The removal of soluble species by warm stratiform clouds

A one-dimensional, time-dependent model has been developed to study the removal of soluble gases from a warm, precipitating stratiform cloud. The model calculates the distributions of water vapor and condensed water, in the form of cloud drops and raindrops, as well as the incloud concentration of a soluble species in the gas and aqueous phases for a specified profile of pressure and temperature and an assumed updraft velocity. Highly soluble gases are found to be rapidly dissolved into cloud droplets and then slowly incorporated into raindrops as cloudwater is converted to rainwater. As this rainwater begins to fall out of the cloud, the total incloud abundance of highly soluble species rapidly decreases, causing the rate of removal of the species from the cloud to also decrease. The rate of removal of highly soluble species continues to decrease until a balance between loss via rainout and transport of new material into the cloud is reached, and an approximate steady state is established. Thus the rainout rate for highly soluble species is found to be ultimately limited by the rate at which new gaseous material can be transferred from the cloud-free air into the cloud. The model calculations indicate that turbulence represents an important mechanism by which highly soluble gases are transported into strati form clouds and thus the calculated rate of removal of highly soluble species from these types of clouds is dependent on the parameterization chosen to simulate this process.