Cirrus: A Modern Perspective

It is now understood that the cirrus clouds inhabiting the upper troposphere play a significant role in regulating the radiation balance of the earth-atmosphere system and so must be recognized as a crucial component in solving the human-induced climate change puzzle (Liou 1986). Because of their high altitudes, these cold, ice-dominated clouds act as a thermal blanket by trapping the outgoing terrestrial (infrared) radiation, but, at the same time, they can be effective at reflecting the incoming solar radiation back out to space. The balance between these two radiative processes, the greenhouse and albedo effects, respectively, determines the net impact of cirrus on our climate system. Which process dominates appears to be quite sensitive to the cloud microphysical and macrophysical properties (e.g., see Stephans et al. 1990). These properties in turn depend on the weather processes that generate cirrus, a function of geographic location, thereby complicating the global view. Of current concern is comprehending how cirrus clouds will respond, or feedback, to the effects of global warming caused by the buildup of carbon dioxide and other greenhouse gases. Would the changing atmosphere produce alterations in cirrus clouds that reinforce, or act to negate, the theoretically predicted global warming surmised from fundamental physics? One must also ask whether increasing jet aircraft traffic is creating more cirrus cloud cover, and if this traffic and agricultural activities are increasing the transport of dust and smoke particles into the upper troposphere and affecting, in a radiatively important sense, those cirrus formed naturally. Settling these issues could be pivotal to making difficult decisions on the future use of the Earth's resources. Fortunately, a new generation of meteorological instrumentation has become available. The need for these new measurement capabilities has helped to spawn and adapt instrumentation for cirrus research. Sophisticated cloud measurement capabilities using in situ probes on jet aircraft, satellite multispectral imaging, and remote sensing with lidar, short-wavelength radar, and passive radiometers, have all greatly facilitated cirrus cloud research. Major advancements have also been made in the field of numerical cloud modeling. As will be reviewed briefly here and in depth in following chapters, these developments have significantly advanced our knowledge of the characteristic properties of cirrus clouds over the past few decades.