Isoprene emissions from vegetation and hydrocarbon emissions from bushfires in tropical Australia

Information from a variety of sources, including an airborne field expedition in November 1985, is used to produce estimates of the annual emissions of some hydrocarbons from bushfires, and isoprene from trees, in tropical Australia. For the continent north of 23° S the annual bushfires (biomass burning) input was estimated, in units of Tg carbon, to be 2 TgC (uncertainty range 0.8–5 TgC), emitted predominantly during the May to October dryseason. Isoprene emissions during this period were estimated also to be 2 TgC (uncertainty range 0.5–8 TgC), but were estimated to be an order of magnitude higher during the November to April wet season, at a level of 23 TgC (uncertainty range 6–100 TgC).The large annual emission of isoprene over the tropical part of the Australian continent yields ppbv levels of isoprene measured at the surface in summertime. Isoprene reactivity with hydroxyl radical is such that at these concentrations isoprene must be a dominant factor in controlling the concentration of OH radical in the convective boundary layer. Simple arguments based on the convective velocity scale suggest that the shape of the isoprene vertical profile in November 1985 would be consistent with available data on the OH-isoprene reaction rate if OH concentration in the boundary layer averaged about 2.5×10⁶ cm^-3 over the middle part of the day.Temporarily at the International Meteorological Institute, Stockholm University, S-106 91, Stockholm, Sweden.     

Variability and decline in the number of severe tropical cyclones making land-fall over eastern Australia since the late nineteenth century

Recent studies have raised concerns that tropical cyclones (TCs), particularly severe TCs, have become more frequent in many places in response to global warming. Other studies discuss errors in TC data that can cause large inaccuracies in some of the observed trends. Additional studies conclude that TCs are likely to become more intense in the future in response to global warming, while regional modelling studies for the south-west Pacific near north-eastern Australia project an intensification of TCs and either a decrease or no change in TC numbers. Here we describe and use a new data base of severe land-falling TCs for eastern Australia derived from numerous historical sources, that has taken over a decade to develop. It provides one of the world??s longest reliable records of tropical cyclone activity, and allows us to document changes over much longer periods than has been done previously for the Southern Hemisphere. Land-fall numbers are shown to vary a great deal on interannual, decadal and longer time-scales. The interannual variability is consistent with previous studies using much shorter data sets: land-fall numbers are well-simulated as a Poisson process and are modulated by the El Ni?o-Southern Oscillation (ENSO). Land-falls occurred almost twice as often in La Ni?a years as they did in El Ni?o years, and multiple land-falls only occurred during La Ni?a years. The statistical link between land-falls and pre-season values of the Southern Oscillation Index provides a modest predictive capability. Decadal variability in ENSO drives some of the decadal variability in land-fall numbers. The sign and magnitude of trends calculated over 30?years periods vary substantially, highlighting that caution needs to be taken in making inferences about trends based on e.g. satellite era data only. The linear trend in the number of severe TCs making land-fall over eastern Australia declined from about 0.45 TCs/year in the early 1870s to about 0.17 TCs/year in recent times??a 62% decline. This decline can be partially explained by a weakening of the Walker Circulation, and a natural shift towards a more El Ni?o-dominated era. The extent to which global warming might be also be partially responsible for the decline in land-falls??if it is at all??is unknown.     

The formation of tropical cyclones

Summary This paper attempts a synthesis of new observations and new concepts on how tropical cyclone formation occurs. Despite many worthy observational and numerical modeling studies in recent decades, our understanding of the detailed physical processes associated with the early stages of tropical cyclone formation is still inadequate; operational forecast skill is not very high. Although theoretical ideas cover a wide range of possibilities, results of new observations are helping us to narrow our search into more specific and relevant topic areas.With 33 FiguresPrologueThis paper is dedicated to Professor Herbert Riehl under whom I studied tropical meteorology at the University of Chicago from 1957–1961 and was later associated with at Colorado State University (CSU). Professor Riehl arranged my first aircraft flights into hurricanes in the late 1950s and gave great encouragment to me to explore the secrets of what causes a tropical disturbance to be transformed into a tropical storm.Herbert would persist in asking me nearly every week or so what causes a hurricane to form? I and my graduate students and research colleagues at CSU have been working to uncover the secrets of tropical cyclone formation ever since. The following article gives my current best estimate of the primary physical processes involved with this topic.     

