High-Resolution Image

Symbols A through D in Fig. 1 represent areas of super cloud clusters.¹ Super cloud clusters are defined as areas of minimal outgoing (from earth) long-wave radiation (OLR). Fig. 2 is a blow-up of these super cloud clusters, which shows the individual cloud clusters that comprise the super cloud clusters. New cloud clusters rise ahead of the pack as trailing ones vanish. A cloud cluster is a group of individual cloud systems. The individual cloud systems that comprise the cloud clusters are not distinguishable in Fig. 2, due to its comparatively coarse resolution.

The super cloud clusters Ð with a distance of 60 to 90 degrees longitude, or 6,600 to 9,900 km, or 4,090 to 6,150 miles, between neighbors Ð move eastward at about 9 meters/second (m/s) (or 32.4 km/hour, or 20 miles/hour). Within a super cloud cluster, individual cloud clusters move westward at about 13 m/s (46.8 km/hour, or 29 miles/hour). The super cloud clusters pictured here are taken from the renowned work of Nakazawa (1988, J. Meteor. Soc. Japan). Super cloud clusters are portions of the tropical intra-seasonal oscillation Ð known as the Madden-Julian Oscillation (MJO) Ð rising from the earthÕs surface. The MJO is a major component of tropical atmospheric variability. Its impact on, among other things, the generation of hurricanes makes it Ð and by extension super cloud clusters Ð an attractive research topic.

A super cloud cluster is an example of a wave packet. A wave packet is defined as a series of waves whose maxima and minima vary from one wave to another. The envelope that circumscribes the waves within a wave packet moves as a single wave; yet, the individual waves within a wave packet move at a different speed (called the phase speed), and often in the opposite direction, from the envelope as a whole. Group velocity is a concept that has been derived by scientists to describe the speed of the envelope. A wave packet will maintain its integrity if the tendency of different component waves to separate, or disperse, and thus destroy the packet, is offset by an interacting forcing among the component waves that holds together the packet. When there is a balance between dispersive and interacting forcing, a wave packet moves as an unchanging unit, termed a Òsolitary waveÓ. Solitary waves move at a constant speed that depends on both group velocity and wave amplitude. Wave packets Ð e.g., the super cloud clusters of the figures Ð should be interpreted as solitary waves. Wave packets, therefore, travel at a speed that depends on both group velocity and wave amplitude Ð not at a speed equal to that of the group velocity.

(Contributed by Winston Chao)

¹ Time-longitude section of transient (seasonal trend removed) outgoing-long-wave-radiation (OLR) averaged between the equator and 5¡N from May to July in 1980. Negative (active convective) regions are contoured. Contour interval is 30 Wm^-2 starting at Ð15 Wm^-2. Low OLR indicates the existence of convective activity (or clouds).