一次降雪过程持续原因分析

利用多种观测资料和中尺度数值模拟资料,对2011年2月14日发生在江淮地区的一次预报失误的持续性降雪过程进行较为全面的分析.结果表明:前期的降水导致近地面维持较大湿度,补充南下的冷平流促使低层大气接近饱和,降雪持续期间,水汽集中在对流层低层浅薄的层次中；对流层中层发展和维持的强冷平流导致降水区上空迅速降温减湿,从而在对流层中低层,逐渐建立起弱对流不稳定层结.而叠置其上的稳定层则将对流活动和水汽的向上输送限制在对流层低层内,使得水汽和能量得以在一定范围内集中；不断补充南下的冷空气强迫近地层风场发生扰动,形成的中尺度切变线,为这种浅薄层次下的弱对流活动提供了触发条件.尽管辐合抬升较弱,但与其它季节相比,气温较低的冬季,在抬升凝结高度较低的大气中,水汽易凝结成云致降水.造成这次预报失误的原因,是忽略了近地层系统的变化.另外,对补充冷空气的影响作用考虑不充分.

Analysis on causes of a persistent snowfall process

Based on a variety of observational data and mesoscale numerical simulation data, The weakness of continuous snowfall process forecasting on February 14, 2011 in Jianghuai region is analyzed comprehensively. Results suggest: rainfall at the early stage leads to more moisture near the surface, southward moving cold advection makes the lower atmosphere close to saturation; vapour mainly concentrated in shallow layer of lower troposphere during persistent snow; development and sustainment of the strong cold advection in the mid-troposphere cause the rapid cooling and humidity reduction in upper precipitation area, thus a weak convective instability stratification is built in the mid-troposphere, over which stable layer limits convective activity and upward transportation of water vapors in the lower troposphere, so vapour and energy concentrate in a certain range ; meso-shear line caused by the stirring of surface layer wind forced by continuously supplied southward cold air provides a trriggering condition for weak convective activity in the shallow layer. Despite convergence lifting is weaker than that in other seasons, vapour is easy to condense into cloud and rain with low lifting condensation level for low winter temperature. The reasons of forecasting weakness are due to setting a limit to evolution of the high-level and mid-level system but ignoring changes of the near-surface one, and underestimating the effect of supplemental cold air.