The preceding 20 chapters reveal cirrus in considerable depth. Just as important, however, is what is not revealed. There are many things that we do not know or understand about cirrus. In this final chapter we present the outstanding scientific issues facing the cirrus research community. Our goal here is to produce a guide for students, scientists, policy makers, and funding organizations who wish to quickly grasp the direction and future needs of cirrus research. The impact of cirrus clouds on climate and how they interact with a climate perturbed by human enterprise is only dimly perceived. Do cirrus clouds, on a regional or global scale, act to cool or warm our planet? By reflecting incoming solar radiation to space, they can cool. Yet as an opacity source in the 10-μm window, they can radiate downward and warm the Earth. Which process dominates, and under what conditions does warming overtake cooling? Does the atmosphere react to cirrus globally or regionally (i.e., can cirrus increase pole-equator temperature differences or mute them)? Are there other mechanisms at work that defeat or amplify temperature changes by cirrus? We do not yet know. Programs such as SUCCESS, ICE, CRYSTAL, INCA, and FIRE/SHEBA will do much to answer questions about contrails and cirrus variability from one part of the world to another. They also will go a long way toward understanding one of the most difficult problems in meteorology: how convection and turbulence are related to cirrus formation and maintenance. In the meantime, existing capabilities are underused. For example, remote sensing techniques for estimating ice water path now exist but have not been assigned enough priority to achieve the necessary breakthroughs. Considerable progress could also be made in data analysis. As in other fields, analyzing existing data has a lower funding priority than designing and building new hardware and flight systems. Three fields of inquiry need more attention before we can claim a sufficient understanding of cirrus: physical properties, radiative properties, and modeling. These fields are interconnected in often subtle ways. Much of what we do not know about cirrus involves the range of properties and their evolution in time.
Keywords: CloudSat, Global Positioning System (GPS), Lidar-Infrared Radiometer (LIRAD), PICASSO-CENA, Rayleigh scattering and theory, airborne (aircraft) measurements, contrails, dropsondes, habit(s), models and modeling
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