一次冷锋后飑线的大振幅重力波特性分析

本文用较密集的观测资料分析江淮地区一次冷锋后的飑线过程。分析表明飑线的生成和发展与中尺度冷涌活动有关，它具有大振幅重力波的特征。重力波在传播过程中于大别山东侧形成边缘锋，边缘锋的传播更类似于沿岸Kelvin波，大振幅重力波在边界层特征更明显。这类被地形拦截的大振幅重力波是江淮地区冷锋后飑线的一种可能激发机制。     

中尺度大气运动中孤立重力波特征的研究

通过对斜压大气(基本风场有垂直切变)运动中重力内波的研究发现:对于小振幅扰动(弱非线性问题),考虑波动之间的非线性相互作用以后,控制重力内波波包(调制波)的演变方程是非线性Schrodinger方程.然后通过对冷锋爆发冷涌过程的计算表明:冷涌主体前沿的宽度约10km,行进速度约为13/s,随着冷涌的增强,其宽度变窄.这些计算结果与实际的观测结果比较一致.     

冷涌结构及冷涌期中低纬环流相互作用的数值试验

本文利用1982年12月下旬一次冷涌过程的数值试验结果,分析了冷涌的结构并初步讨论了冷涌期中低纬环流的相互作用。结果表明,冷涌仅在近地面反映清楚,水平流场上表现为由大地形激发的开尔文波引起的东北大风沿青藏高原东侧迅速南下,而冷涌前沿则是以重力波特征向南传播。Hadley环流随冷涌南进不断加强。冷涌在涡度场和散度场上分别为一条状北倾的正涡度区和辐合区,它对应于冷锋南下时造成的上升运动区。冷涌能引起赤道地区的扰动加强和中纬度对流层高层西风急流的增强。冷涌期中低纬地区能量增大,通过能量的频散作用把中低纬环流连结起来了。      

冬季风期间南海地区冷涌的动力学分析

利用一个部分非线性的二维经向重力内波动力模式，通过求行波解，对冬季风期间南海冷涌的传播形式，传播特征和传播速度进行了分析，并讨论了大尺度经向环流及其垂直结构、热力作用、层结稳定状况和初始扰动条件对冷涌传播的作用。主要结论是：冷涌的传播表现为具有非线性效应的无频散或弱频散的重力行波，其向南传播速度远大于平流速度而接近于重力内波波速；不同流场和位温的初始扰动状况决定了冷涌将以周期重力行波或者以孤立重力     

对流层与下平流层大气重力波的气候及差异特征研究——以太原为例

大气重力波对全球大气的动力、热力结构具有重要影响,研究重力波参数气候特征是研制全球大气模式中重力波参数化的一个重要环节,通过引入重力波的影响,有利于提高大气模式的预报能力。本文利用2014-2017年太原地区(112.55°E,37.78°N)高垂直分辨率探空资料,对其对流层(2～9 km)、下平流层(17～24 km)大气重力波参数的气候特征及其之间差异特征进行研究。结果表明:(1)在1-12月,相对于对流层,下平流层的平均重力波水平波长、周期、固有相速、能量上传百分比均偏大,垂直波长均偏小,但群速在2月、5-9月偏大,其他月份偏小;(2)对流层与下平流层之间的重力波参数偏差、绝对差较大,相关性弱;(3)对流层、下平流层的重力波参数及其之间的偏差,在不同区间范围内的占有率分布特征存在明显差异;通过研究太原地区上空对流层、下平流层大气重力波参数的气候及其之间差异特征,进一步补充了中国区域的大气重力波参数气候特征,有利于研制更适合中国区域数值预报模式的重力波参数化方案。     

冬季风时期南海冷涌特征的初步分析

本文利用冬季风时期的资料,对在南海爆发的冷涌过程进行了统计分析,指出了南海冷涌的月际变化特征、话跃期及间隙期的周期特征。同时也讨论了冷涌过程中要素场、高低叠流场、温度场和逆温层的变化特征。所得结果,有助于对南海冷涌的认识。     

