Under What Conditions Can We Detect a Microphysical Response in Clouds Seeded with AgI? Lessons from SNOWIE

The Seeded and Natural Orographic Wintertime clouds: the Idaho Experiment (SNOWIE) provided clear, unambiguous signals of ice initiation and growth and of precipitation fallout to the surface within super-cooled clouds that had been seeded with silver iodide (AgI). Observations collected from airborne in situ probes and air- and ground-based radar in three cases have allowed us to quantify the amount of ice produced in seeded regions of cloud and the timescales required to produce that ice and for it to fall to surface as snow (French et al 2018, Tessendorf et al 2019). For example, for these cases we have shown that ice is first detectable in seeded regions roughly 15 – 20 minutes after the seeding material is released. Ice crystals produced through seeding remain in relatively low concentration in the SNOWIE clouds, ranging from only a few to about 15 L^-1; but, at least in some cases, appear to be continually regenerated well downwind (>50 km) and well after (> 75 minutes) the AgI was released. We also demonstrated where and how fast snow falls to the surface and that it varies largely from case to case (Friedrich et al 2019).

Airborne seeding occurred in 18 cases and despite the aforementioned successes from SNOWIE, only those three cases contained such strong, unambiguous signals. Here we examine in situ and radar observations from several other cases in SNOWIE in which conditions appeared conducive for successful cloud seeding. In a few of these cases, there appear signals consistent with the expected seeding response, in regions where they are expected to occur. However, such signals are not apparent for all seeding lines within a case and may be the result of natural ice processes. The presentation of these observations include comparisons between ice crystal concentration, size, and ice/liquid mass for regions that have been seeded and regions of clear natural ice production. Further we examine differences between natural ice regions and those in seeded regions to provide first estimates of when seeding signatures can reasonably be detected with radar and in situ observations. Understanding how background (ie natural) conditions influence our ability to detect seeding signatures is a critical step for complete analysis of the SNOWIE data set.