改进的Holroyd云粒子形状识别方法及其应用

云降水粒子形状是影响云微物理过程的重要因素,准确的云粒子形状信息是诸多云微物理参量计算的前提。为获取机载云粒子成像仪（CIP）所测云粒子的形状信息,文中提出了一种改进的Holroyd云粒子形状识别方法,即先对云粒子形状进行预分类,然后针对预分类后的完整粒子和可识别的部分状粒子,分别选出合适的参数及其阈值再进行具体的分类,最终可将云粒子分为微小状、线形状、聚合状、霰、球形、板状、不规则和枝状。利用实测数据对原始的Holroyd方法和改进的Holroyd方法进行识别效果对比验证。结果表明改进的Holroyd方法在云粒子形状识别的准确度方面比原Holroyd方法有较大的提高。将所提方法应用于太原地区一次降水性层状云的云微物理飞机观测资料以分析不同的降水阶段云中冰晶粒子的形状分布、增长机制、冰晶粒子数浓度以及冰水含量的垂直分布特征,所获取的云中冰晶粒子属性表明新提出方法有助于云微物理分析。      

Aircraft Observations of Orographic Cloud and Precipitation Features over Southern Baffin Island,Nunavut, Canada

This study evaluates cloud and precipitation features over the orography of southern Baffin Island in the southeast Canadian Arctic during the Storm Studies in the Arctic (STAR) field project in autumn 2007. Three case studies provide the basis for a comparative analysis of how cloud and precipitation features from upstream ocean regions are modified by the orography, in addition to the variability of these features over diverse synoptic and sea-ice conditions. Using data collected by a research aircraft with an onboard W-band Doppler radar and microphysical instrumentation, multiple factors were found to play roles in enhancing and/or reducing cloud and precipitation over the orography of the region. Gravity waves, terrain shape, atmospheric stability, and atmosphere–ocean exchanges were all associated with precipitation enhancement. In addition, several factors that reduce precipitation were identified, including sublimation, high sea-ice extent, and low-level blocking in the upstream environment. Accretion and aggregation were identified as important particle growth mechanisms over the orography. By increasing particle density and/or mass, the probability of ice particles precipitating to the surface increased. These results indicate that the complexity of these critical features over terrain in high-latitude regions poses considerable challenges for modelling.     

Sensitivities of tornadogenesis to drop size distribution in a simulated subtropical supercell over eastern China

ABSTRACT Numerical simulations with the Advanced Regional Prediction System （ARPS） model were performed to investigate the impact of microphysical drop size distribution （DSD） on tornadogenesis in a subtropical supercell thunderstorm over Anhui Province, eastern China. Sensitivity experiments with different intercept parameters of rain, hail and snow DSDs in a Lin-type microphysics scheme were conducted. Results showed that rain and hail DSDs have a significant impact on the simulated storm both microphysically and dynamically. DSDs characterized by larger （smaller） intercepts have a smaller （larger） particle size and a lower （higher） mass-weighted mean fall velocity, and produce relatively stronger （weaker） and wider （narrower） cold pools through enhanced （reduced） rain evaporation and hail melting processes, which are then less favorable （favorable） for tornadogenesis. However, tornadogenesis will also be suppressed by the weakened mid-level mesocyclone when the cold pool is too weak. When compared to a U.S. Great Plain case, the two microphysical processes are more sensitive to DSD variations in the present case with a higher melting level and deeper warm layer. This suggests that DSD-related cloud microphysics has a stronger influence on tornadogenesis in supercells over the subtropics than the U.S. Great Plains.     

The cloud-microphysical cause of torrential rainfall amplification associated with Bilis (0604)

After its landfall in China’s mainland in 2006, Typhoon Bilis brought about torrential rainfall amplification at the edge of Guangdong, Jiangxi, and Hunan provinces, causing severe disasters. From a cloud-microphysical perspective, we discuss the differences of cloud-microphysical processes before and during the precipitation amplification and possible causes of the rainfall amplification by using high-resolution simulation data. The results show that the cloud-microphysical characteristics during the above two periods are significantly different. With the distinct increase in the rainfall intensity, the cloud hydrometeor contents increase markedly, especially those of the ice-phase hydrometeors including ice, snow and graupel, contributing more to the surface rainfall. The clouds develop highly and vigorously. Comparisons of conversion rates of the cloud hydrometeors between the above two periods show that the distinct increases in the cloud water content caused by the distinct enhancement of the water vapor condensation rate contribute to the surface rainfall mainly in two ways. First, the rain water content increases significantly by accretion of cloud water by rain water, which thus contributes to the surface rainfall. Second, the accretion of cloud water by snow increases significantly the content of snow, which is then converted to graupel by accretion of snow by graupel. And then the graupel melts into rain water, which subsequently contributes to the surface rainfall amplification. In summary, a flow chart is given to clarify the cloud-microphysical cause of the torrential rainfall amplification associated with Bilis.     

