Giant sea-salt aerosols and warm rain formation in marine stratocumulus

The concentrations and sizes of smaller aerosols (radius smaller than 0.5 micron) in the marine atmosphere vary due to natural and anthropogenic factors. The concentrations and sizes of giant and ultra-giant aerosols vary primarily due to wind-speed dependent wave breaking. In climate models the formation of warm rain from marine stratocumulus clouds is usually parameterized based on the drops that form on the smaller aerosols.

Calculations including giant and ultra-giant salt aerosols show that these are the dominant microphysical cause of rain-rate variability, whereas the smaller natural or anthropogenic aerosols contribute far less to rain-rate variability.

Using a dynamically simple, but microphysically highly complex model, the main results are:

(1) Drops initially formed on giant and ultra-giant aerosol particles dominate the rainfall rate at cloud base. Many other coalescence drops result from coalescence of droplets initially formed on smaller sulfate particles, but these coalescence drops remain small in comparison to those initially formed on the larger sea-salt particles. Thus the main droplet peak is for all intents and purposes merely "food" for the growth of the drops formed on giant and ultra-giant aerosol particles.

(2) The natural variability in wind speed causes large variations in the sizes and concentrations of giant and ultra-giant aerosol particles, and this leads to a variation in rainfall rate that is larger than the variation in rainfall rate caused by the natural and anthropogenic variations in smaller sulfate particles. This makes the sea-salt aerosol variability a first-order effect and the variability in smaller sulfate aerosols a second-order effect for the development of rain in warm marine stratocumulus.

(3) Simple calculations of the effects of enhanced droplet growth caused by mixing-generated broad condensational spectra point to this effect as being third-order for the development of precipitation in warm marine stratocumulus.

(4) Merely judging the state of precipitation development from the droplet effective radius is not substantiated. This is because the small droplets (formed on small aerosol particles) dominate the effective radius calculation, whereas the few large drops (formed on giant aerosol particles) do not contribute significantly to the droplet effective radius. The variability in precipitation rate caused by variability in giant sea-salt particles is thus 'masked' by the smaller cloud droplets when calculating the droplet effective radius.