An evaluation of the impact of the introduction of sea salt aerosol source and sink routines in the microphysics scheme of the Regional Atmospheric Modeling System

The Regional Atmospheric Modeling System developed at Colorado State (RAMS@CSU, Cotton et. al., 2003) has recently had two new routines (Carrio and Cotton, 2010) introduced into its microhphysics scheme. The first routine provides a prognostic scheme to take into account sea salt aerosol sources based on a set of empirical formulas (O'Dowd, 1999); and the second routine provides a scavenging mechanism (aerosol sink) of sea salt particles by rain and drizzle drops. The introduction of these two new routines has prompted the evaluation of their impact on the overall microphysics simulation within RAMS.

This is a report on the results of that evaluation based on the simulation of the development of an idealized tropical cyclone using a set of experiments that selectively enable and disable the new sea salt aerosol source and sink routines. Preliminary results show distinct changes in the time progression of microphysical quantities due to the enabling of either of the new sea salt aerosol routines. The sea salt aerosol source routine introduces an increase in the average cloud droplet concentration (#/cc) in a volumetric region near the eyewall. The sea salt aerosol sink introduces a low frequency oscillation (on the scale of hours) in the amounts of supercooled cloud droplet mass, mean supercooled cloud droplet diameter and total surface precipitation in the outer rainband region.

We believe that the increase in the average cloud droplet concentration in the eyewall region due to the sea salt aerosol source happens for the following reasons. As the storm intensifies, high speed winds cover more of the surface, especially near the eyewall, and these winds become strong enough to create sea spray which provides the source for sea salt aerosols. This ultimately results in the formation of more cloud droplets due to the activation of the additional sea salt aerosols in the near eyewall region. In addition we hypothesize that the oscillatory response in the microphysical quantities due to the sea salt aerosol sink occurs by the following sequence. The introduction of CCN in the rainband region suppresses the cloud droplet collision/coalescence processes by changing the droplet size distribution to a large number of small droplets with similar sizes. In the rainband region where convection is active, these droplets are carried aloft above the freezing level which increases the amount of supercooled cloud droplets. This enhances updrafts due to the release of latent heat as these droplets freeze which ultimately increases the precipitation rate. Then the increased precipitation will scavenge more CCN at low levels increasing the size of the droplets which allows an increase in collision/coalescence to take place, reducing the supercooled cloud droplets aloft, reducing the updrafts and ultimately reducing the precipitation. Once the precipitation diminishes the scavenging decreases and allows the CCN to build up thus starting the cycle again.