Convective Cloud Bands Downwind of Mesoscale Mountain Ridges
Convective Cloud Bands Downwind of Mesoscale Mountain Ridges
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DOI：
10.1175/jas-d-18-0211.1
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发表时间：
2018-11
影响因子：
3.1
通讯作者：
D. Kirshbaum;D. Schultz
中科院分区：
文献类型：
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作者：
D. Kirshbaum;D. Schultz
最近在中纬度山脊背风处观测到能够产生强降水的细长和准静止云带。本文采用理想化的显式对流模拟来研究这类波段。对环境参数空间的系统采样表明，这些波段受多层上游静态稳定剖面的影响，其中有条件不稳定的中层覆盖在绝对稳定的表层上。这样的剖面促进了背风侧液压跃变的形成，深度垂直上升引发了升高的潮湿对流。在光滑的脊上，由于跨屏障和沿屏障的压力梯度的局部叠加，孤立带在每个脊末端发展。与穿过山脊中段的地块相比，这种叠加增强了背风侧的垂直位移。在北半球，科里奥利力（相对于来流）由于经过两个脊端相反的相对涡度扰动而偏向于左带而不是右带。而过左端的负涡度异常增强了上升强迫，过右端的正涡度异常则抑制了上升强迫。对于本文所考虑的环境流动，模拟带在中等高度（1.5 km高）的山脊上最为持久，与较高或较短的山脊相比，中等高度（1.5 km高）的山脊会产生更强的背风上升。在更崎岖的地形上，由于水平压力梯度的局部叠加，额外的带形成过去的深隙或山谷。与最近一些地形云带的研究相反，这些模拟云带的存在归功于湿润静态不稳定性的释放，表明它们的形成既不需要倾斜也不需要惯性不稳定性。
Elongated and quasi-stationary cloud bands capable of producing heavy precipitation have recently been observed in the lee of midlatitude mountain ridges. Herein, idealized explicit-convection simulations are used to investigate such bands. A methodical sampling of environmental parameter space reveals that the bands are favored by a multilayer upstream static-stability profile, with a conditionally unstable midlevel layer overlying an absolutely stable surface-based layer. Such profiles promote the formation of leeside hydraulic jumps, with deep upright ascent that initiates elevated moist convection. Over smooth ridges, isolated bands develop past each ridge end due to a local superposition of cross-barrier and along-barrier pressure gradients. This superposition enhances leeside vertical displacements compared to parcels traversing the ridge midsection. In the Northern Hemisphere, the Coriolis force favors the left band over the right band (relative to the incoming flow) due to opposite-signed relative-vorticity perturbations past the two ridge ends. Whereas the negative vorticity anomaly past the left end enhances forcing for ascent, the positive vorticity anomaly past the right end suppresses it. For the environmental flows considered herein, the simulated bands are the most persistent over medium-height (1.5-km high) ridges, which force stronger leeside ascent than taller or shorter ridges. Over more rugged terrain, additional bands form past deep gaps or valleys, again due to a local superposition of horizontal pressure gradients. In contrast to some recent studies of orographic cloud bands, these simulated bands owe their existence to the release of moist static instability, indicating that neither slantwise nor inertial instability is required for their formation.