Factors affecting boundary-layer wind shear over the Indian Ocean during FGGE

Statistics on the vertical wind shear in the boundary layer over the Indian Ocean were examined for the causes of regional and seasonal changes. Low-level cloud motions and surface ship wind reports were used to define the vertical shear. Temperature data from the ship reports were analyzed for boundary-layer stability related to the observed shears. Smaller wind shears were found in areas of large negative air-sea temperature difference (unstable boundary layers). The thermal wind effects were very small over most of the tropical Indian Ocean. The largest factor affecting the speed shear was the strength of the wind itself. Larger speed shear was found under high wind conditions. A small reduction in the direction difference between cloud and ship observations also was found under higher speeds. The scatter of cloud-ship comparisons around the mean (dispersion) also decreased for higher wind speeds. Daily gridded cloud motion and ship wind speed data had a correlation coefficient of 0.8 with a scatter of 1.9 m s^-1 (r.m.s.) around the mean difference.     

The feasibility of estimating large-scale surface wind fields for the summer MONEX using cloud motion and ship data

Cloud motion data were compared to ship observations over the Indian Ocean during the summer monsoon, 1 May to 31 July 1979, with the objective of using the cloud data for estimating surface wind and ultimately the wind stress on the ocean. The cloud-ship comparison indicated that the cloud motions could be used to estimate surface winds within reasonable accuracy bounds, 2.6 m s^-1 r.m.s. speeds and 22° to 44° r.m.s. directions (22° r.m.s. for winds < 10 m s^-1). A body of statistics is presented which can be used to construct an empirical boundary layer with the eventual goal of producing a stress analysis for the summer MONEX from cloud motion data.     

一次弱对流天气降雹成灾的雷达回波特征分析

通过对新疆玛河流域一次冰雹天气雷达资料及地面实况资料的分析,研究弱对流云降雹的雷达反射率因子与径向速度场的回波特征,进一步探讨弱对流云降雹与0℃层高度较低及垂直风切变的关系：弱对流天气当日零度层高度值可作为分析弱冰雹云的依据;高低空风速的差异形成的风切变造成的动压力产生垂直加速度,使对流运动得到加强和维持,也是此次降雹的动力机制。分析此次天气,揭示了新疆玛河流域弱对流云降雹天气过程生消的一些重要特征和变化,对雹云的识别和分析具有一定的指导意义。     

Ship-board measurements and estimations of air-sea fluxes in the western tropical pacific during TOGA COARE

Direct air-sea flux measurements were made on R/V Kexue #1 at 4 ° S, 156 ° E during the Tropical Ocean Global Atmosphere (TOGA) Coupled Ocean-Atmospheric Response Experiment (COARE) Intensive Observation Period (IOP). An array of six accelerometers was used to measure the motion of the anchored ship, and a sonic anemometer and Lyman-α hygrometer were used to measure the turbulent wind vector and specific humidity. The contamination of the turbulent wind components by ship motion was largely removed by an improvement of a procedure due to Shao based on the acceleration signals. The scheme of the wind correction for ship motion is briefly outlined. Results are presented from data for the best wind direction relative to the ship to minimize flow distortion effects. Both the time series and the power spectra of the sonic-measured wind components show swell-induced ship motion contamination, which is largely removed by the accelerometer correction scheme. There was less contamination in the longitudinal wind component than in the vertical and transverse components. The spectral characteristics of the surface-layer turbulence properties are compared with those from previous land and ocean results. Momentum and latent heat fluxes were calculated by eddy correlation and compared to those estimated by the inertial dissipation method and the TOGA COARE bulk formula. The estimations of wind stress determined by eddy correlation are smaller than those from the TOGA COARE bulk formula, especially for higher wind speeds, while those from the bulk formula and inertial dissipation technique are generally in agreement. The estimations of latent heat flux from the three different methods are in reasonable agreement. The effect of the correction for ship motion on latent heat fluxes is not as large as on momentum fluxes.     

