A Mechanism Study of Cloud Droplet Activation Near Cloud Top over the Southeast Pacific

Friday, December 18, 2015 - 08:00
Add to Calendar

In the conventional view, cloud droplet number concentrations (QN) decrease with height as most cloud droplet activation occurs near the cloud base. However, the aircraft observation in the 2008 VOCALS-REx (VAMOS Ocean-Cloud-Atmosphere-Land Study Regional Experiment) over the southeast Pacific (SEP) showed that QN increase with height occasionally (~ 45% of the vertical profiles detected by the C130 outbound flight), suggesting possible cloud droplet activation around the cloud top. To investigate the mechanism for this and its impact, the WRF-Chem Model was coupled with a new two-moment physics-based microphysical scheme due to the lack of parameterization of cloud droplet activation around the cloud top in the default model framework. Results show that the model can simulate the characteristics of QN increasing with height. High QN near cloud top mainly come from the local aerosol activation into cloud droplets, rather than transported from other polluted regions. However, the frequency of QN increasing with height in the simulations is much higher than that in the observations. Overstrong longwave cooling due to the overestimated liquid water content may account for this to some extent. Further investigation is needed to identify other causes of high QN near cloud top.