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I I I I I I I I I I I I I I I I I I I 3.5 Opportunities in Cold Orographic Clouds The mountains of the eastern TVA region generate many orographic clouds, which in winter often take the form of extensive sheets of stratus or stratocumulus clouds. Our interest is mainly in the orographic clouds, which are defined here as orographic clouds with cloud top temperatures below 0 oC. Reynolds (1987) reviews the major scientific questions related to seeding shallow cold orographic clouds. He states that shallow cold orographic clouds are inefficient in converting all cloud water to precipitation before the con- densate passes over the barrier and evaporates to the lee. The term static seeding describes attempts to increase the precipitation efficiency of cloud systems without influencing the cloud dynamics. One way to do this in cold orographic clouds is to introduce artificial ice nuclei to produce additional ice crystals, which may grow into precipitation particles before passing over the crest. In order to implement this concept, seeding material must be intro- duced at the proper time, place and quantity to initiate the precipitation pro- cess efficiently. Ground or aerial releases are two methods available for this. Dependent upon the temperature structure of the cloud, silver iodide released from the ground may be an effective method of treatment. The temperature requirement is that the -5 oC level (threshold activation temperature of AgI) be no more than 3000 ft (1000 m) above the release point. This is because mechani- cal mixing induced by air flowing over complex terrain is effective at lofting the seeding material only about 1000 m in this fairly. stable cloud type. If conditions are warmer than this, aerial releases would be required or a dif- ferent seeding agent e.g., liquid propane is needed for ground releases. The seeding of cold orographic clouds has yielded very consistent results in different places. Both direct physica1 observations of the seeding and precipi- tation statistics show that seeding produces about 0.01 in/hr additional preci- pitation when supercooled liquid water is present. The results are consistent in that the concentration of supercooled liquid water is similar (0.1-0.2 g/m-3) from one location to another and it usually exists within the temperature range of 0 to -10 oC. It is anticipated that comparable liquid water concentrations would be observed in similar clouds over the eatern TVA region. The combination of satellite imagery, radar data, precipitation data, and moun- taintop weather information has provided valuable information on the structure, extent and characteristics of the cold orographic stratiform clouds. These clouds are of large horizontal extent, quite often extending back into central Ohio and Indiana and generally covering the entire eastern TVA region. The clouds are thinner or more sparse on the North Carolina side due to downslope flow such a cloud deck is fairly shallow, say 3 km or less, generally has cloud top temperatures warmer than -15 oC and cloud base temperature colder than +5 oC, and is yields only light precipitation. The data suggest the dominant pre- cipitation mechanism is the ice-phase process, however, a mixed phase precipita- tion process may occur when cloud base temperatures are warmer than +5 oC. The clouds with a mixed phase process appear to provide heavier precipitation. They xviii