在台风业务系统中使用卫星云导风资料的试验

在国家气象中心台隐数值预报业务系统中引入国家卫星气象中心提供的卫星云导风资料,进行两项预报试验。第一项是把卫星云导风资料作为观测资料进行最优插值客观分析;第二项是将云导风资料加入到轴对称的人造台风模型风场上,使之产生非对称风场。试验结果表明：卫星云导风资料作为观测资料进入模式后对减小台风路径预报误差效果明显;用云导风形成人造台风模型非对称风场可进一步提高台风路径预报精度。     

Future Climate Change of the Subtropical North Atlantic: Implications for the Cloud Forests of Tenerife

This paper is concerned with climate change in the region of the Canary Islands and the potential implications for the laurel forests of Tenerife. Frequent orographic cloud formation during the dry season is of vital importance to the altitudinal distribution of the laurel forests, because it maintains a semi-humid environment in the otherwise semi-arid climate of the Canary Islands. The distinctive environmental conditions in conjunction with the location of the Canary Islands on the Northern poleward edge of the Hadley Circulation make these ecosystems potentially highly sensitive to regional changes in climatic conditions. To explore this sensitivity, we first quantify observed trends in humidity and temperature across an altitudinal transect at the base of the Anaga peninsular, and second, analyse the results of three GCM experiments (CGCM1, ECHAM4 and CSIRO) to develop alternative climate change scenarios, and third, use these data to assess likely shifts in the elevational distribution of the laurel forest climate envelope. We report a significant increase in relative humidity and decreases in the diurnal temperature range on Tenerife at altitudes below the trade wind inversion within the last 30 years during the dry season, which suggests an increased occurrence of low-level clouds. There is also partial evidence for a drying trend across the trade wind inversion, which may be linked to an increased subsidence. Overall, the models suggest a downward shift of the area climatically suitable for laurel forests, which may be driven by changes in temperature and moisture supply in the region as well as by larger-scale changes in the atmospheric circulation. Our findings contrast with previously published findings for a tropical montane cloud region, which predict an upward shift of the cloud base. This suggests, following the assumptions inherent in the models applied, that the ecological consequences of climate change for cloud forests may be linked to their relative location in the Hadley Circulation.     

Studying geoengineering with natural and anthropogenic analogs

Solar radiation management (SRM) has been proposed as a possible option for offsetting some anthropogenic radiative forcing, with the goal of reducing some of the associated climatic changes. There are clearly significant uncertainties associated with SRM, and even small-scale experiments that might reduce uncertainty would carry some risk. However, there are also natural and anthropogenic analogs to SRM, such as volcanic eruptions in the case of stratospheric aerosol injection and ship tracks in the case of marine cloud albedo modification. It is essential to understand what we can learn from these analogs in order to validate models, particularly because of the problematic nature of outdoor experiments. It is also important to understand what we cannot learn, as this might better focus attention on what risks would need to be solely examined by numerical models. Stratospheric conditions following a major volcanic eruption, for example, are not the same as those to be expected from intentional geoengineering, both because of confounding effects of volcanic ash and the differences between continuous and impulsive injection of material into the stratosphere. Nonetheless, better data would help validate models; we thus recommend an appropriate plan be developed to better monitor the next large volcanic eruption. Similarly, more could be learned about cloud albedo modification from careful study not only of ship tracks, but of ship and other aerosol emission sources in cloud regimes beyond the narrow conditions under which ship tracks form; this would benefit from improved satellite observing capabilities.     

