Impact of Moisture Conditions on Twomey Effect
October 26, 2012 15:25:31
Description of Problem
In the Twomey effect, increasing aerosol concentration enhances cloud albedo by acting as cloud condensation nuclei (CCN) and creating more, smaller cloud droplets when the liquid water content (LWC) is constant. However, the impact of varying LWC on the Twomey effect is not determined yet, as well as the impacts of other variables characterizing moisture conditions, such as relative humidity (RH) and mixing ratio that affect the CCN nucleation and droplet growth. Between January and June 2009, an aircraft field campaign aiming at Routine AAF Clouds with Low Optical Water Depths (CLOWD) Optical Radiative Observations (RACORO) was conducted in the vicinity of the ACRF Southern Great Plains site. During the campaign, 57 flights were taken, and half of them were inside the boundary layer clouds. The obtained long-term representative statistics of cloud microphysical and aerosol properties of the atmosphere make the statistical average methods possible. This provides a more accurate measurement than the satellite, as the cloud 3D effect and other retrieving algorithms limitation do not exist here. These data could also support model simulations of boundary layer clouds.
Scientific Objectives and Approach
Making use of the observation data inside the boundary layer liquid-water cloud, we characterized the strength of Twomey effect by the slope of correlation between CCN and droplet effective radius (Re). However, because CCN data are not as many as the cloud droplet number concentration (CDNC), correlation between CDNC and Re was also studied here to complement the results. The varying slope value under different LWC, mixing ratio and RH reflects the effects of moisture conditions on the Twomey effect. For the further study, model simulations are used to understand the cloud microphysical mechanism underlying the observed phenomenon and to examine these effects in other types of clouds with different LWC, RH and mixing ratio value.
In the statistical averaging study, significant impacts of LWC and mixing ratio on Twomey effect were found, revealing that increasing LWC leads to a stronger Twomey effect while mixing ratio acts in an opposite way. There is no clear trend for the RH impact. The self-growing of droplet by condensation in water vapor rich atmosphere might be responsible to migrating effect of mixing ratio as the peak position of droplet size distribution does not change with increasing the mixing ratio. In addition, the LWC impact on the Twomey effect seems to rely on high mixing ratio values (better correlation with weaker negative slope of the correlation between Re and CCN when mixing ratio is large). As the data sets are not enough, further model simulations on this cloud type are needed.