Fast descent routes from within or near the stratosphere to Earth's surface
Abstract. By using high concentrations of 7Be as an indicator, we clarify fast descent routes from within or near the stratosphere to Earth's surface, with the study site being in Fukuoka, Japan. Most routes arise from high latitudes through the following processes. First, the descent associated with a tropopause fold occurs, followed by southward movement with slow descent at the rear side of a strong trough. Because this motion occurs along an isentropic surface, the descending air parcels nearly conserve the potential temperature. As an extension, a strong descent associated with a sharp drop in the isentropic-surface height occurs at the south edge of the trough; this transports air parcels to low altitudes. This process involves irreversible phenomena such as filamentation and cutoff of potential vorticity. Finally, upon meeting appropriate near-surface disturbances, parcels at low altitudes are transported to Earth's surface. In some cases, parcels descend within mid-latitudes. In such routes, because the potential temperature is much higher at high altitudes than at low altitudes, strong descent with conservation of the potential temperature is impossible, and the potential temperature decreases. In these cases, the entire flow does not move downward; instead, only part of the flow in a diffluent field descends. When parcels descend, they push low isentropic surfaces, and their potential temperature decreases upon mixing with parcels having low potential temperature in the lower layers. The prevalence of the high-latitude route is explained as follows. In the mid-latitude route, because parcels at high and relatively low altitudes mix, the high concentrations of 7Be included in high-altitude parcels are difficult to maintain. Therefore, for parcels to arrive at low altitudes in the mid-latitude while maintaining high concentrations of 7Be, i.e., conserving the potential temperature, their area of origin should be high altitudes in high latitudes where the potential temperature is almost the same as that in the arrival area. Moreover, the initial descent must occur, because parcels cannot descend in the stratosphere when moving from high to mid-latitudes; parcels must already have descended from the stratosphere to the troposphere in high latitudes for effective descent with the movement to mid-latitudes. In spring, tropopause folds are frequent in high latitudes, disturbances in the southward transport of parcels are strong, and disturbances occur by which parcels descend to the surface. Therefore, high concentrations of 7Be occur most frequently in spring.
