Climatic Response to Large Summertime Injections of Smoke into the Atmosphere: Changes in Precipitation and the Hadley Circulation

摘要

An atmospheric general circulation model (AGCM) has been initialized with a 150 Tg summertime injection of smoke from post-war fires over Europe, Asia and North America. The smoke is subject to large-scale and convectice transport, dry deposition, coagulation and precipitation scavenging. The Hadley circulation is shown to respond in three stages. In the first stage, which lasts about one week depending on initial conditions, the Hadley circulation doubles in intensity. As the smoke spreads across the equator, and as the troposphere becomes more stable, the Hadley cell then weakens until it becomes actually weaker than in the control climate. In the final stage, as the smoke is removed, the Hadley cell gradually returns towards the control. Surface precipitation generally decreases as a result of the smoke. By the fourth week following the injection, zonal-mean surface precipitation in the tropics and summer hemisphere midlatitudes are about half of those in the control climate. The decrease is most notable over land, ocean precipitation being reduced only in the tropics. Penetrating convective precipitation is greatly reduced at all latitudes; large-scale precipitation is enhanced, becoming the dominant mode of precipitation in the simulation. Precipitation scavenging is shown to be the dominant removal process for particles larger than one micron in diameter. As a result, the lifetime of large particles increases several-fold due to the reduction in precipitation and the ''self-lofting'' of the smoke. For particles smaller than one micron in diameter, precipitation scavenging is found to be a much less efficient removal mechanism than both coagulation, which is important during the first week following the injection, and dry deposition at later times. 16 refs., 23 figs. (ERA citation 10:052310)