Structure of low‐level jet streams ahead of mid‐latitude cold fronts
Structure of low‐level jet streams ahead of mid‐latitude cold fronts
复制标题
中纬度冷锋前的低空急流结构
DOI：
--
复制
发表时间：
1973
期刊：
影响因子：
--
通讯作者：
C. Pardoe
中科院分区：
文献类型：
--
作者：
K. Browning;C. Pardoe
几个案例研究显示了结构的一个冷锋在不列颠群岛。使用在锡利群岛获得的数据对其中一个案例进行了详细分析，以避免地形造成的任何混乱影响。所有描述的锋面的特征都是在表面冷锋处有一条狭窄的浅而强的对流带;这种对流基本上是二维的，被称为线对流。在每一种情况下，线对流的前方都有一个风速达25 ~ 30 m s−1的低空急流作为边界;在线对流之后，风速突然减小。低空急流嵌入在“对流边界层”中，在900至850毫巴时达到最大速度，它由一股异常温暖的潮湿空气组成，其轨迹越过更温暖的海洋。
结果表明，线对流可以看作是低空急流中尺度“右旋螺旋环流”的一部分。线对流构成了急流的一个侧面;它的特征是非常强的气旋性切变，并由最低100毫巴内急流核心下方的空气摩擦会聚提供。一些上升的线对流内的空气随后向前流动，并在低空急流的上部作为螺旋形环流的一部分逐渐消退;然而，大部分上升的线对流内的空气被挤出边界层，并在倾斜冷锋区上方作为倾斜对流向后上升。在倾斜对流福尔斯中生长的降水落在地面冷锋后方的冷空气中，降水的部分蒸发所产生的热汇可能使地面冷锋低空急流边界的尖锐性增强。
Several case studies are presented showing the structure of ana-cold fronts over the British Isles. One of the cases is analysed in detail using data acquired on the Isles of Scilly to avoid any confusing effects due to topography. All of the fronts described are characterized by a narrow band of shallow but vigorous convection at the surface cold front; this convection is essentially two-dimensional and is termed line convection. In each case the line convection is bounded on its forward side by a low-level jet reaching 25 to 30 m s−1; behind the line convection the winds decrease abruptly. The low-level jet is embedded within a ‘convective boundary layer’, reaching its maximum velocity at 900 to 850 mb, and it consists of a tongue of anomalously warm, moist air, which has had a trajectory over an even warmer sea.
It is shown that the line convection can be regarded as part of a mesoscale ‘right-hand corkscrew circulation’ within the low-level jet. The line convection constitutes one flank of the jet; it is characterized by very strong cyclonic shear and is fed by frictional convergence of air from beneath the jet core within the lowest 100 mb. Some of the air which ascends within the line convection subsequently flows forward and gently subsides within the upper part of the low-level jet as part of the corkscrew circulation; however, most of the air which ascends within the line convection is extruded from the boundary layer and ascends in a rearward direction as slantwise convection above the inclined cold frontal zone. Precipitation grown in the slantwise convection falls into the cold air behind the surface cold front and the heat sink resulting from the partial evaporation of this precipitation probably accentuates the sharpness of the boundary of the low-level jet at the surface cold front.