The motion of binary tropical cyclones

Summary When two tropical cyclones are present simultaneously in the same region they show as a rule a counterclockwise rotation around each other. A theoretical explanation of this motion is given. The theory permits the computation of the rate of rotation. A discussion of nine examples with sufficiently reliable and complete observational data gives a satisfactory agreement between theoretical and observed rates of rotation.

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Zusammenfassung Es ist häufig beobachtet worden, daß zwei tropische Zyklonen sich entgegen dem Uhrzeigersinne umeinander bewegen. Eine theoretische Erklärung dieser Bewegung kann gegeben werden auf Grund der Annahme, daß sich jedes Zyklonenzentrum unter dem Einfluß des Windfeldes der anderen Zyklone bewegt; auf Grund dieser Theorie läßt sich die Rotationsgeschwindigkeit des Zyklonenpaares berechnen. An Hand von neun Fällen mit genügend zuverlässigem und vollständigem Beobachtungsmaterial wird gezeigt, daß gute Übereinstimmung zwischen Theorie und Beobachtung besteht.

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Résumé On a souvent observé que deux cyclones tropicaux se meuvent l'un par rapport à l'autre dans le sens inverse des aiguilles d'une montre. On peut expliquer le fait en admettant que chaque centre cyclonique se déplace sous l'influence du champ de vent de l'autre. Grâce à cette hypothèse on peut calculer la vitesse de rotation de la paire de cyclones. Neuf exemples richement documentés montrent un bon accord entre la théorie et l'observation.

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With 3 Figures.     

On the formation and development of tropical cyclones

A method of diagnostic calculation of the maximal wind speed in the tropical cyclone, its radius, and central pressure is proposed taking into account large-scale air motions in a low-gradient baric field between subtropical anticyclones. The results of such calculations are considered. A conclusion is made about sufficient accuracy of calculations of the parameters within the tropical zone using only dynamic factors.     

孟加拉湾风暴Mala结构及对云南强降水的影响

利用实时观测资料和NCEP(1°×1°)的6 h再分析资料,对2006年春季发生在孟加拉湾的超强风暴Mala的移动路径、强度变化、环流背景以及风暴温湿场、动力场特征等进行分析。结果表明:Mala在阿拉伯副热带高压和西太平洋副热带高压两高间辐合区生成、加强,并沿西太平洋副热带高压西侧西南或偏南气流移动。风暴发展、成熟到消亡,湿度对风暴的作用比温度明显;动力场结构除具有台风结构的一般特征外,在风暴发展期,中心附近散度场从低层到高层为辐合和辐散交替结构,表明风暴内部高空辐散抽吸作用对于风暴发展起到重要作用。登陆后风暴低压内自身能量和水汽与冷空气共同作用下,在冷暖交汇处出现强烈的上升运动和激发出中尺度辐合线是造成云南强降水主要原因。     

Fine-resolution regional climate model simulations of the impact of climate change on tropical cyclones near Australia

Fine-resolution regional climate simulations of tropical cyclones (TCs) are performed over the eastern Australian region. The horizontal resolution (30 km) is fine enough that a good climatological simulation of observed tropical cyclone formation is obtained using the observed tropical cyclone lower wind speed threshold (17 m s^–1). This simulation is performed without the insertion of artificial vortices (bogussing). The simulated occurrence of cyclones, measured in numbers of days of cyclone activity, is slightly greater than observed. While the model-simulated distribution of central pressures resembles that observed, simulated wind speeds are generally rather lower, due to weaker than observed pressure gradients close to the centres of the simulated storms. Simulations of the effect of climate change are performed. Under enhanced greenhouse conditions, simulated numbers of TCs do not change very much compared with those simulated for the current climate, nor do regions of occurrence. There is a 56% increase in the number of simulated storms with maximum winds greater than 30 m s^–1 (alternatively, a 26% increase in the number of storms with central pressures less than 970 hPa). In addition, there is an increase in the number of intense storms simulated south of 30°S. This increase in simulated maximum storm intensity is consistent with previous studies of the impact of climate change on tropical cyclone wind speeds.     

