Impact of Moisture Conditions on Twomey Effect
October 26, 2012 15:25:32
Description of Problem
Generally, aerosols suppress shallow cloud development and precipitation process through Twomey effect and aerosol second indirect effect. However, it is not always the case if the meteorological variables change. Previous studies have shown the possibility of anti-Twomey effect. This study attempts to identify and evaluate the dependence of aerosol effects on meteorological variables using both RACORO (Routine AAF Clouds with Low Optical Water Depths (CLOWD) Optical Radiative Observations) aircraft field campaign data, long-term GOES (Geostationary Operational Environmental Satellite) and surface observational data in ARM SGP site. During the RACORO campaign (January to June, 2009), 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. These data could also support model simulations of boundary layer clouds. Various factors such as cloud water mixing ratio, RH, and liquid water content (LWC) are investigated.
Scientific Objectives and Approach
Making use of the aircraft 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). Only the data collected in the core area of clouds were used because the droplets at the edge part suffer from the entrainment and evaporation. Cloud albedo and Re from satellite observation and aerosol concentration from ground observation are also used to provide long-term evidence. 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 aircraft statistical averaging study, significant impacts of cloud water mixing ratio on Twomey effect were found, indicating that increasing total cloud water mixing ratio leads to a weaker Twomey effect while there is no clear trend for the LWC and RH impact. The weak competition among cloud droplets for water vapor in humid environment might be responsible to migrating effect of total cloud water mixing ratio. The long-term study also revealed that Re is larger in the humid half year than that in arid half year, and that cloud albedo slightly increases with aerosol concentration in the arid half year while decreases in the humid half year. Both the aircraft and satellite data analysis prove that Twomey effect is weaker in humid environment. Model simulation in the future could provide more evidence of our results.
Refereed Journal Publications
“ Evaluation of the aerosol indirect effect in marine stratocumulus clouds: Droplet number, size, liquid water path, and radiative impact”; Twomey et. al.; J. Geophys. Res; 110, D08203, doi:10.1029/2004JD005116
“The influence of pollution on the shortwave albedo of clouds” ; Twomey et. al.; J. Atmos., 34, 1149– 1152
“Increase of cloud droplet size with aerosol optical depth: An observation and modeling study”; Tianle Yuan et. al.; Journal of Geophysical Research, 113, D04201, doi:10.1029/2007JD008632
Other Publications and Conferences
Poster titled by “Dependence of aerosol effect on meteorological variables” in ASR 2012 Science Team Meeting
Talk titled by “Dependence of aerosol effect on meteorological variables” in breakout session in ASR 2012 Science Team Meeting