Coalescence Enhancement in large Multicell Storms Caused by the Emissions from a Kraft Paper Mill

Abstract

While conducting a randomized seeding experiment, a storm was selected whose microphysical characteristics were so far from what was expected, given the existing thermodynamic environment, that some explanation was needed to account for the apparent errant behavior of this storm. More than a decade of sampling at ?10°C using the project Lear jet has led to a simple classification technique, based on cloud-base temperature and buoyancy, which predicts the absence or presence, and to some extent the degree of coalescence (coalescence-freezing) precipitation growth in local convective storms. The unusual nature of this storm was recognized against this extensive microphysical database. The probable cause is attributed to emissions from a Kraft paper mill 10 km south of the storm's position. The mill had recently undergone an expansion program that had quadrupled its output of paper products. Using the radar characteristics of this unusual storm as a guide, a search of one season of radar data revealed the existence of five other similar storms, all within about 30 km of the paper win. These records indicated that the storms apparently modified by the paper mill tended to last longer, grow taller, and rain harder than any other storms recorded on that day. The new awareness of this apparent inadvertent weather modification by the paper mill led to launching missions to intercept clouds growing in the vicinity of the mill. Storms, apparently altered by the mill, were sampled and compared to other nearby storms. The most singular feature of the modified storms was the appearance of lame (>4 mm) drops at the most common sampling level (?10°C), indicating an accelerated or enhanced coalescence precipitation formation process. Measurements in a field of cumuli indicated a broadening of the cloud-base droplet spectra in clouds affected by the emission of the mill. It is the addition of this ?long tail? to the cloud-base droplet spectra that is apparently turning on or at least enhancing coalescence in affected storms.