2009年6月一次飑线过程灾害性大风的形成机制

对2009年6月3～4日一次产生地面大风的人字形强飑线过程进行了观测资料分析和数值模拟研究。观测资料的分析表明: 人字形回波系统的右半支的结构与一般的飑线系统类似, 在系统成熟阶段地面存在明显的雷暴高压、 冷池、 出流边界、 尾流低压等特征; 人字形回波的左半支对应的地面风速比右半分支弱, 且强对流区后部没有对应层状云、 地面雷暴高压、 冷池等; 灾害性大风的产生主要由这个人字形系统的右半支造成的。高分辨率模拟结果的分析表明: 系统由线状转变为人字形系统的原因是由于气旋扰动的冷暖切变的作用, 冷、 暖切变上分别形成了有层状云和无层状云的飑线分支。系统的右半分支在发展阶段和成熟阶段对流区有比较强烈的下沉气流, 系统的后部的中层入流可能会加强这个下沉气流。中层入流是地面大风形成的重要原因之一; 成熟阶段垂直于飑线系统主要有三股气流, 包括从飑线前部向后的入流和中层从后部到前部的入流, 以及前部的低层入流到高层的出流。     

风沙流中风速廓线的数值模拟与实验验证

如何描述风沙流中被风沙运动改变了的风速廓线是风沙相互作用研究中的关键问题之一.该文中将跃移风沙流视为一种颗粒拟流体,将跃移颗粒对气流产生的阻力用颗粒流的阻力系数来表达,建立了描写两场相互作用的数学模型.颗粒流的阻力系数采用了前人在液态流化床研究中得出的阻力系数表达形式,通过引入一个修正系数,使其适用于风沙流(气-固两相流).将风沙边界层划分为跃移颗粒所产生的阻力不可忽略的内边界层和跃移颗粒阻力可以忽略但受内边界层影响的外边界层,分别建立了内边界层和外边界层的风速廓线表达式.应用所建立的数学模型,根据由风洞实验测定的跃移风沙流的浓度分布和速度分布资料,计算了跃移风沙流中的风速廓线,并与风洞实测结果进行了对比.结果表明,计算风速廓线与实测风速廓线吻合得比较好,在半对数图上均为上凸的曲线,有别于无风沙运动时的直线.跃移边界层外风速分布可较好地用对数函数来描述.对风沙流中风速廓线的进一步分析证实了风沙物理学奠基人Bagnold在其早期观测风沙流中的风速廓线时提出的"结点现象"(Bagnold结),该结点的高度随风速的增大而升高,随颗粒粒径的增大而降低.根据数值模拟和模拟实验,可以认为有风沙运动的动床剪切风速是综合反映风场与跃移层以及地表之间相互作用的物理量.     

雹云的判别方法与短时预报统计模式

本文利用江苏省强对流天气的雷达回波资料、天气与灾情实况和探空资料,讨论了降雹概率与回波顶高的关系。用独立参数降雹条件概率公式判别雹云,并对条状和带状回波移向与高空风向间的偏角进行了统计分析。所建立的统计模式,具有一定实用价值。     

Sea surface winds derived from cloud motion vectors over the tropical Atlantic

The relationship between satellite-derived low-level cloud motion, surface wind and geostrophic wind vectors is examined using GATE data. In the trades, surface wind speeds can be derived from cloud motion vectors by the linear relation: V = 0.62 V _s + 1.9 m s^–1 with a mean scatter of ±1.3 m s^–1. The correlation coefficient between surface and satellite wind speed is 0.25. Considering baroclinicity, i.e., the influence of the thermal wind, the correlation coefficient does not increase, because of the uncertainty of the thermal wind vectors. The ratios of surface to geostrophic wind speed and surface to satellite wind speed are 0.7 and 0.8, respectively, with a statistical uncertainty of ±0.3. Calculations of the ratio of surface to geostrophic wind speed on the basis of the resistance law yield V/V _g = 0.8 ± 0.2, in agreement with experimental results. The mean angle difference between the surface and the satellite wind vectors amounts to - 18 °, taking into account baroclinicity. This value is in good agreement with the mean ageostrophic angle - 25 °.     