2007年3月3—5日辽宁省暴雪和大风天气的中尺度分析

使用中尺度数值模式WRFV2.2.1对辽宁省2007年3月3-5日的暴雪和大风天气过程进行了数值模拟,结合10min一次的地面自动观测站资料和数字化多普勒天气雷达探测资料,研究了中尺度重力波的结构及其环境场特征,探讨了波动的激发机制.对流层上层中尺度重力波生成在350-250 hPa(约9-11 km),周期为2-3h,水平波长30-40 km,波动沿水平方向传播约9h.地面气压扰动振幅约为2 hPa,周期为2-3 h,波动由西南向东北方向传播,方向与地面风向相反.沿波的传播方向,地面观测的逐时降水量呈波动特征,周期约为2h.对流层上层中尺度重力波减弱后,雷达降水回波强度出现显著的波动特征.对流层上层中尺度重力波生成在朝向脊区传播的高空急流出口区下方,300 hPa环境场具有显著的切变不稳定特征.波动生成在理查逊数小于0.25的地区,在中尺度重力波生成的高度上,暖平流强,风速低,风切变大.中尺度重力波生成地区出现显著的不平衡气流,拉格朗日罗斯贝数大于0.7,水平散度倾向出现明显的大值,其中-▽w·(e)v/(e)v的量级明显大于其他各项,表明对流层上层重力波的生成及发展与环境场的显著风切变有关.     

东亚冬季风的统计研究

利用ECMWFl979—1984年5个冬季(12—2月)的资料对东亚和西太平洋地区的冬季风进行了统计研究。得到的主要结果有: (1)东亚和西太平洋地区有两个冷涌区;从东海到南海等东亚沿岸地区和菲律宾以东西太平洋地区。其中以前者最明显。冷涌主要是700hPa以下的现象,近地面层北风最强,随高度迅速减弱。 (2)伴随着冷涌向南扩展,冷空气经华南沿中南半岛沿岸区一直向南扩展到近赤道地区; (3)在近地面层,西伯利亚高压的活动和强度与冷涌有密切的联系; (4)东亚的冬季风或冷涌有明显的年际变化,这与西伯利亚高压和南支槽活动的年际变化有关。      

青藏高原地区夏季两次强降水过程中重力波特征分析

为了更为全面地认识高原地区重力波对暴雨发生发展的影响,利用中尺度数值模式WRF(V3.5.1)对青藏高原东部地区2007年8月25日和2011年8月14日发生的两次强降水过程进行了模拟,分析了降水过程中的重力波特征及其与强降水之间的联系。结果表明,模拟结果较好地再现了这两次强降水过程,降水过程中的垂直速度场和散度场交替式运动完整地表现出波的传播特征,其伴随的上升运动和降水也有着密切的联系。分析Richardson数表明,5~6 km高度上垂直切变不稳定出现的时间先于这些波传播的发生,随着波动的增强,垂直切变逐渐减弱,垂直切变的不稳定气流为重力波提供能量,垂直切变和Richardson数可作为下游地区暴雨预报的指标和依据。同时,Morlet小波分析的结果表明,两次过程中主要是中尺度重力波,其中较长波长的重力波出现时间早,持续时间长,对强降水过程的加强起主要作用。这些表明中尺度重力波是该地区中尺度强降水发生的动力机制之一,在该地区夏季强降水过程中扮演着重要角色。重力波的分析有助于对该地区强降水过程的理解,进而提高青藏高原地区降水的预报能力。     

一次暴雨过程的重力波特征分析

选取四川省的一次暴雨个例分析研究大气中对流活动和重力波的相互内在联系。针对这次暴雨过程,采用MICAPS探空资料和天气雷达资料,并且结合天气学分析以及重力波指数的分析方法对暴雨的形成原因和重力波的机制进行了研究。结果表明:重力波与暴雨有一定的内在联系,重力波引导了暴雨的发展;非线性平衡方程对于诊断大幅度的重力波有很好的指示作用。     

冬季中国东北极端低温事件环流背景特征分析

利用1961~2014年CN05.2逐日温度数据,对冬季东北极端低温事件进行了定义,并按其发生时冷空气对中国东部（105°E以东）的影响范围,将其分为第一类和第二类极端低温事件,其中前者局限在东北,而后者则扩展至中国东部大部分地区。分析表明,在年代际时间尺度上,第一类极端低温事件强度减弱,而第二类的则增加;对持续天数而言,第二类极端低温事件的在减少,而第一类在1990年代以前也持续减少,但是1990年代之后急剧增加;2月份总的极端低温事件发生天数最多,其在1990年代以前要远大于12月和1月份,且在1990年代以前总体在减少,以后则增加;850 hPa风场分析显示,第二类极端低温事件中来自贝加尔湖的西北路径冷空气比第一类的要强,而来自鄂霍次克海的东北路径冷空气则相反;在300 hPa的E-P通量散度场上,这两类极端低温事件中东北都处于波动能量辐散中心,第二类极端低温事件发生时罗斯贝波波动能量传播比第一类时的要弱,第一类发生时则纬向传播比较明显,而且波动中心值都比较大;在位势高度距平场的谐波分析中,长波槽同位相扰动叠加在超长波槽上更易导致极端低温事件的发生。     

