Snow from shallow clouds seeded from power plant smoke stacks

Occasionally anthropogentic sources of pollutants and warm moist air can alter local conditions to produce or enhance precipitation in larger mesoscale or synoptic scale conditions that would otherwise be precipitation free. Others in the past have noted precipitation enhancement downwind of paper mills and other types of smoke stacks. On many occasions the author has seen accounts of cumulus clouds sitting over or just downwind of smoke stacks while looking out the window of aircraft. Similarly, he has seen the same over major grass or forest fires. On 31 January 2010 a coal-fire power plant smoke stack polluted and moistened a column of air that blew down stream in a narrow shallow plume and initiated a strip of precipitation about 5-8 km wide and 25 to 35 km long in western Oklahoma City, Oklahoma that continuously produce light to moderate snow for over ten hours. The purpose of this paper is to discuss the synoptic, mesoscale and microscale conditions of the event, and show the shallow plume of radar reflectivity that lasted so long and produced snow.

An approximately 600 m thick cloud layer with very low ceilings, resulted in very dense freezing fog (visibility less than 1 km) and patchy freezing drizzle over most of Oklahoma on 31 January 2010. A narrow strip of reflectivity about 5-8 km wide and 25 to 35 km long began to continuously produce light to moderate snow around 19 UTC over western Oklahoma City, Oklahoma. The narrow snow band was oriented in the direction of the low-level (lowest 600 m) steering winds out of the south-southeast. The reason for this snow band appears to be related to additional particulate matter from a coal-fired power plant perhaps acting as ice nuclei, and additional warm moisture air rising up and into, and possibly just above the cloud deck. These together produced small ice crystals that seeded the thicker cloud deck below. With plentiful cloud droplets, some large owing to the reports of freezing drizzle, the reflectivity band intensified and produced snow for over 10 hours. In this paper the radar reflectivity from the KTLX radar at 0.48° elevation is shown in this paper. Another elevation scan was available at 0.49°, but it was essentially identical to the 0.48° elevation scan. Above the 0.49° elevation scan the next available was the 1.49° elevation scan, which was free of weather related radar reflectivity because of very dry air in place at approximately those levels as detected by the KTLX radar.