Momentum flux in the subcloud layer of a microburst-producing thunderstorm determined from JAWS Dual-Doppler data

Dual-Doppler winds at 1647 MDT for the 14 July, 1982 convective storm collected during the Joint Airport Weather Studies (JAWS) project at Denver's Stapleton International Airport were objectively analyzed to produce a three-dimensional wind field. The domain of interest had dimensions of 10 × 10 × 8.5 km centered on the microburst. Vertical velocities were computed by integrating the anelastic continuity equation downward from the storm's top. A variational approach was then employed to adjust the derived three-dimensional wind field. Subsequently, fields of deviation perturbation pressure and virtual temperature were retrieved from a detailed wind field using the three momentum equations. These retrieved fields were subjected to internal consistency checks to determine the level of confidence before interpetation. The fields were then used to calculate the generation of the vertical transport of horizontal momentum in the subcloud layer of a microburst-producing storm during the quasi-steady mature stage. Results show that the microburst occurrence in the atmospheric boundary layer (ABL) enhances eddy transfer of momentum. Direct calculation of the vertical transport of u- and v-momentum reveals that momentum was being transferred downward from the mid-levels of the storm to the microburst. The dominant processes contributing to the generation/dissipation of horizontal momentum flux were the total buoyancy production, pressure effects, vertical mean wind shear and vertical transport of momentum. The above processes play an important role in maintaining the strength of the microburst outflow in the ABL during the quasi-steady mature stage of the microburst life cycle.     

The subcloud-layer eddy kinetic energy budget of a microburst-producing thunderstorm determined from jaws dual-Doppler measurements

Dual-Doppler data collected from 1646 to 1648 MDT on 14 July, 1982 in Colorado were employed to study the eddy kinetic energy budget in the subcloud layer of a microburst-producing thunderstorm during its mature stage. Each term in the budget equation was computed from the Doppler-derived winds and retrieved thermodynamic fields within the 10 by 10 km horizontal domain. Results show that in the atmospheric boundary layer (ABL) where the microburst dominates, the turbulent flow extracts energy from the mean flow in order for the microburst to maintain its strong diverging outflow at low levels. The vertical transport of eddy kinetic energy is predominantly downward in the low layer due to the organized downdrafts in the microburst area. The horizontal flux convergence (divergence) of eddy kinetic energy by the mean and eddying motions is approximately balanced by that of the vertical flux divergence (convergence). Similarly, the contributions from the pressure and buoyancy production terms are nearly in balance. As a result, a net change of the eddy kinetic energy generation in the subcloud layer is relatively small in comparison with the individual term in the budget equation.     

Comparison of tropical and midlatitude thunderstorm characteristics anchored in thermodynamic and dynamic aspects

Thunderstorms prevailing over tropics and midlatitudes depict dissimilar features relating to the thermodynamic and dynamic aspects. The identification of the physical characteristics of the tropical and midlatitude thunderstorms is the main objective of the present study. The stations Kolkata (22.6°N, 88.4°E) and Denver (39.47°N, 104.32°W) are selected from the tropics and midlatitudes for the comparative analyses. The study reveals that the average storm relative helicity (SRH) and the lapse rate between 700 and 500 hPa level is much higher over Denver compared to Kolkata during thunderstorm days. The study further reveals that the surface to mid troposphere (upto 500 hPa) become drier (～2 times) over Denver than Kolkata prior to the occurrence of thunderstorms while the upper tropospheric (300–100 hPa) humidity remains comparable for both the locations.     

On the relationship of thunderstorm ice hydrometeor characteristics and total lightning measurements

Satellite-borne and ground-based devices for the detection of lightning offer the opportunity to explore relationships–on all significant scales up to global–between lightning frequency, f, and other thundercloud parameters. Calculations predict that f is proportional to the product of the downflux p of solid precipitation and the upward mass flux, I, of ice crystals. This prediction has received support from limited computational studies. The physical reasons for such a relationship are explained in terms of the paramount role of ice in the electrification of thunderstorms. Herein, this prediction is subjected to further, preliminary examination through analysis of lightning and dual-polarimetric radar data collected during the STERAO experiment conducted in Northern Colorado during the summer of 1996. The analysis has yielded some highly provisional support for this flux hypothesis. Computed trends of radar derived hydrometeor fractions of solid precipitation and small ice show correlation to the total lightning frequency and raise the possibility of determining values of p and/or I from lightning measurements.It is shown that the extent to which the observed correlations between f and both solid precipitation and small ice trends are or are not strong can provide an indication as to whether the lightning activity is limited by the available concentrations of precipitating or non-precipitating ice in the upper regions of the charging zone of the thundercloud, where most of the charge transfer occurs. It is demonstrated that the most accurate determinations of precipitation rate p from measurements of lighting frequency f are likely to be for conditions where the field-growth is limited by the availability of graupel pellets. It is shown that the simultaneous time variations of f and solid precipitation trends of the type obtained in the STERAO experiment could enable us to determine the nature of the dominant glaciation process operative in the thunderclouds studied.     