Outcome regimes of binary raindrop collisions

This study delineates the physical conditions that are responsible for the occurrence of main outcome regimes (i.e., bounce, coalescence, and breakup) for binary drop collisions with a precipitation microphysics perspective. Physical considerations based on the collision kinetic energy and the surface energies of the colliding drops lead to the development of a theoretical regime diagram for the drop/raindrop collision outcomes in the We–p plane (We — Weber number, p — raindrop diameter ratio). This theoretical regime diagram is supported by laboratory experimental observations of drop collisions using high-speed imaging. Results of this fundamental study bring in new insights into the quantitative understanding of drop dynamics, applications of which extend beyond precipitation microphysics. In particular, results of this drop collision study are expected to give impetus to the physics-based dynamic modeling of the drop size distributions that is essential for various typical modern engineering applications, including numerical modeling of evolution of raindrop size distribution in rain shaft.     

华南冷锋云系的中尺度和微物理特征模拟分析

利用中国气象科学研究院（CAMS）中尺度云分辨模式,结合实测地面雨量、卫星和雷达资料,对发生在2004年3月31日~4月1日的华南春季冷锋降水过程进行模拟分析。模拟云带的出现时间、位置、形状与走向以及随时间的演变均与卫星观测一致。模拟的雷达回波分布同实测一致,回波主要出现在地面锋线以及锋后冷空气一侧,呈西南-东北带状分布,锋面云系的不同部位回波单体的差异很大。模拟的主要降水时段内的地面雨量分布范围以及大小同实测接近,中尺度雨带呈西南-东北带状结构,随着冷锋的移动逐渐向东南方向移动。在中尺度雨带上有4个生命史超过3小时的强降水中心,强降水中心基本都是向东略偏南的方向移动,与回波单体的移动方向一致。锋面云系的垂直运动深厚,且基本与云区对应,云系产生在低层辐合、正涡度,高层辐散和高相当位温的区域。地面锋线附近的上升速度大,云水含量高,冰相粒子的淞附和雨滴碰并云滴是云中的主要微物理过程,暖雨过程和冷雨过程都重要;而在高空锋区宽雨带部分低层为下沉气流,上升气流只出现在高层,主要是过冷云水、霰和雪晶组成的混合云,雪晶是霰增长的主要源项,降水主要由霰的融化产生,冷云降水过程比较重要。     

云微物理参数在飞机积冰分析和预报中的应用研究

对飞机积冰预报的现状进行了分析 ,介绍了积冰预报中云微物理参数的应用概况 ,并就积冰预报中云微物理参数的应用前景进行了分析和讨论。将模式模拟和卫星反演的微物理参数引入飞机积冰分析和预报 ,将进一步提高飞机积冰预报的准确性和合理性     

Numerical sensitivity studies of axis ratio changes of colliding ice crystals

Numerical sensitivity studies were carried out to investigate the influence of shape variations of colliding ice particles on the collisional behavior. Therefore, two general ice particle shapes were considered: ice plates and ice columns. The changing of the shape of the ice particles was described by the changing of the axis ratio of the particles. Numerical growth calculations with a cloud model were carried out to show the impact of the axis ratio variations on the collectional growth in comparison to calculations with a constant axis ratio. Ice plates show significant differences between the two cases, whereas for ice columns the effect of the axis ratio variations is less important.     

Approximation formulas for the microphysical properties of saline droplets

Microphysical theory has proven essential for explaining sea spray's role in transferring heat and moisture across the air–sea interface. But large-scale models of air–sea interaction, among other applications, cannot afford full microphysical modules for computing spray droplet evolution and, thus, how rapidly these droplets exchange heat and moisture with their environment. Fortunately, because the temperature and radius of saline droplets evolve almost exponentially when properly scaled, it is possible to approximate a droplet's evolution with just four microphysical endpoints: its equilibrium temperature, T_eq; the e-folding time to reach that temperature, τ_T; its equilibrium radius, r_eq; and the e-folding time to reach that radius, τ_r.Starting with microphysical theory, this paper derives quick approximation formulas for these microphysical quantities. These approximations are capable of treating saline droplets with initial radii between 0.5 and 500 μm that evolve under the following ambient conditions: initial droplet temperatures and air temperatures between 0 and 40 °C, ambient relative humidities between 75% and 99.5%, and initial droplet salinities between 1 and 40 psu.Estimating T_eq, τ_T, and τ_r requires only one-step calculations; finding r_eq is done recursively using Newton's method. The approximations for T_eq and τ_T are quite good when compared to similar quantities derived from a full microphysical model; T_eq is accurate to within 0.02 °C, and τ_T is typically accurate to within 5%. The estimate for equilibrium radius r_eq is also usually within 5% of the radius simulated with the full microphysical model. Finally, the estimate of radius e-folding time τ_r is accurate to within about 10% for typical oceanic conditions.     

Microphysics of clouds at Kleiner Feldberg

During a field measuring campaign at Kleiner Feldberg (Taunus) in 1990, microphysical characteristics of clouds have been measured by Forward Scattering Spectrometer Probes (FSSP). The aim was to study the influence of aerosol and meteorological factors on droplet size and number. The results are: More mass in the accumulation size range of the aerosol leads to more droplets in stratocumulus clouds and to higher soluble masses in droplets of stratus clouds. However, the aerosol distribution was coarser in the stratus clouds compared to the stratocumulus clouds. Within the first 200 m from cloud base, the droplets grow while their number decreases. The growth results in a stable size of about 14 µm diameter over a large distance from cloud base in many stratocumulus clouds. Two types of mixing processes were observed: processes with reductions in the number of droplets (inhomogeneous mixing) and with reductions in the size of the droplets (homogeneous mixing).