Influence of springtime North Atlantic Oscillation on crops yields in Northeast China

Forecasting grain production is of strategic importance in considerations of climate change and growing population. Here we show that the springtime North Atlantic Oscillation (NAO) is significantly correlated to the year-to-year increment of maize and rice yield in Northeast China (NEC). The physical mechanism for this relationship was investigated. Springtime NAO can induce sea surface temperature anomalies (SSTAs) in the North Atlantic, which display a tripole pattern and are similar to the empirical mode pattern in spring. The spring Atlantic SSTA pattern that could persists to summer, can trigger a high-level tropospheric Rossby wave response in the Eurasia continent, resulting in atmospheric circulation anomalies over the Siberia-Mongolia region, which is unfavorable (favorable) for cold surges that affect NEC. Weaker (stronger) cold surges can accordingly reduce (increase) cloud amount, resulting in an increase (a decrease) in daily maximum temperature and a decrease (an increase) in daily minimum temperature, thereby leading to an increase (a decrease) in diurnal temperature range. And summer-mean daily minimum temperature and diurnal temperature range are most significantly related to the NEC crop yields.     

Revisiting wintertime cold air intrusions at the east of the Andes: propagating features from subtropical Argentina to Peruvian Amazon and relationship with large-scale circulation patterns

This study investigates the spatial and temporal characteristics of cold surges that propagates northward along the eastern flank of the Andes from subtropical to tropical South America analysing wintertime in situ daily minimum temperature observations from Argentina, Bolivia and Peru and ERA-40 reanalysis over the 1975–2001 period. Cold surges usually last 2 or 3 days but are generally less persistent in the southern La Plata basin compared to tropical regions. On average, three to four cold surges are reported each year. Our analysis reveals that 52 % of cold episodes registered in the south of La Plata basin propagate northward to the northern Peruvian Amazon at a speed of around 20 m s^?1. In comparison to cold surges that do not reach the tropical region, we demonstrate that these cold surges are characterized, before they reach the tropical region, by a higher occurrence of a specific circulation pattern associated to southern low-level winds progression toward low latitudes combined with subsidence and dry condition in the middle and low troposphere that reinforce the cold episode through a radiative effect. Finally, the relationship between cold surges and atmosphere dynamics is illustrated for the two most severe cold intrusions that reached the Peruvian and Bolivian Amazon in the last 20 years.     

Characteristics of Atmospheric Circulation Associated with Cold Surge Occurrences in East Asia： A Case Study During 2005/06 Winter

The characteristics of the upper-level circulation and thermodynamical properties for the period when two distinct cold surges broke out over East Asia during the 2005/06 winter are investigated. From early December 2005 to early January 2006, exceptionally cold weather lasted for approximately one month due to two successive cold surges that took place on 2 December 2005 and 2 January 2006, respectively. This study reveals that both involve the upper-tropospheric circulation, which induces the amplification and expansion of the surface Siberian high toward East Asia, but arose from different causes： the former is caused by the upper-level blocking originated from the North Pacific and the latter is caused by the upper-level wave train across the Eurasian Continent. In particular, it is suggested that the lower-tropospheric anomalous wind caused by upper-level circulation anomalies and a steep meridional temperature gradient amplified by phase-locked annual cycle combined to induce very strong cold advection in East Asia, which resulted in exceptionally cold weather that lasted for several weeks. The present results emphasize that the characteristics of the upper-tropospheric circulation can be considered as important precursors to cold surge occurrences in East Asia.     

Numerical simulations of hailstorm feeder‐cloud initiation

Abstract

A simple, moist two‐dimensional numerical model is developed and used to study the mechanisms for the formation of hailstorm feeder clouds. By using high spatial resolution, the model is able to simulate low‐level storm outflows and reproduce the observed separation between hailstorm feeder clouds. The cold outflow emanating from the storm is induced by evaporation of precipitation. The simulations show that feeder clouds develop in response to the gravity waves generated by wind shear that is inherent in the ambient environment and/or created by the interaction between the low‐level inflow and the storm outflow. The subcloud moisture also plays an important role in the formation of a wave‐like line of feeder clouds. It contributes in the cloud development as well as in the enhancement of gravity waves.     

