Case Studies of Mixed-phase Winter Orographic Clouds with High Liquid Water Content over Idaho

Abstract:

Wintertime orographic clouds have been shown to contain supercooled liquid water (SLW) as observed by radiometers and simulated by numerical models. The presence of SLW is often an indication that the precipitation process is not efficient, possibly due to a lack of ice nuclei able to be activated into ice crystals. Natural ice nuclei often do not become activated until temperatures are colder than -15 C, however silver iodide has been shown to activate at subfreezing temperatures as warm as -5 C (DeMott 1999, Hoose and Mohler 2012). As such, the precipitation from relatively warm mixed-phase orographic clouds with SLW could potentially be enhanced using silver iodide. Idaho Power Company (IPC) has been operating a cloud seeding program in the Payette River Basin of western Idaho for over 15 years aimed at enhancing the precipitation from winter orographic clouds. During the past 5 years, IPC and the National Center for Atmospheric Research (NCAR) have been conducting research aimed at better understanding the cloud physics of the winter orographic clouds in the region and their potential for cloud seeding. From this research, several cases have been identified that have very high amounts of SLW, based on radiometer observations and numerical modeling. In one case, in situ measurements from the University of Wyoming King Air were also collected. This paper will present observations and modeling results of two cases with high SLW and discuss the implications that such cases have on aircraft icing and how seeding them with silver iodide might impact their precipitation production.