Measurement of the Sensible Eddy Heat Flux Based on Spatial Averaging of Continuous Ground-Based Observations

Using the standard eddy-covariance (EC) method to quantify mass and energy exchange at a single location usually results in an underestimation of vertical eddy fluxes at the surface. In order to better understand the reasons for this underestimation, an experimental set-up is presented that is based on spatial averaging of air temperature data from a network of ground-based sensors over agricultural land. For eight days during the 34-day observational period in May and June 2007, additional contributions to the sensible heat flux of more than 50Wm⁻² were measured in the lower surface layer by applying the spatial EC method as opposed to the standard temporal EC method. Smaller but still significant additional sensible heat fluxes were detected for four more days. The additional energy is probably transported in organised convective structures resulting in a mean vertical wind velocity unequal to zero at the tower location. The results show that convective transport contributes significantly to the surface energy budget for measurement heights as low as 2–3 m. Since these structures may be quasi-stationary, they can hardly be captured by a single-location measurement. The spatial EC set-up presented here is capable of quantifying contributions to the sensible heat flux from structures up to the scale of our spatial sensor network, which covered an area 3.5 × 3.5 km. For future experiments aiming at closing the energy balance, the spatial EC method should be employed to measure both the sensible and latent heat fluxes. Experimental determination of the horizontal advection of sensible and latent heat should also be considered, since such transport must occur due to convergence and divergence related to convection.     

对流强度对雷暴云微物理发展和电荷结构影响的数值模拟

为了探讨对流强度大小对雷暴云内微物理发展和起电过程的影响,基于已有的二维积云起、放电模式,改变其扰动温度进行敏感性试验。试验结果表明:对流强度对雷暴云内微物理过程、起电率及后续电荷结构的产生均有一定程度的影响:1）对流强度较小时,冰晶粒子极大值在高温区（高于-13.8℃）出现,对流强度较大时,上升风明显增强,将更多的水汽带入高空,气溶胶活化过程明显增强,使得云滴粒子明显增多,冰晶粒子较早产生,冰晶粒子极大值在低温区（低于-13.8℃）出现,发展过程更为剧烈;同时,较高的对流强度也使得降雨量增多,霰粒子数目也在对流发展旺盛时期显著增多。2）非感应起电率主要和冰晶-霰的碰并分离过程有关,对流强度较大时,非感应起电率较大,电荷结构持续时间较长,过程明显,感应起电率也较强。3）对流强度较大时,电荷结构更为复杂,雷暴云发展初期基本呈现为三极性,发展旺盛时期底部正电荷区域嵌入一个较小的负电荷区,呈现四极性电荷结构,雷暴云发展末期基本呈现偶极性电荷结构;对流强度较小时,发展初期、旺盛时期均呈现三极性电荷结构,发展末期呈现偶极性电荷结构。     

“8.18-19”山西中南部暴雨天气特征分析

利用自动站加密资料、FY-2C卫星产品以及实况探测资料,对2010年8月18-19日发生在山西中南部多站暴雨和2个测站大暴雨的天气过程进行了综合分析。结果表明:(1)此次大范围强降水主要受500hPa切断低压、槽线和副热带高压(下称副高)的共同影响,副高边缘不断生成和消亡的中小尺度对流云团(2个一般对流云团和6个中尺度对流云团(包括1个MCC和5个MCS))是导致暴雨、大暴雨的直接系统;暴雨站上空TBB绝对值变化与降水变化有着较好的一致性。(2)此次降水过程,暴雨区非常分散,存在6个中心,分别表现出典型的β-中尺度和γ-中尺度特征,其分布呈现出双峰、单峰和孤立单峰等多种型式,具有先沿着500hPa引导气流向东北发展,再随着副高撤退又向东南压的特点。(3)地面自动站风场资料显示,此次暴雨天气过程存在涡旋降水、切变降水和辐合降水3种类型,其降水强度和持续时间均存在明显的差异,但暴雨落区却与它们和副高的相对位置有关。(4)A指数诊断揭示,高湿气团在高不稳定层结背景下,沿副高边缘不断抬升到高空,与来自高纬地区的强冷空气相遇,导致了强降水;在对流层中、高层,转化项是涡度发展的主要因子,使得垂直运动不断加强,成为不稳定大气的动力强迫因素。     

2004年江苏火箭人工增雨降温分析

为缓解夏季高温,减轻用电负荷,2004年江苏省实施了火箭人工增雨降温作业.本文利用多普勒天气雷达和地面各项观测资料,对2004年11 d 27次火箭增雨降温作业前后降水与降温情况进行统计分析,在此基础上重点分析了8月5日常州市的火箭人工增雨降温作业过程,分析了这次作业的效果,讨论了强对流云催化降温的机理和火箭增雨降温作业的着眼点,为以后此项工作的开展提供依据.     

