Abstract
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 southern 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 midlatitudes. In such routes, because the potential temperature is much higher at high altitudes than at low altitudes, descent with conservation of the potential temperature is impossible, and the potential temperature decreases along the trajectories through mixing.The prevalence of the high-latitude route is explained as follows. In the midlatitude 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 midlatitude 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.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.
Highlights
Stratosphere-to-troposphere transport (STT) has attracted much research interest in the fields of atmospheric dynamics and chemistry and environmental science
We conclude from our analysis that it is rare for parcels to be transported within midlatitudes from high to low altitudes while maintaining high concentrations of 7Be included in high-altitude parcels (Kownacka, 2002; Land and Feichter, 2003): in this route, as mentioned in Sect. 4.2.2, parcels at high altitudes necessarily mix with those at low altitudes
By using high concentrations of 7Be as an indicator, fast descent routes from within or near the stratosphere to Earth’s surface, with the study site being in Fukuoka, Japan, were elucidated
Summary
Stratosphere-to-troposphere transport (STT) has attracted much research interest in the fields of atmospheric dynamics and chemistry and environmental science. The average features of the STT have been clarified considerably (e.g., Holton et al, 1995; Stohl et al, 2003; US Environmental Protection Agency, 2006). The intrusion of stratospheric air into the extratropical troposphere involves synoptic-scale and mesoscale processes, among which tropopause folds and cutoff lows are the most important (Stohl et al, 2003; Sprenger et al, 2003, Nieto et al, 2008). Many STT studies have focused on stratospheric air infiltration to the troposphere and have only examined the transport process to the upper and mid-troposphere.
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