太行山东麓一次强对流降雹过程中的地形强迫

利用WRF模式对2011年5月26日发生在太行山东麓的一次强对流降雹过程进行数值模拟,探讨了太行山及周边地形在本次强对流过程的作用。结果表明,控制华北平原的偏东暖湿气流受太行山阻挡并与切变线东南侧的西南暖湿气流汇合,在太行山东侧形成水汽高值区。太行山东坡下垫面向上热通量明显高于华北平原,午后850hPa高度山坡与平原的假相当位温梯度达到0.2 K·km~(-1),850~600 hPa假相当位温垂直梯度达4 K·km~(-1),对应上坡风的垂直速度大于1 m·s~(-1),热力环流为太行山东麓对流的发生提供了动力条件。太行山东侧暖湿气层之上为偏西干冷气流,由此形成的强热力不稳定与水汽高值区、上坡风共同造成太行山东麓强对流过程的发生。局地小尺度地形抬升与重力波共同促使太原盆地有对流单体生成,该单体移经太行山西侧迎风坡受阻挡抬升而增强,越过山顶后与维持在太行山东侧的对流单体发生合并,从而导致对流云的强烈发展。

The orographic impact of a severe convection over the Taihang Mountains

The Taihang Mountains are located between the Shanxi Plateau and the North China Plain,and extend over 400 km from north to south.Since the range rises steeply from the North China Plain to an elevation of approximately 1 000 to 1 200 m,hail is much more common along the eastward slope of the mountains.It was found that the upper level observations at 00 UTC on May 25th,2011 showed that the 500 hPa synoptic pattern in North China was dominated by a northwesterly flow,with a cold advection behind the pressure trough.A cyclonic wind shear was located at 850 hPa,with a southwesterly warm/moist conveyor belt to the east of the shear line.It was determined that,from 17:00 to 21:00 (BST),convective precipitation occurred in Xingtai,at the eastern foot of the Taihang Mountains,with amounts of up to 29.3 mm observed.Also,hail of up to 5 cm in diameter was observed in the steep eastward-facing slope.A Weather Research and Forecasting(WRF) model was utilized in an attempt to determine if the dynamical and physical impacts of the Taihang Mountains and its surrounding terrain may have contributed to the triggering and development of the severe convection.The impact of the resolution effects of the terrain data of the topographic height on the precipitation simulation in this special scenario were evaluated by a comparison between resolution of the 5-minute and 30-second terrain resolution with the model''s horizontal grid space of 1.67 km.It was observed in this study that the finer representation of the complex terrain improved the precipitation simulation.The simulated results indicated that the easterly warm moist air flow on the north side of the Huanghai low was blocked by the Taihang Mountains,and joined the southwesterly warm moist air flow from the southeast side of low level shear line.This led to a high value in the water vapor mixing ratio at the eastern side of the Taihang Mountains.The upward heat flux at the eastern slope of the Taihang Mountains was apparently higher than that of the North China Plain,due to the larger daily shortwave radiation warming on the slope surface.This resulted in the pseudo-equivalent potential temperature gradient reaching 0.2 K·km^-1 at the 850 hPa level between the mountain slope and the air over the plain.Also,the vertical gradient of the pseudo-equivalent potential temperature reached 4 K·km^-1 between the 850 and 600 hPa levels during the afternoon.The high thermal gradients of the pseudo-equivalent potential temperature increased the horizontal and vertical pressure gradient forces in such a way as to drive the wind in an upslope direction.The resulting upslope winds led to a strong ascending velocity(above 1 m·s^-1),and created dynamic conditions for the convection development on the eastern slope of the Taihang Mountains.The dry and cold westerly flow passed over the warm moist air layer on the eastern side of Taihang Mountains,which resulted in a strong static instability.The occurrence and intensification of the local convection on the eastern side of the Taihang Mountains were caused by a combination of the strong instability,high values of the water vapor mixing ratio,and upslope winds.Due to the local small-scale orographic uplift and gravity wave which were generated by the Luliang Mountains,weak convection cells appeared at the Taiyuan Basin.Then,a weak convection cell developed when it moved to the western slope of the Taihang Mountains due to the uplift caused by the blocking.When the eastward moving convection cell passed the Taihang Mountains,it combined with a local strong convection on the eastern side of the Taihang Mountains,which resulted in the intense development of a combined convection.