Diurnal Sensitivity of Cloud Liquid Path to Number Concentration Variations

A key uncertainty in constraining aerosol indirect effects on clouds is the response of the cloud liquid water path to variation in cloud droplet number. One reason for this uncertainty is that different cloud processes can influence the cloud liquid water path with different signs and time scales. In particular, precipitation suppression tends to increase the cloud water path while entrainment drying tends to decrease the cloud water path. We use a combination of Lagrangian observations of stratocumulus from geostationary satellite data and a Lagrangian large eddy simulation forced by weather reanalysis to investigate the influence of these two processes on the sensitivity of stratocumulus cloud liquid water path to number concentration variation.

Both observations and large eddy simulations show that the stratocumulus cloud liquid water path tends to decrease as cloud droplet number concentration is increased. However, our results show a sensitivity that is smaller than many previous studies have found. Furthermore, the magnitude of the sensitivity has a distinct diurnal cycle. The water path reductions are largest during the day and are significantly reduced or even positive at night. Using process denial experiments we show that while both precipitation-suppression and entrainment drying peak at night the magnitude of the diurnal amplitude of the precipitation-suppression sensitivity is significantly larger than that of the entrainment drying thus modulating the overall diurnal influence of number concentration.

Our results exemplify the non-negligible influence of the diurnal forcing on the sensitivity of stratocumulus clouds to aerosol perturbations. We extrapolate that prior studies that use idealized, non-precipitating modelling with no diurnal cycle may overestimate the liquid water path reductions associated with the entrainment drying effects.