Reconciling observed and predicted droplet concentration at the Indian summer monsoon cloud base

A droplet closure study was done using the sub-cloud aerosol and cloud condensation nuclei (CCN) spectra measurements and cloud base droplet measurements collected during the Cloud Aerosol Interaction and Precipitation Enhancement Experiment (CAIPEEX) Phase IV. Six cases with varying aerosol and cloud droplet number concentrations were identified for the study. An adiabatic parcel model with kinetic limitations of aerosols and two commonly used droplet nucleation parameterizations were used to predict the cloud base droplet number concentration. The parcel model provided better closure, especially at higher updrafts, which signifies the role of kinetic limitation during cloud formation. The underprediction at higher updrafts by one of the parametrizations was improved to some extent by increasing the Aitken mode aerosol hygroscopicity to 0.25. Further, attempts were made to test whether region specific mean aerosol characteristics can be used to predict the cloud base droplet number concentration instead of individual observations. The regional mean aerosol size distribution and hygroscopicity estimates during low and high aerosol loading conditions were taken from Varghese et al. (2023) and tuned. 50 nm - 3 μm aerosols described as three lognormal modes were sufficient to produce the observed droplet number concentration within measurement uncertainties for the monsoon cases. At the same time, finer aerosols (20-50 nm) were crucial for the two cases with continental airmass back trajectories. This droplet closure study is the first of its kind from the Indian subcontinent. It highlights the need for accurately representing aerosols in cloud-resolving models and demonstrates the potential of utilizing regional aerosol characteristics to achieve this goal.