East Asian winter monsoon: results from eight AMIP models

 This study evaluates simulations of the East Asian winter monsoon in eight GCMs that participated in the Atmospheric Model Intercomparison Project (AMIP). In addition to validating the mean state of the winter monsoon, the cold surge and its transient properties, which includes the frequency, intensity, preferred propagation tracks, and the evolution patterns of the surges, are examined. GCM simulated temporal distribution of the Siberian high and cold surges is also discussed. Finally, the forcing of the cold surges on the tropical surface wind and convection, along with their interannual variation is analyzed. The mean state of the winter monsoon is generally portrayed well in most of the models. These include the climatological position of the Siberian high, the 200 hPa divergent center, and the large-scale wind patterns at the surface and the 200 hPa. Models display a wide range of skill in simulating the cold surge and its transient properties. In some of the models, the simulated cold surge trajectory, intensity, frequency, propagation patterns and source regions are in general agreement with those from the observed. While in others, the models cannot adequately capture these observed characteristics. The temporal distribution of the Siberian high and cold surges were realistically reproduced in most GCMs. Most models were able to simulate the effect of the cold surges on the tropical surface wind, although a few models unrealistically generated subtropical southerly wind in the mid-winter. The relationship between cold surges and the tropical convection was not satisfactorily simulated in most models. The common discrepancies in the winter monsoon simulation can be attributed to many factors. In some models, the reason is directly related to the improper location of the large-scale convective center near the western Pacific. The satisfactory simulations of the monsoon circulation and the cold surges are partly due to the topographical characteristics of the East Asian continent, i.e., the Tibetan Plateau to the west and the oceans to the east. The correct simulation of the interannual variation of the surface wind near the South China Sea (SCS) and the maritime continent is a demanding task for most of the models. This will require adequate simulations of many aspects, including tropical convection, the Siberian cold dome, the extratropical-tropical linkage, and the air-sea interaction. The discrepancies noted here furnish a guide for the continuing improvement of the winter monsoon simulations. Improved simulations will lead to an adequate delineation of the surface wind and convection near the maritime continent, which is essential for portraying the winter monsoon forcing in a coupled model. Received: 10 March 1997/Accepted: 4 June 1997     

Long-term variability of cold surges in Korea

Cold surge statistics have been analyzed from the 52-year (1961–2012) winter temperature data archived at Korea Meteorological Administration. Despite a significant winter warming in South Korea, there is no statistically affirmable sign of the occurrence frequency, duration and total days of cold surges to have changed in the record; there is little abatement in the frequency, duration, and total days of cold surges. Thermal advection anomalies were also derived from the NCEP/NCAR and ERA interim reanalysis datasets. Cold surges defined in terms of thermal advection anomalies do not exhibit any statistically significant change either. The increased mean and the decreased variance of thermal advection both indicate that cold advection from north has, in fact, decreased gradually in South Korea. It appears that cold surges are statistically rare enough to be affected by increased mean and decreased variance of thermal advection. Polar warming is often interpreted as weakening of jet stream and increasing southward flux of cold air. Analysis of thermal advection, on the other hand, does not show increased advection of cold air from north in South Korea.     

南半球冷空气入侵与热带气旋的形成

文中采用NCAR/PSU研制的非静力中尺度模式MM 5 ,研究了南半球冷空气入侵在热带气旋形成中的作用。初始场为纬向平均场 ,不含任何扰动 ,但为热带气旋的发生提供了基本条件 ;通过改变设在赤道的南边界条件 ,设计系列数值试验反映南半球不同强度冷空气的入侵。数值试验结果表明 :南半球冷空气侵袭后 ,在菲律宾以东洋面上形成热带气旋 ;没有冷空气入侵时 ,只有扰动产生 ,没有热带气旋形成。在对流不稳定的背景场中 ,即使没有冷空气入侵 ,低层小尺度辐合引起强上升运动 ,产生的非绝热加热 ,在热带洋面上也能形成扰动。但是非绝热加热使得稳定度增加 ,没有低层强辐合的支持 ,对流不能持续 ,扰动不能发展成为热带气旋。南半球冷空气的入侵 ,一方面气温降低 ,使得中低层层结稳定度降低 ;另一方面 ,冷空气形成向北的气压梯度 ,在低纬度产生南风 ,导致低层强辐合。稳定度因子和低层辐合的共同作用 ,驱动深厚的垂直环流 ,产生十分显著的非绝热加热 ,形成了暖心的热带气旋。上述研究结果一定程度上肯定了存有疑义的冷空气学说