In situ measurements of contributions to the global electrical circuit by a thunderstorm in southeastern Brazil

The global electrical circuit, which maintains a potential of about 280 kV between the earth and the ionosphere, is thought to be driven mainly by thunderstorms and lightning. However, very few in situ measurements of electrical current above thunderstorms have been successfully obtained. In this paper, we present dc to very low frequency electric fields and atmospheric conductivity measured in the stratosphere (30–35 km altitude) above an active thunderstorm in southeastern Brazil. From these measurements, we estimate the mean quasi-static conduction current during the storm period to be 2.5 ± 1.25 A. Additionally, we examine the transient conduction currents following a large positive cloud-to-ground (+ CG) lightning flash and typical − CG flashes. We find that the majority of the total current is attributed to the quasi-static thundercloud charge, rather than lightning, which supports the classical Wilson model for the global electrical circuit.     

用双多普勒雷达分析华南一次飑线系统的中尺度结构特征

飑线是中国春、夏季常见的一种天气现象.2007年4月23-24日华南地区的一次强飑线过程,因强度强、持续时间长,给广东部分县市造成巨大的经济损失.利用广州和深圳的新一代多普勒天气雷达的探测资料,采用双多普勒雷达风场反演方法和热力动力反演方法,研究此次飑线内部的三维风场、动力和热力结构.在此基础上,通过质量、动量诊断探讨其维持机制.同时,结合地面、高空常规观测及自动站资料分析系统伴随的环境和地面中尺度特征.结果表明,系统发生于中等强度垂直风切变(0-3 km,18 m/s)和中等强度对流不稳定度的环境中,其移动速度最快达17 m/s,属快速移动的飑线.飑线过境时,地面测站呈现风向突变、风速骤增、气压上升、温度与露点骤降等现象.其成熟期的雷达回波和风场结构与过去观测的热带、副热带快速移动飑线系统特征相似.由前(前方指系统移动方向)向后,降水结构依次为对流区、过渡带和层云区.其中,对流区由多个对流单体组成,对应强回波和上升运动,而单体之间为弱下沉运动.气流特征呈准二维结构,包括系统前方深厚的从前向后气流和系统后部低层的从后向前气流.这两支气流在系统前缘低层辐合形成动力高压,触发新对流单体,是系统长时间维持的主要机制.在对流区倾斜上升气流的下方,存在一凝结潜热释放增暖引起的低压区.另外,在对流系统西侧不断有新对流单体形成,并被平流至系统中,促进对流维持.进一步对对流区的质量和动量通量诊断揭示,垂直于飑线系统的水平动量的垂直输送有逆梯度输送的特性,将增加垂直飑线方向的环境垂直风切变,有利于系统维持.相对的,平行飑线方向的水平动量具有顺梯度输送特征,使该方向上风切变分量有被均滑趋势.     

双多普勒雷达风场反演误差和资料的质量控制

提出了用双多普勒雷达观测资料进行回波强度、径向速度和方位定位的质量控制方法,以及利用概率分布法订正配对的双多普勒雷达回波强度的方法。以2001年973“中国暴雨”外场试验期间获取的双多普勒雷达观测资料为例,比较了合肥、马鞍山和宜昌、荆州的两对双多普勒雷达同步观测的回波强度、径向速度和方位的改变对观测数据对比的影响;分析了这两对双多普勒雷达径向速度测量误差引起的风场反演的误差。结果表明:两对双多普勒雷达观测的回波强度和径向速度的空间位置和变化趋势比较一致,合肥与马鞍山雷达的回波强度有一定差异,径向速度也有1～2 m/s的差异。风场反演的误差与风场的方向、大小、空间位置等有关。在两个径向速度夹角在40°～140°范围内,共面上的风场的反演误差在1～2倍的雷达探测径向速度的误差范围内。     

Variations and trends in annual and seasonal air temperatures in Greece determined from ground and satellite measurements

