Importance of Microphysical Processes in Simulating Tropical Mesoscale Convective Systems


High clouds associated with tropical mesoscale convective systems (MCSs) can extend thousands of kilometers and last tens of hours, strongly impacting the global radiation budget. Accurately representing these clouds in climate models requires understanding of microphysical processes that control cloud properties and lifetime. U.S. Department of Energy scientists at Brookhaven National Laboratory and their collaborators performed cloud resolving model simulations using three microphysics
parameterizations of varying complexity, evaluating them against
satellite-retrieved cloud properties. A new algorithm to identify and
track MCSs was also developed and applied to observations and model
simulations over the Tropical Western Pacific (TWP) to track the full
lifetime of individual cloud systems. The results demonstrated that
MCS simulations are sensitive to microphysics parameterizations. The
most crucial element was the fall velocity of frozen particles (i.e.,
ice, snow, and graupel). While model simulations all had similar
updraft characteristics, microphysics parameterizations that produced
particles with lower fall velocities produced a larger buildup of ice
in the upper troposphere, leading to longer lasting and/or larger
MCSs than observed in satellite observations. In terms of cloud
properties, the performance of more complex two-moment schemes was
not superior to that of the simpler one-moment schemes for these
tropical cloud systems. This result indicates that improvements to
microphysical parameterizations need to focus on better
representation of processes such as ice nucleation and aggregation of
ice crystals into snowflakes that affect number concentration in
tropical high clouds.


Van Weverberg, K., A. M. Vogelmann, W. Lin, E. P. Luke, A. Cialella, P. Minnis, M. Khaiyer, E. Boer, and M. P. Jensen. 2013. “The Role of Cloud Microphysics Parameterization in the Simulation of Mesoscale Convective Systems and Anvil Clouds in the Tropical Western Pacific,” Journal of the Atmospheric Sciences, DOI: 10.1175/JAS-D-12-0104.1.