation of CALIPSO-derived CCN number concentrations (N-CCN). We found a reasonable agreement between CALIPSO and the in-situ measurements with a correlation coefficient of 0.72. However, the overall comparison suggests that the CALIPSO overestimates the N-CCN by 54 +/- 33%. We further analyzed CALIPSO-derived N-CCN for the in-situ stations. A strong seasonal variation of N-CCN at SS 0.4% is observed in all six stations. The mean N-CCN is maximum (minimum) during the post-monsoon (pre-monsoon) for Pathankot 2592 +/- 1026 cm(-3) (1636 +/- 809 cm(-3)), Bareilly 3618 +/- 1326 cm(-3) (2078 +/- 792 cm(-3)) stations whereas maximum (minimum) during winter (monsoon) for Guwahati 3696 +/- 1400 cm(-3) (2661 +/- 1096 cm(-3)), Pune 3422 +/- 1628 cm(-3) (2241 +/- 1086 cm(-3)), Hyderabad 3765 +/- 1366 cm(-3) (2498 +/- 1084 cm(-3)) and Bangalore 2956 +/- 1274 cm(-3) (2401 +/- 1071 cm(-3)).

Automated summary

The estimation of cloud condensation nuclei (CCN) concentration is crucial for aerosol-cloud interaction (ACI) studies. However, acquiring long-term CCN measurements is challenging, leading to the use of aerosol optical properties as proxies of CCN. In this study, we used CALIPSO data and an algorithm called OMCAM to estimate height-resolved CCN and validated it with in-situ CCN measurements from CAIPEEX. Results showed a reasonable agreement but also an overestimation of N-CCN by CALIPSO.