Modeled aerosol-cloud indirect effects and processes based on an observed partially glaciated marine deep convective cloud case

A tropical maritime case of deep convective clouds was studied using a state-ofthe-art aerosol-cloud model in order to evaluate the microphysical mechanisms of aerosol indirect effects (AIE). The aerosol-cloud scheme used is a hybrid bin/bulk model, which treats all phases of clouds and precipitation allowing a detailed analysis of process-level aerosol indirect effects on targeted cloud types. From the simulations, a substantially huge total AIE on maritime clouds of - 17.44 ±6.1 Wm−² was predicted primarily because maritime clouds are highly sensitive to perturbations in aerosol concentrations because of their low background aerosol concentrations. This was evidenced by the conspicuous increases in droplet and ice number concentrations and the subsequent reductions in particle mean sizes in the present-day. Both the water-only (-9.08 ±3.18 Wm−² ) and the partially glaciated clouds (-8.36 ±2.93 Wm−²) contributed equally to the net AIE of these maritime clouds. As for the partially glaciated clouds, the mixed-phase component (-14.12 ±4.94 Wm−²) of partially glaciated clouds was dominant, whilst the ice-only component (5.76 ±1.84 Wm−²) actually exhibited a positive radiative forcing at the top of the atmosphere (TOA). This was primarily because ice water contents aloft were diminished significantly owing to increased snow production in the present-day

Aerosol Indirect Effects on Glaciated Clouds. Part II: Sensitivity Tests Using Solute Aerosols

Sensitivity tests were performed on a midlatitude continental case using a state-of-the-art aerosol–cloud model to determine the salient mechanisms of aerosol indirect effects (AIE) from solute aerosols. The simulations showed that increased solute aerosols doubled cloud-droplet number concentrations and hence reduced cloud particle sizes by about 20% and consequently inhibited warm rain processes, thus enhancing the chances of homogeneous freezing of cloud droplets and aerosols. Cloud fractions and their optical thicknesses increased quite substantially with increasing solute aerosols. Although liquid mixing ratios were boosted, there was, however, a substantial reduction of ice mixing ratios in the upper troposphere, owing to the increase in snow production aloft. The predicted total aerosol indirect effect was equal to −9.46 ± 1.4 W m−2. The AIEs of glaciated clouds (−6.33 ± 0.95 W m−2) were greater than those of water-only clouds (−3.13 ± 0.47 W m−2) by a factor of two in this continental case. The higher radiative importance of glaciated clouds compared with water-only clouds emerged from their larger collective spatial extent and their existence above water-only clouds. In addition to the traditional AIEs (glaciation, riming and thermodynamic), the new AIEs sedimentation, aggregation and coalescence were identified

Aerosol indirect effects on glaciated clouds. Part I: Model description

Various improvements were made to a state-of-the-art aerosol–cloud model and comparison of the model results with observations from field campaigns was performed. The strength of this aerosol–cloud model is in its ability to explicitly resolve all the known modes of heterogeneous cloud droplet activation and ice crystal nucleation. The model links cloud particle activation with the aerosol loading and chemistry of seven different aerosol species. These improvements to the model resulted in more accurate prediction especially of droplet and ice crystal number concentrations in the upper troposphere and enabled the model to directly sift the aerosol indirect effects based on the chemistry and concentration of the aerosols. In addition, continental and maritime cases were simulated for the purpose of validating the aerosol–cloud model and for investigating the critical microphysical and dynamical mechanisms of aerosol indirect effects from anthropogenic solute and solid aerosols, focusing mainly on glaciated clouds. The simulations showed that increased solute aerosols reduced cloud particle sizes by about 5 μm and inhibited warm rain processes. Cloud fractions and their optical thicknesses were increased quite substantially in both cases. Although liquid mixing ratios were boosted, there was however a substantial reduction of ice mixing ratios in the upper troposphere owing to the increase in snow production aloft. These results are detailed in the subsequent parts of this study

Aerosol–fog interaction and the transition to well-mixed radiation fog

We analyse the development of a radiation fog event and its gradual transition from optically thin fog in a stable boundary layer to well-mixed optically thick fog. A comparison of observations and a detailed large-eddy simulation demonstrate that aerosol growth and activation is the key process in determining the onset of adiabatic fog. Weak turbulence and low supersaturations lead to the growth of aerosol particles which can significantly affect the visibility but do not significantly interact with the long-wave radiation, allowing the atmosphere to remain stable. Only when a substantial fraction of the aerosol activates into cloud droplets can the fog interact with the radiation, becoming optically thick and well mixed. Modifications to the parameterisation of cloud droplet numbers in fog, resulting in lower and more realistic concentrations, are shown to give significant improvements to an NWP model, which initially struggled to accurately simulate the transition. Finally, the consequences of this work for common aerosol activation parameterisations used in climate models are discussed, demonstrating that many schemes are reliant on an artificial minimum value when activating aerosol in fog, and adjustment of this minimum can significantly affect the sensitivity of the climate system to aerosol radiative forcing