A Critical Analysis of the Wind Climatology of the Adriatic Sea from Different Sources

Summary  We consider the wind climatology of the Adriatic Sea derived from three different sources: an operational meteorological model, a simplified wind model, and traditional ship reports. The simplified wind model is found to provide reliable results in storm conditions, but it partly smoothes the fields when the meteorological pattern is not well defined and fairly uniform at the basin scale. The ship reports show a strong evident bias towards low values. This is partly interpreted as a tendency for ships to avoid rough weather and not to report in these types of conditions. A second reason is a bias present in the transfer from the Beaufort to the metric scale. The best results are provided by the operational meteorological model, after its results have been corrected for a bias in wind speed, associated with the resolution of the model with respect to the dimensions of the basin under study. Received October 23, 1997 Revised August 10, 1998     

河北省冬季低槽冷锋层状云结构特征和可播性分析

2018年1月6日河北省出现一次低槽冷锋天气系统,利用两次云粒子垂直探测资料分析层状云结构特征和演化规律,探讨冬季低槽冷锋系统层状云催化条件和催化时机。结果表明：天气系统初期西南风风速中心发生在6500 m高空时,风速随高度呈不连续分布,层状云云顶温度为-29.2℃,中低云层稀薄;3000 m以上云层粒子基本被冰化,云内过冷水含量小于0.05 g·m^-3,固态含水量为0.1 g·m^-3左右,3000 m以下云层有大量自然冰晶,过冷水低于0.15 g·m^-3,不具备催化条件。700—3000 m高度层西南风加强,云内过冷水含量普遍大于0.1 g·m^-3;2200 m风速中心风速达16 m·s^-1,该高度最大液态水含量达0.38 g·m^-3,冰晶浓度为6 L^-1,温度为-9℃,适宜催化。低槽冷锋天气系统层状云结构特征和催化条件受槽前西南风强弱和风速中心高度影响,天气系统初期层状云云顶过高、温度过低时,层状云为不可播云。随槽线东移,风速中心高度降低,3000 m以下西南风加强,层状云转变为可播云。     

The effects of changing solar angles,cloud regimes,and air temperatures on the temperatures of contrasting surfaces

A steady-state energy budget and a solar radiation-cloud attenuation model have been linked. This has permitted a systematic examination of the changing relative surface temperatures and differences between surface and air temperatures as functions of varying solar altitudes, selected cloud types, cloud covers, and air temperatures. This analysis was performed for two contrasting surfaces: barren and grassy. The results are presented in a series of three-dimensional graphs. The grassy landscape reacted more conservatively to changing solar altitudes (for all cloud types and amounts) compared with the barren surface. The results are assumed to encompass most of the possible extremes encountered in real landscapes exposed to similar changes in solar angle, air temperatures, and cloud regimes.Dr. O'Rourke is currently a Post-Doctoral Scholar at UCLA.     

利用静止卫星窗区红外亮温反演晴空区风场的初步研究

云导风反演明显的局限是在晴空区无法获取风场。为克服这种局限,提出利用静止卫星红外图像资料反演晴空区风场的方法—时间序列差值法,利用LOWTRAN7对其进行可行性分析,并应用云导风实验系统CWIS进行晴空区导风实验。结果表明：实验结果与NCEP资料低空风场有很好的一致性。因此,在传统的“云导风”基础上,增加晴空区卫星导风,可为卫星遥感资料在台风、梅雨和强对流等天气系统分析预报中的定量应用提供更多的卫星风矢。     

