Bifurcation of volcanic plumes in a crosswind

Bent-over buoyant jets distorted by a crosscurrent develop a vortex pair structure and can bifurcate to produce two distinct lobes which diverge from one another downwind. The region downwind of the source between the lobes has relatively low proportions of discharged fluid. Factors invoked by previous workers to cause or enhance bifurcation include buoyancy, release of latent heat at the plume edge by evaporating water droplets, geometry and orientation of the source, and the encounter with a density interface on the rising path of the plume. We suggest that the pressure distribution around the vortex pair of a rising plume may initially trigger bifurcation. We also report new experimental observations confirming that bifurcation becomes stronger for stronger bent-over plumes, identifying that bifurcation can also occur for straight-edged plumes but gradually disappears for stronger plumes which form a gravity current at their final level and spread for a significant distance against the current. Observations from satellites and the ground are reviewed and confirm that volcanic plumes can show bifurcation and a large range of bifurcation angles. Many of the bifurcating plumes spread out at the tropopause level and suggest the tropopause may act on the plumes as a density interface enhancing bifurcation. Even for quite moderate bifurcation angles, the two plume lobes become rapidly separated downwind by distances of tens of kilometers. Such bifurcating plumes drifting apart can only result in bilobate tephra fall deposits. The tephra fall deposit from the 16 km elevation, SE spreading, bifurcating volcanic plume erupted on 15 May 1981 from Mt Pagan was sampled by previous workers and clearly displayed bilobate characteristics. Examples of bilobate tephra fall deposits are reviewed and their origin briefly discussed. Bilobate deposits are common and may result from many causes. Plume bifurcation should be considered one of the possible mechanisms which can account for come examples of bilobate tephra fall deposits.     

风对赛艇等户外静水项目有利赛道选择的影响

利用山东日照水上运动训练基地奥林匹克水上公园静水赛道两岸自动观测站近两年的逐10 min最大风实况数据,对两岸风向、风速的变化特征进行了对比分析。结果表明,水面对风的加速作用使赛艇、皮划艇等水上静水项目的比赛需要根据不同风况设定或调整有利赛道次序,并给出了弱风环境下的赛艇、皮划艇等静水项目比赛,根据不同风向风速判断和选择有利赛道的方法。研究得出：沿赛道走向,顺风、逆风或侧风风速小于3 m/s时,风对有利赛道的选择影响不大;侧风风速稳定大于3 m/s时位于水面上风方的赛道可依次设定或调整为有利赛道;而在风速稳定大于6 m/s时,静水项目的比赛可能无法正常进行。分析结果可为此类水上赛事裁判组制定和调整有利赛道提供依据。