Stratospheric Dynamics: Wave Measurements and Modeling

Leonhard Pfister and K. Roland Chan


The lower stratosphere is the region where significant ozone depletion has occurred as a result of human activities (the prime example is the Antarctic ozone hole). Chemical reaction rates in this region are slow; therefore, determining the origin and properties of air motions in the lower stratosphere is vitally important to understand the distribution of ozone depletion. Waves, a particular type of air motion, drive the overall stratospheric circulation and, thus, govern the global distribution of ozone.

The waves of greatest interest to this project have sizes ranging from a typical house lot to 100-200 miles. Too small to be resolved by satellites, their properties are detected by instruments on high-flying aircraft, such as NASA's ER-2. Two of the most important causes for these waves are depicted in the illustrations on the reverse. Figure 1 shows waves excited by air flow over mountains, as measured by the Ames Meteorological Measurement System (MMS) aboard the ER-2. Like ocean waves, these "mountain" waves are able to travel substantial distances. In this case, they travel upward about 12 miles to the flight level of the ER-2. Upon reaching this level, the mountain wave interacts with the flow to produce a long string of wavelets following the main wave at 17!E. The details of this interaction and its significance for the overall stratospheric circulation are currently under study.

Figure 2 shows waves excited by convective clouds, in this case a tropical cyclone (hurricane) off northern Australia. Here, dark green and black denote regions with the highest clouds and strongest convection; yellow and red areas indicate weaker convection. The white contours depict the structure of the wave, as determined by a model simulation based on ER-2 MMS measurements made above the highest clouds. The wave is similar in many ways to the waves caused by a pebble thrown into a stream, with the pebble being the hurricane and the stream being the stratospheric flow (in this case, from right to left). These waves drive the overall circulation at all altitudes in the tropical stratosphere. Model calculations show that they are partially responsible for the vertical flow between the lower atmosphere (troposphere) and the stratosphere.

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