A numerical study of the marine stratocumulus cloud layer

A one-dimensional grid-level model including longwave radiative transfer and a level-4 second-order turbulent transfer closure which contains prognostic equations for turbulent quantities, is used to study the physics and dynamics of inversion-capped marine stratocumulus clouds.A set of numerical experiments had been performed to examined the role of sea surface temperature, large-scale vertical velocity, wind speed, and vertical wind shear in the formation and the structure of low-level clouds. For a given sea surface and geostrophic wind speed, stratocumulus clouds can grow higher with smaller large-scale subsidence as less dry air entrains into the cloud. Clouds grow higher with higher sea surface temperature for a given geostrophic wind speed and large-scale subsidence as a result of enhanced moist convection. In high wind speeds, the entire cloud deck is lifted up because of larger surface energy flux. In the budget studies of the turbulent kinetic energy (TKE), the buoyancy term is a major source term when the wind speed and the vertical shear are small across the inversion top. When the wind speed and the vertical wind shear across the inversion top become large, the mixed layer is decoupled into a cloud and a subcloud layer. In the TKE budget studies, the shear generation term becomes an important term in the budgets of the TKE and the variance of vertical velocity.     

On formation of convective roll vortices by internal gravity waves: A theoretical study

Summary The linearized atmospheric equations system is solved analytically in a two layer model. The solutions show that the thermal disturbance located at the interface can induce internal gravity wave, which propagates downstream in the stable layer and brings about flow disturbances in the lower unstable layer. Motion of roll vortices with flow pattern similar to that found in the convective cloud street forms in the lower part of the upper layer and the upper part of the lower layer. If proper content of water vapor exists the cloud lines presenting small angle with the mean wind appear at the top of the lower layer. The effects of the wind speed and the temperature structures of the atmosphere in the lower convective layer and the overlying stable layer on the characteristics of the roll vortices are also discussed in this study.With 7 Figures     

用改进的光流法和卫星TBB资料进行对流临近预报

为了在缺少雷达观测的地区开展对流临近预报,利用光流法和半拉格朗日外推法进行了卫星云图外推实验,同时利用无辐散约束改进光流矢量来避免云图辐散失真。(1) 光流法反演的风场能够准确反映出台风涡旋环流结构,采用半拉格朗日方案进行外推,可以保持云系的旋转特性,具有良好的稳定性和精度,但随着外推时间的增加光流矢量中的噪声会导致云图辐散失真。(2) 无辐散约束减少了风矢量杂乱无序现象,弥补了缺失的光流,还能对风速进行平滑,使风速空间梯度更平滑。(3) 用改进后的风场进行外推,避免了云系辐散失真,在保持其形态不被破坏的同时,还能减少云团TBB虚假增长。(4) 检验表明改进后的外推预报方案,具有更小的平均绝对误差,MAE提高了4%,临界成功指数提升了9%。     

On the Atmospheric Response to Urbanization and Open-Pit Mining under Various Geostrophic Wind Conditions

Summary  Simulations alternatively assuming a real landscape with and without open-pit mines and grown settlements were performed with a non-hydrostatic meteorological model of the meso-β-scale to elucidate whether the atmospheric response to such land-use changes is sensitive to the direction and magnitude of geostrophic wind. The results of simulations with the same geostrophic wind conditions substantiate that the daily domain-averages of the variables of state hardly differ for the different landscape realizations, except for cloud- and precipitating particles. However, land-use changes may significantly affect the local conditions over and downwind of the altered surfaces. The significant differences in the cloud- and precipitating particles, however, are not bound to the surroundings of land-use changes. The vertical component of wind vector, which is modified by the different heating of converted land-use, strongly affects cloud- and precipitation formation by the interaction cloud microphysics-dynamics. The magnitude of atmospheric response changes under the various directions and speeds of geostrophic wind for most of the field quantities and fluxes. Received February 10, 1999     

两种导风方法在台风路径分析中的 应用比较研究

基于云导风使用的最大相关法（已经广泛应用）和TCFM法（傅立叶相位分析法和最大相关法相结合）两方法的原理,开发了利用FY-2C静止气象卫星30min间隔卫星云图资料获得云迹风的可视化导风实验系统。利用该系统处理2005年夏季麦莎、卡努、龙王等3个台风期间的卫星云图,对得到的云迹风进行台风路径反演分析。结果表明,TCFM导风方法得到的结果优于最大相关法。