A mixed rain-snow storm associated with a strong burst of cold air and development of an extratropical cyclone occurred over North China from 3 to 5 November 2012.This early snowfall event was characterized by a dramatic drop in temperature,strong winds,high precipitation intensity,broad spatial extent,and coexistence of multi-phase precipitating hydrometeors.This study investigates the multi-scale interactions between the large-scale circulation background and the synoptic-scale weather systems associated with the storm.The results are as follows.（1） The Arctic Oscillation （AO） had been in its negative phase long before the event,leading to southward advection of cold air into North China in advance of the storm.（2）The large-scale atmospheric circulation experienced a decreased number of long waves upstream of North China prior to the storm,resulting in reduced wave velocity and an almost stagnant low pressure system （extratropical cyclone） over North China.（3） An Ω-shaped blocking high over East Asia and the western Pacific obstructed the eastward movement of an upstream trough,allowing the corresponding surface cyclone to stabilize and persist over Beijing and its neighboring areas.This blocking high was a major factor in making this event a historically most severe precipitation event in autumn in Beijing for the past 60 years.（4） Baroclinic instability at lower levels gave rise to rapid development of the cyclone under the classical ＂second type＂ development mechanism for extratropical cyclones.（5） Moisture originated from the Yellow Sea entered the slowly-moving cyclone in a steady stream,creating fairly favorable water vapor supply for the heavy rainfall-snowfall,especially during the later stage of the cyclone development.（6） Moisture transport and frontal lifting triggered low-level instability and updrafts.Intensification of the front enhanced the vertical wind shear,causing conditional symmetric instability （CSI） to expand upward within the unstable lower tropos
Two methods for initialization of ensemble forecasts are compared, namely, singular vector (SV) and conditional nonlinear optimal perturbation (CNOP). The comparison is done for forecast lengths of up to 10 days with a three-level quasi-geostrophic (QG) atmospheric model in a perfect model scenario. Ten cases are randomly selected from 1982/1983 winter to 1993/1994 winter (from December to the following February). Anomaly correlation coefficient (ACC) is adopted as a tool to measure the quality of the predicted ensembles on the Northern Hemisphere 500 hPa geopotential height. The results show that the forecast quality of ensemble samples in which the first SV is replaced by CNOP is higher than that of samples composed of only SVs in the medium range, based on the occurrence of weather re-gime transitions in Northern Hemisphere after about four days. Besides, the reliability of ensemble forecasts is evaluated by the Rank Histograms. The above conclusions confirm and extend those reached earlier by the authors, which stated that the introduction of CNOP improves the forecast skill under the condition that the analysis error belongs to a kind of fast-growing error by using a barotropic QG model.
Effects of vertical wind shear, radiation, and ice clouds on cloud microphysical budget associated with torrential rainfall during landfall of severe tropical storm Bilis （2006） are investigated by using a series of analysis of two-day grid-scale sensitivity experiment data. When upper-tropospheric upward motions and lower-tropospheric downward motions occur on 15 July 2006, the removal of vertical wind shear and ice clouds increases rainfall contributions from the rainfall type （CM） associated with positive net condensation and hydrometeor loss/convergence, whereas the exclusion of cloud radiative effects and cloud-radiation in- teraction reduces rainfall contribution from CM. The elimination of vertical wind shear and cloud-radiation interaction increases rainfall contribution from the rainfall type （Cm） associated with positive net conden- sation and hydrometeor gain/divergence, but the removal of cloud radiative effects and ice clouds decreases rainfall contribution from Cm. The enhancements in rainfall contribution from the rainfall type （cM） as- sociated with negative net condensation and hydrometeor loss/convergence are caused by the exclusion of cloud radiative effects, cloud-radiation interaction and ice clouds, whereas the reduction in rainfall contri- bution from cM results from the removal of vertical wind shear. When upward motions appear throughout the troposphere on 16 July, the exclusion of all these effects increases rainfall contribution from CM, but generally decreases rainfall contributions from Cm and cM.
