ABSTRACT

Acknowledgments................................................................ 412 References............................................................................ 412

5.1. INTRODUCTION

5.1.1. Lidar Study of the Middle and Upper Atmosphere

The middle and upper atmosphere is a complex and important region. It contains a wealth of important geophysical phenomena, for example, the Earth’s coldest environment — the mesopause; polar mesospheric clouds (PMCs) and noctilucent clouds (NLCs); the meteoric metal layers of Na, K, Ca, Li, and Fe; the airglow layers of OH, O, and O2; and planetary, tidal, and gravity wave activities that play vital roles in overall global atmosphere circulation. The growing interest in the middle atmosphere is also tied to this region’s importance for detecting and understanding global climate change. Global climate change is not confined to the lower atmosphere, but extends into the middle and upper atmosphere as well. While greenhouse gases such as CO2 and CH4 help warm the lower atmosphere by absorbing infrared radiation, they are also efficient radiators of heat and result in cooling in the middle and upper atmosphere (Roble and Dickinson, 1989). Doubling the CO2 concentration is predicted to cool the stratopause (50 km) by 10 to 12 K and the mesopause region (80 to 100 km) by 6 to 12 K (Portman et al., 1995). The observed increases in the geographic extent and brightness of PMCs and NLCs in the Polar Regions during the past decade are also believed to be evidence of cooling temperatures and increasing water-vapor concentration in the mesopause region caused by increases of greenhouse gases (Gadsden, 1990; Thomas, 1996). Therefore, the middle and upper atmosphere thermal structure and polar mesospheric clouds are recognized to be important and sensitive indicators of global climate change. Modeling the middle atmosphere is hampered by the lack of observational data and the uncertainties in the parameterization of gravity waves. Observational studies of the middle and upper atmosphere provide crucial tests of

atmospheric general circulation models, which underlie our understanding of the global climate change, thereby leading to improvements in these models and our knowledge of the global atmosphere.