Research Interests: Airflow over and around mountains, cyclogenesis, frontal formation and propagation, monsoon meteorology.
Teaching Interests: Dynamic meteorology, waves and instabilities, earth system science.
Dr. Bannon explores the influence of mountains on the earth's weather and climate. The interaction of the atmosphere with the underlying topography encompasses a rich variety of phenomena. Among them are the formation of storms downstream of mountain ranges (lee cyclogenesis), the role of high plateaus as elevated heat sources, the blocking of the low-level flow to form trapped boundary currents, the exchange of momentum with the solid earth, and the propagation of fronts over mountains. Other research focuses on the influence of physical processes (friction, precipitation) on the formation, structure, and propagation of fronts and cyclones.
A major theme in Bannon's research is the role that mountains play in our weather and climate. A mountain range can have both a mechanical influence or a thermal influence, or a combination of the two.
Mechanically, a mountain can force the air to go up and over the range or it can block the air and make it move laterally around the obstacle. Air parcels that have been displaced vertically often undergo a wavy motion downwind of the mountain. These "mountain waves" produce thin lens-shaped clouds as well as updrafts that glider pilots use for soaring. Sophisticated computer models used for weather prediction make better forecasts when they include the energy transports due to mountain waves. Bannon and a graduate student have documented the strength of Pennsylvania's mountain waves using high resolution data of the state's topography that includes Tussey and Nittany Mountains.
Mountains also affect the path of low-pressure cells or storms. Typically a storm system off the west coast of the U.S. is deflected to the north, bringing rain to Seattle but sparing San Diego. The storm tends to weaken over the Rockies, only to reform and intensify over Colorado and Kansas. Bannon has developed a mathematical model of the formation of this "lee cyclone". He has also studied the distortion of a cold front as it encounters a long mountain range like the Appalachians. A front is slowed upstream of a mountain, weakens as it move over the crest only to reappear stronger and steeper downstream.
Bannon has also investigated the role of mountains in the summer monsoon of South Asia. There the giant plateau of Tibet acts as an elevated heat source. During summer, the air over the plateau is warmed by the sun's radiation. As the warmed air rises, air from the surrounding lowlands of India and Bangladesh are drawn towards the Himalayas. This moist air is forced upward, helping to generate the heavy monsoon rains. Thus, the Asian monsoon is more like a giant mountain breeze than a sea breeze. Using a blend of satellite, upper air, and ground based observations, Bannon and a graduate student have quantified the surface radiation budget for the monsoon and shown that the radiative warming over Tibet is twice that over the surrounding regions.
Mountains also help create deserts. Air dried by its ascent over the Tibetan plateau arrives over central Asia with too little moisture to produce precipitation. In this "rain shadow," the Gobi desert forms. In South America, the blocking of the surface winds by the Andes prevents rising motion and precipitation. This results in the coastal desert of Chile and Peru, where the town of Iquiqui has gone as long as ten years without measurable precipitation.
While primarily a theoretician, Bannon has also participated in the Alpine Experiment. This international field program studied mountain waves, lee cyclones, and frontal distortion over the European Alps. Flights in specially equipped research aircraft that can withstand hurricane force winds flew over Europe and the Mediterranean sea gathering data to test the theories.
Bannon, P.R., 1986: Linear development of quasi-geostrophic baroclinic disturbances with condensational heating. Journal of the Atmospheric Sciences, 43, 2261-2274.
Bannon, P.R., 1989: Linear baroclinic instability with the geostrophic momentum approximation. Journal of the Atmospheric Sciences, 46, 402-409.
Bannon, P.R., 1991: Aspects of rotating shear flow over a mountain ridge. Journal of the Atmospheric Sciences, 48, 211-216.
Bannon, P.R., 1992: A model of Rocky Mountain lee cyclogenesis. Journal of the Atmospheric Sciences, 49, 1510-1522.
Abreu, M.L., and P.R. Bannon, 1993: Dynamics of the South American coastal desert. Journal of the Atmospheric Sciences, 50, 2952-2964.