Summary The variations and trends in annual and seasonal air temperatures in Greece were examined on the basis of ground measurements for 25 stations during the period 1951–1993, and satellite measurements for the south eastern Mediterranean during the period 1979–1991. Data were smoothed using a 5-year running mean and were thereafter examined by regression analysis to define trends in the long duration time series. Data were also examined to detect abrupt changes and trends in the long duration annual, winter and summer series of mean maximum, mean minimum and mean temperatures. An overall cooling trend was detected for the majority of stations in winter over the entire period; the same cooling trend was also recognised for the annual and summer mean values, although a reverse warming trend was detected around the mid-70s at several stations. Satellite measurements indicate a slight warming trend, although this is not statistically significant. Considering the results of the regression analysis and the statistical tests applied to the 25 stations, it may be concluded that annual mean temperatures are dominated by an overall cooling trend, with the exception of stations in urban areas where urbanisation effects may have resulted in a warming trend. Summer temperatures, however, exhibit a warming trend roughly after 1975 at most stations.With 5 Figures     

Carbon tetrachloride lifetimes and emissions determined from daily global measurements during 1978–1985

Seven years of daily gas chromatographic measurements of CCl₄ at the five globally distributed ALE/GAGE surface sites are reported. It is determined that CCl₄ has been accumulating in the atmosphere at a rate of 1.3±0.1%/yr over the period 1978–1985 and that the releases of CCl₄ into the atmosphere have remained fairly constant, with the smallest releases in 1981–1982. Using an inversion scheme based on a nine box model of the atmosphere, we infer a CCl₄ lifetime of approximately 40 yr, an inventory on 1 July 1978 of (2.08±0.07)×10⁹ kg and an average rate of release over the period 1978–1985 of (0.9±0.9)×10⁷ kg/yr. These results produce excellent agreement with a release scenario derived from global production estimates for CCl₄ and the major CCl₄ byproduct, the chlorofluorocarbons. However, to obtain this consistency, it is necessary that our current ALE/GAGE absolute calibration standard be reduced approximately 25% thus bringing it into agreement with measurements by Yokohata et al. (1985) and Hanst et al. (1975).     

一次夏季雷暴天气过程中闪电活动特征分析

利用探空资料、多普勒天气雷达和闪电定位仪数据,分析了2009年7月30日发生在南京地区一次雷暴天气过程的雷达及闪电数据时空演变特征。结果表明：对流有效位能Ecap比起K指数（IK）等对于对流潜势预报具有更明显的指示作用,0 ℃层和-10 ℃层高度的降低有利于雷暴云的雷电活动;整个过程以负闪为主导,闪电强度越大,闪电频数也越高,每次闪电峰值后,都对应一次谷值;闪电数据与多普勒天气雷达回波叠加后分析发现,回波的生消演变对应着闪电频数和强度的生消演变,负闪主要落在强回波中心区域,正闪零星分布于回波强度梯度较大的区域;雷达径向速度图像特征变化更能揭示闪电发生发展的机制,逆风区对应雷暴中心区域,不仅是强降水的中心区也是闪电的中心区,对逆风区的识别监测能够更好的指导雷暴预警报工作;利用雷达数据计算的云底动能施力参量,能够很好的描述支持闪电起电的热动力特征,云底动能施力对雷电增长的贡献有一段持续传输过程,该参量峰值比频闪峰值和强度峰值都约有0.5 h的提前量。     

双多普勒雷达反演强风暴三维风场的数值试验

两部雷达测得的径向速度再加上质量连续方程可以求解大气风场，利用这一原理，以1996年6月29日发生在北京东北部京冀交界处的一次强风暴过程为例，用模式反演出风暴体的三维风场结构，误差分析表明和实况基本一致。利用模式输出的三维风场，先模拟两部多普勒雷达扫描的数据，在此基础上，进行三维风场反演，结果表明，反演得到的流场和模拟的流场总体趋势完全一致，风暴的中心位置吻合较好，上升气流和下沉气流的分布也很接近，水平速度分量反演的误差很小，基本上可以反映三维风场的真实情况。垂直速度的平均离差在各个高度上都较小，反演结果较好。     

雷达、闪电资料在典型雷暴单体中的应用

用闪电定位资料和雷达回波资料进行分析,即从雷达产品的4个参数与地闪频数的关系进行相关分析,分析结果表明:(1)反射率因子的回波面积大小和强回波中心的移动与闪电发生的时间、地点有很好的一致性。(2)闪电发生前最大回波顶高有明显的增大,闪电发生的位置和最大回波顶高的位置有很好的对应关系。(3)地闪频数的变化与VIL(垂直累积液态含水量)最大值的变化并不完全对应,但VIL最大值下降到25 kg/m2以下时,可以考虑作为雷暴消散的一个因子。VIL大值区的移动以及VIL大值区面积的变化与地闪频数的整体变化是一致的。(4)雷达VWP产品的无数据区破坏的时间提前于闪电的发生和地闪频数的增长,其恢复滞后于闪电消失。     

