MOBILE MICROWAVE RADIOMETER OBSERVATIONS - SPATIAL CHARACTERISTICS OFSUPERCOOLED CLOUD-WATER AND CLOUD SEEDING IMPLICATIONS

Previous studies of the spatial distribution of supercooled liquid wat er in winter storms over mountainous terrain were performed primarily with instrumented aircraft and to a lesser extent with scans from a st ationary microwave radiometer. The present work describes a new techni que of mobile radiometer operation that was successfully used during n umerous winter storms that occurred over the Wasatch Plateau of centra l Utah to determine the integrated depth of cloud liquid water relativ e to horizontal position on the mountain barrier. The technique had th e advantage of being able to measure total liquid from the terrain upw ard, without the usual terrain avoidance problems that research aircra ft face in cloudy conditions. The radiometer also collected data durin g several storms in which a research aircraft could not be operated be cause of severe turbulence and icing conditions. Repeated radiometer t ransects of specific regions of the plateau showed significant variabi lity in liquid water depth over 30-60-min time periods, but also revea led that the profile of orographically generated cloud liquid was cons istent, regardless of the absolute quantities. Radiometer liquid depth generally increased across the windward slope of the plateau to a pea k near the western edge of the plateau top and then decreased across t he relatively flat top of the plateau. These observations were consist ent with regions where maximum and minimum vertical velocities were ex pected, and with depletion of cloud liquid by accretional ice particle growth across the mountain barrier. A comparison of data from the mob ile radiometer and a stationary radiometer verified the general decrea se in liquid depth from the windward slope to the top of the plateau a nd also showed that many liquid water regions were transient mesoscale features that moved across the plateau. Implications of the results, relative to the seeding of orographic clouds, were that seeding aeroso ls released from valley-based generators could at times be inhibited b y stable conditions from reaching appropriate supercooled liquid water regions and, as found by others, the region of cloud most likely to b e encountered by an AgI seeding agent released from the ground was als o relatively warm compared to the ice-forming capability of the partic ular agent used in these experiments. Also, one convective case study that exhibited relatively warm temperatures in the cloud layer indicat ed that, even in conditions that permit vertical transport to supercoo led liquid zones, sufficient time for ice particle growth and fallout from seeded plumes on this plateau may be lacking.