利用对地闪的观测估算对流性天气中的降水

地闪与对流性天气中的降水量之间存在着较好的相关性,本文利用1997年6～8月间在陇东地区以平凉雷电与雹暴实验站为中心对对流性天气过程进行地闪与降水的观测,解决以下三个问题：(1)单个地闪所对应的对流性天气中的降水量;(2)相对于地闪发生位置的对流性降水空间分布;(3)相对于地闪发生时刻的对流性降水时间分布,实现了利用地闪资料更加精确地估算对流性天气中的降水量。     

Laboratory Simulation Of Vertical Plume Dispersion Within A Convective Boundary Layer

Mesoscale measurements of the vertical dispersion coefficient ₂ by using a composite turbulence water tank were validated through a comparison with CONDORS (Convective Diffusion Observed with Remote Sensors) field data, and were analysed with respect to the intensity of the thermal flux, mechanical turbulence, and plume release height.It seems possible to correct the plume _z values for different release heights below 0.5z_i (z_i is the mixing height) by applying an equation expressing the height dependency of turbulence intensity. The downwind distance where the plume's mass centre height approaches its final level was also analysed with respect to the above three parameters, and an empirical equation to estimate the downwinddistance derived.     

洛阳地区强对流天气过程分析

通过对洛阳地区11次强对流天气进行分析，发现强对流天气分两类，并分别和不同的中尺度系统相联系，两强对流天气发生在不同的大尺度环境场中，不仅环流形势不同，环境特征也有明显差异，因而对他们的预报思路和着眼点也应区别对待。     

Observations Of The Morning Transition Of The Convective Boundary Layer

The morning transition between the stable nocturnal situation and the daytime convective boundary layer (CBL) is of interest both for basic understanding and for initializing prognostic models. While the morning growth phase of the CBL has been studied in detail, relatively little has been published on the transition itself. In this paper, conventional observations of surface temperature, humidity, and turbulent fluxes,and data from a meteorological tower, are combined with measurements of the onset of convection by boundary-layer wind profilers to explore the timing and behaviour of the transition period. The transition is defined here as the period between sunrise and the time at which the depth ofconvection reaches about 200 m AGL. Diagnostic relationships based on surface heat flux, the temperature difference between 2 m and 200 m, and bulk Richardson number are explored. The transition is foundto be enabled by surface heating relaxing the surface stability, while the warming of the layerbetween 2 m and 200 m is in large part due to shear-driven entrainment.     

磁暴期间热层中性大气受热抬升的观测例证

采用AE-D卫星上中性大气数密度的观测资料,分析了1975年11月内多次磁暴期间热层大气中氦和原子氧与氮分子之数密度比值的变化.结果表明,磁暴对中性大气加热的理论是符合实际情况的.低热层大气受热抬升使较高高度上的大气中质量较轻成分的数密度相对比例有明显下降,而较重成分的百分比则有增加.与地磁宁静期间相比,在140-10km高度上n(He)/n(N₂)之比值降低了一个数量级,而n(Ar)/n(N₂)的增幅则略大于一个数量级.此比值的变化幅度大小随高度而变,但各高度上暴时变化的发展趋势是基本一致的.     

Analysis of the severe group dust storms in eastern part of Northwest China

Based on the available original dust storm records from 60 meteorological stations, we discussed the identification standard of severe dust storms at a single station and constructed a quite complete time series of severe group dust storms in the eastern part of Northwest China in 1954–2001. The result shows that there were 99 severe group dust storms in this region in recent 48 years. The spatial distribution indicates that the Alax Plateau, most parts of the Ordos Plateau and most parts of the Hexi Corridor are the main areas influenced by severe group dust storms. In addition, the season and the month with the most frequent severe group dust storms are spring and April, accounting for 78.8% and 41.4% of the total events respectively. During the past 48 years the lowest rate of severe group dust storms occurred in the 1990s. Compared with the other 4 decades, on the average, the duration and the affected area of severe group dust storms are relatively short and small during the 1990s. In 2000 and 2001, there were separately 4 severe group dust storms as the higher value after 1983 in the eastern part of Northwest China.