铜仁市一次雷暴大风过程分析

利用常规站资料、ERA5资料（0.25°×0.25°）以及务川雷达和铜仁新一代多普勒天气雷达资料对2022年4月24日发生在贵州铜仁市多个区（县）的一次雷暴大风过程进行分析,结果表明：（1）此次天气过程是发生上干下湿的不稳定环境中,高空槽、南支槽、低涡、切变线和地面辐合线都为此次强对流天气过程提供了触发条件。（2）大的DCAPE值、中层中等强度的垂直风切变和低层较强的垂直风切变以及较大的850hPa与500hPa的温差都是利于大风产生的条件。（3）大风常常出现在弓形回波前部突出处,高悬的强回波、弱回波区、高的回波顶高以及径向速度中出现逆风区和强并且深厚的中层径向辐合等都是出现大风天气的雷达产品特征。     

Development and structure of a maritime continent thunderstorm

Summary The evaluation of a maritime continent thunderstorm complex (Hector) occurring over Bathurst and Melville Islands north of Darwin, Australia (12° S, 131° E) is investigated primarily using Doppler radar data. Thunderstorm formation follows the development of sea breeze circulations and a period of shallow non-precipitating convection. Evidence exists for initiation of long-lived and organised convection on the sea breeze fronts, although short-lived, scattered convection is apparent earlier in the day. Merging of the convective systems is observed in regions of enhanced low-level convergence related to sea breeze circulations. The merged convective complex is initially aligned in an almost east-west direction consistent with the low-level forcing. The merged complex results in rapid vertical development with updraughts reaching 40 m s^– and echo tops reaching 20 km height. Maximum precipitation production occurs during this merger phase. On the perimeter of the merged convective complex, evidence exists for front-to-rear updraughts sloped over lower-level downdraughts with rear-to-front relative flow and forward propagating cold pools. The mature phase is dominated by this convection and the complex re-orientates in the prevailing easterly vertical shear to an approximate north-south direction, then moves westward off the islands with the classic multicellular squall-like structure.The one-dimensional cloud model of Ferrier and Houze (1989) used with a four class ice formulation reproduced the cloud top height, updraught structure and echo profile very well. To test the importance of ice physics upon thunderstorm development, several sensitivity tests were made removing the effects of the ice phase. All of these model clouds reached nearly 20 km, although simulations without the effects of ice had updraughts reduced from about 40 m s^–1 to 30 m s^–1. The simulated convection was more sensitive to changes in environmental conditions and parameterised cloud dynamics. The strong intensity of the convection was largely accounted for by increasing equivalent potential temperatures due to diurnal heating of the surface layer. The vertical velocity and radar structure of the island thunderstorm has more similarity with continental rather than oceanic convection. Maximum vertical velocities, in particular are almost an order of magnitude greater than typical of oceanic convection. With the intense updraughts, even in the low shear environment, there is evidence for mesoscale circulations within the convection.With 17 Figures     

Three-Dimensional Wind Field Retrieved from Dual-Doppler Radar Based on a Variational Method: Refinement of Vertical Velocity Estimates

In this paper,a scheme of dual-Doppler radar wind analysis based on a three-dimensional variational method is proposed and performed in two steps.First,the horizontal wind field is simultaneously recovered through minimizing a cost function defined as a radial observation term with the standard conjugate gradient method,avoiding a weighting parameter specification step.Compared with conventional dual-Doppler wind synthesis approaches,this variational method minimizes errors caused by interpolation from radar observation to analysis grid in the iterative solution process,which is one of the main sources of errors.Then,through the accelerated Liebmann method,the vertical velocity is further reestimated as an extra step by solving the Poisson equation with impermeable conditions imposed at the ground and near the tropopause.The Poisson equation defined by the second derivative of the vertical velocity is derived from the mass continuity equation.Compared with the method proposed by O’Brien,this method is less sensitive to the uncertainty of the boundary conditions and has better stability and reliability.Furthermore,the method proposed in this paper is applied to Doppler radar observation of a squall line process.It is shown that the retrieved vertical wind profile agrees well with the vertical profile obtained with the velocity–azimuth display(VAD)method,and the retrieved radial velocity as well as the analyzed positive and negative velocity centers and horizontal wind shear of the squall line are in accord with radar observations.There is a good correspondence between the divergence field of the derived wind field and the vertical velocity.And,the horizontal and vertical circulations within and around the squall line,as well as strong updrafts,the associated downdrafts,and associated rear inflow of the bow echo,are analyzed well.It is worth mentioning that the variational method in this paper can be applied to simultaneously synthesize the three-dimensional wind field from multiple-Doppler radar observations.     

