New Method to Calculate Entrainment from Ground-Based Observations
As convective clouds grow, they mix with drier air through the entrainment process. Entrainment reduces a
cloud’s liquid water content, lowering its buoyancy, increasing its decay rate, and altering its microphysical characteristics. Regional and global climate models require assumptions about entrainment to simulate cloud properties and lifetimes, so observations of entrainment rate are needed. Entrainment observations have typically been made using aircraft, which are expensive and therefore limited. U.S. Department of
Energy researchers have developed a new method to calculate entrainment rate in shallow cumulus clouds using long-term measurements at the Atmospheric Radiation Measurement (ARM) Southern Great Plains (SGP) facility. The method combines measurements from four different remote sensing instruments, the
aerosol observing system (AOS) and a simple cloud parcel model. An iterative process adjusts the entrainment rate in the model until the modeled cloud characteristics converge to the observations. The
method also produces uncertainty estimates on the retrieved entrainment rate values. In this initial study, the researchers have illustrated the method by applying it to three months of data at the SGP site. Next, they will apply the method to the many years of historical data at the SGP site and to the ARM site in Darwin,
Australia, providing an unprecedented database of entrainment rates in shallow cumulus for analysis and model evaluation.
Wagner, T. J., D. D. Turner, L. K. Berg, and S. K. Krueger. 2013. “Ground-Based Remote Retrievals of Cumulus Entrainment Rates,” Journal of Atmospheric and Oceanic Technology, DOI:10.1175/JTECH-D-12-00187.1.