CLEX SCIENCE OBJECTIVES

This experiment addresses a better physical understanding of the morphology of extensive layers of non-precipitating clouds in the middle and upper troposphere. These are in a sense the "forgotten" clouds on Earth. National weather services do not forecast these clouds and with a few exceptions, the civilian community generally ignores them. Within the climate research community, only a subset of "CLEX clouds" - layers of "radiatively significant" cirrus - have been identified as a high-priority for study and understanding regarding their role in the climate system. However, in the last five years three realizations have caused us to greatly elevate the scientific priority of understanding the origin and duration of complex single and multi-layer cloud systems. The first was a practical example of the important effects of these clouds on several key DoD operations during Desert Shield and Desert Storm. Various post-campaign reports and publications have documented the importance of understanding and forecasting such cloud layers and their details will not be repeated here. Secondly, the advent of new 3-D and 4-D variational assimilation of satellite radiance data into operational forecast models - and the improved forecast skill that resulted (Burridge, ECMWF, 1996; and Friday, NWS, 1996, personal communications) - focuses special attention on proper model representation of these clouds (Tiedke, 1993) and forward radiances from them. Third is a realization from the climate research community that the layer-to-layer and layer-to-surface radiative effects of extended cloud layers not only may play a key role in their morphology but may also strongly impact regional atmospheric dynamics.

For these reasons we developed the Complex Layered-Cloud research project (Task 10) in the DoD Center for Geosciences, Phase II at CSU. Planning and early studies began on this research in mid-1994. It soon became obvious that several questions being addressed under "Task 10" were also of special importance to two other Center for Geosciences tasks. The following subsections recall the hypotheses, objectives and plans of these three research tasks which together form the basis for the CLEX. Additional details and background may be found in Vonder Haar et al. (1994) and (1996).