1951-2010年大连市雷暴气候统计特征

利用1951-2010年大连雷暴资料,通过数理统计、Mann-Kendall和Morlet小波分析等方法研究大连市雷暴的气候特征。结果表明：近60 a年大连市年平均雷暴日较多,年际和年代际变化较大,年雷暴日呈增加趋势,每10 a雷暴日增加0.5 d。初雷日明显提前,终雷日明显推迟。雷暴有很强的季节性特点,集中出现在5-9月,而6-8月占近6成;夏季多于秋季,冬季最少。逐月平均雷暴分布与月平均气温、月降水量呈同相单峰形,7月雷暴出现高峰值,10月雷暴锐减。雷暴出现频率以02-06时为最高,16-20时为最低。雷暴日数的年际和年代际变化存在不显著的2-4 a和14-16 a振荡周期,并且在1972年发生突变。     

Lifetime of the thunderstorm electric energy in the global atmospheric circuit and thunderstorm energy characteristics

The lifetime of electric energy in the atmosphere is introduced and investigated as is the total electric energy of the atmosphere related to the total mean rate of electric energy dissipation. This lifetime, as determined from general estimations and convenient analytical expressions, turns out to be very small – from about 10 to about 100 s, depending on the assumptions on the control parameters of principal sources in the global electric circuit. In particular the energy lifetime is less than the relaxation time of the “global condenser” and field relaxation time near the ground surface. It is explained by the high dissipative rate of the electric energy in the atmosphere, taking into account that the regions mainly contributing to the total energy and its dissipative rate are connected to the altitudes of active parts of electrified (thunderstorm) clouds in the atmosphere with exponentially increasing conductivity.     

The effects of the Arctic haze as determined from airborne radiometric measurements during AGASP II

The interaction of the Aretic winter aerosol (Arctic haze) with solar radiation produces changes in the radiation field that result in the enhancement of scattering and absorption processes which alter the energy balance and solar energy distribution in the Arctic atmosphere-surface system. During the second Arctic Gas and Aerosols Sampling Project (AGASP II) field experiment, we measured radiation parameters using the NOAA WP-3D research aircraft as a platform. State-of-the-art instrumentation was used to measure in situ the absorption of solar radiation by the Arctic atmosphere during severe haze events. Simultaneously with the absorption measurements, we determined optical depths, and total, direct, and scattered radiation fields. All optical measurements were made at spectral bands centered at 412, 500, 675, and 778 nm and with a bandpass of 10 nm. With this selection of spectral regions we concentrated on the measurement of the radiative effects of the aerosol excluding most of the contributions by the gaseous components of the atmosphere. An additional measurement performed during these experiments was the determination of total solar spectrum fluxes. The experimentally determined parameters were used to define an aerosol model that was employed to deduce the absorption by the aerosols over the full solar spectrum and to calculate atmospheric heating rate profiles. The analyses summarized above allowed us to deduce the magnitude of the change in some important parameters. For example, we found changes in instantaneous heating rate of up to about 0.6 K/day. Besides the increased absorption (30 to 40%) and scattering of radiation by the atmosphere, the haze reduces the surface absorption of solar energy by 6 to 10% and the effective planetary albedo over ice surfaces by 3 to 6%. The vertical distribution of the absorbing aerosol is inferred from the flux measurements. Values for the specific absorption of carbon are found to be around 6 m²/g for externally mixed aerosol and about 11.7 m²/g for internally mixed aerosol. A complete study of the radiative effects of the Arctic haze should include infrared measurements and calculations as well as physics of the ice, snow, and water surfaces.     

山东省一次积层混合云暴雨三维风场的双多普勒雷达探测

使用地基双多普勒天气雷达监测资料和常规探测资料,对2005年9月19日凌晨发生在鲁中地区的一次暴雨进行分析,结果表明：本时段暴雨是积层混合云降水所致,中低层存在切变线和辐合线,风场中尺度结构造成小尺度对流单体的发展,这些对流单体以带状结构组成回波群镶嵌在大面积的层状云中;伴随弱冷空气侵入,风场的中尺度结构主要出现在4km以下的层次,中低层切变线和冷空气的侵入是强回波单体发展的动力因素;在垂直方向上,强回波区有明显的垂直运动。风场的中尺度结构对鲁中强降水的发生和维持有重要作用。