Modification of the Atmospheric Boundary Layer by a Small Island: Observations from Nauru

Long, C. N., Pacific Northwest National Laboratory

Cloud Distributions/Characterizations

Radiative Processes

Matthews, S., J. M. Hacker, J. Cole, J. Hare, C. N. Long, and R. M. Reynolds, (2007): Modification of the atmospheric boundary layer by a small island: observations from Nauru, MWR, Vol. 135, No. 3, pages 891–905.

Figure 1. Illustration of daytime heating producing a thermal internal boundary layer effect over Nauru, which in turn produces cumulous clouds above the boundary layer.

Figure 2. Illustration of Nauru heat-island produced by convective rolls forming cloud streets.

Figure 3. Satellite images of Nauru on December 13, 2000 showing the cloud street extending hundreds of km (bottom) with cloud- free areas on either side (top) consistent with convective rolls.

A plausible cause of the Nauru cloud plume is presented, along with the details of the island heat effect Nauru experiences.

Nauru, a small island in the tropical Pacific, generates plumes of clouds (called "streets") that may grow to several hundred kilometers in length. This study uses observations from the ARM Nauru99 Field Experiment to examine the mesoscale disturbance of the marine atmospheric boundary layer by the island that initiates these cloud streets. Observations were made from two ships in the vicinity of Nauru and from instruments on the island, as well as aircraft flights.

During the day, the island surface layer is often warmer than the marine surface layer, and wind speed is lower than over the ocean. Surface heating forces the growth of a thermal internal boundary layer, above which cumulus clouds form. A plume of warm-dry air was observed over the island during Nauru99 that extended 15 to 20 km downwind.

Previous research has demonstrated that long cloud streets could be generated by a pair of convective rolls originating in the convergence zone in the region downwind of a heat-island of finite width, such as that produced by Nauru. Observations during the experiment, including aircraft passes under the Nauru cloud street, showed temperature and humidity traces that were characteristic of convective rolls, although conclusive evidence was not available. So it is likely that a pair of convective rolls is sometimes generated by Nauru that form and maintain the observed cloud street.

The Nauru cloud street is likely produced by a pair of convective rolls (vortices) produced by the Nauru heat-island perturbation of the low-level air flow. This self-sustaining mechanism then often produces a long cloud street extending hundreds of kilometers downstream of the island, which is visible in satellite imagery.