Science Seminars

1/20/2011 – The Open-Cellular Cloud System as a Coupled Oscillator

Speaker: Graham Feingold (NOAA ESRL CSD)
Date: Thursday, Jan 20, 2011
Time: 4:00pm
Location: Duane D-142

Seminar Abstract:

We explore the underlying principles of self-organization behind closed- and open-cellular mesoscale cloud structures in cloud fields off the west coast of continents. Early surface observations in the 1930s drew parallels to Rayleigh-Benard convection but it was only with the advent of meteorological satellites that observations of mesoscale cellular organization became commonplace. Recent evidence has shown that aerosol particles — through their influence on precipitation formation — help to determine whether cloud fields take on closed or open cellular patterns. Here we use satellite imagery, lidar, and numerical models to show how precipitating clouds produce an open cellular cloud pattern that oscillates between different, weakly stable states. The oscillations are a result of precipitation causing downward motion and outflow from clouds that were previously positively buoyant. The evaporating precipitation drives air down to the Earth’s surface, where it diverges and collides with the outflows of neighboring precipitating cells. These colliding outflows form surface convergence zones and new cloud formation. In turn, the newly formed clouds produce precipitation and new colliding outflow patterns that are displaced from the previous ones. As successive cycles of this kind unfold, convergence zones alternate with divergence zones and new cloud planforms emerge to replace old ones. The result is an oscillating, self-organized system with a characteristic cell size and precipitation frequency.