热带气旋螺旋云带动力不稳定的性质

热带气旋螺旋云带的不对称特征,在热带气旋的路径和强度变化中起着重要作用,对其动力性质的研究是整个热带气旋研究中的重要组成部分.文中分别对一个正压无辐散涡旋模型和正压原始方程涡旋模型进行线性化,采用标准模方法计算扰动的谱点和谱函数,研究扰动在基本流场中的不稳定问题,从而讨论了热带气旋中螺旋云带动力不稳定的性质.将一指定的基流廓线代入这两个模型,均会出现不稳定扰动.前者的流动为涡旋运动,仅在不稳定扰动的两个峰值之间可以看出螺旋状的结构特征,在距涡旋中心140 km的外围,不稳定扰动沿径向没有波动分布,没有螺旋云带状结构.此处相应于涡旋Rossby波的停滞半径(stagnation radius),在此半径之内出现的螺旋结构称为内螺旋云带,而在此半径之外出现的螺旋云带称为外螺旋云带.也就是说前者仅出现了眼壁(最大风速半径之内的最大扰动中心)、内螺旋云带,而后者则出现了眼壁、内螺旋云带和外螺旋云带.这说明滤去重力惯性波的正压无辐散涡旋模型(前者)只适合于解释热带气旋不稳定内螺旋云带的形成和结构,当综合考虑不稳定内、外螺旋云带的形成时,水平辐合、辐散的作用不能忽略,此时必须要用正压涡旋模型(后者).在该模型中因最不稳定扰动随涡旋半径的不同,其分别体现了涡旋Rossby波和重力惯性波的特点,故其是不稳定的涡旋Rossby-重力惯性混合波,其不稳定的性质是非平衡的.由此可知,要同时解释内、外螺旋云带的生成和结构,则非平衡的涡旋Rossby-重力惯性混合波不稳定理论应是更合适的选择.     

Oceanic thermal forcing of North Atlantic tropical cyclones

The relationship between North Atlantic tropical cyclone (TC) peak intensity and subsurface ocean temperature is investigated in this study using atmospheric and ocean reanalysis data. It is found that the peak intensity of basin-wide strong TCs (Categories 4 and 5) is positively correlated with subsurface ocean temperature in the extratropical North Atlantic. A possible physical mechanism is that subsurface ocean temperature in the extratropical North Atlantic can affect local sea surface temperature (SST); on the other hand, the moisture generated by the warming SST in the extratropical North Atlantic is transported to the main region of TC development in the tropics by a near-surface anticyclonic atmospheric circulation over the tropical North Atlantic, affecting TC peak intensity. Moreover, coastal upwelling off Northwest Africa and southern Europe can affect subsurface ocean temperature in the extratropical North Atlantic. Therefore, the peak intensity of strong TCs is also found to be directly correlated with the water temperature in these two upwelling regions on an interdecadal timescale.摘要利用大气与海洋再分析数据等相关资料, 本项研究发现, 北大西洋强台风 (Saffir–Simpson分类中的第4和第5类) 的最大强度与亚热带北大西洋的次表层海温呈正相关. 由于亚热带北大西洋的次表层海温会影响当地的海表温度, 该地区海面产生的水汽通过近地面的反气旋大气环流可被输送到位于热带的台风主要发展区域, 进而影响台风的最大强度. 与此同时, 位于西非北部和南欧的近岸涌升流会影响亚热带北大西洋的次表层海温. 因此, 强台风的最大强度也被发现与上述两个涌升流区域的海温具有相关性, 但是这种相关性主要体现在年代际时间尺度上.     

双台风相互作用的一种分析

一、引言 台风移动主要受背景流场的“引导气流”操纵。当间距足够近的两个或多个台风同时存在时,由于台风的位置变化比较显著,由它们引起的那部份“引导气流”的变化也就较大,这就使得在多台风情况下台风路径比较复杂。 在我们的统计动力学方案中,虽然在选取样本时已经有意识地剔除了一些影响明显的双台风个例。但是,从拟合误差的初步分析中,仍然很清楚地看出,产生较大误差极     

阿拉伯海热带气旋生成特征分析

利用印度气象局（India Meteorological Department,IMD）、国际气候管理最佳路径档案库（International Best Track Archive for Climate Stewardship,IBTrACS）提供的1982—2020年阿拉伯海热带气旋路径资料,美国国家环境预报中心（National Centers for Environmental Prediction,NCEP）再分析资料,对近39 a阿拉伯海热带气旋源地和路径特征、活跃区域、频数及气旋累积能量（accumulated cyclone energy,ACE）指数的季节特征和年际变化特征进行分析,并结合环境因素,说明其物理成因。结果表明：阿拉伯海热带气旋多发于10°~25°N,65°~75°E海域,5—6月、9—12月发生频数较高且强度较强,1—4月、7—8月发生频数较低且气旋近中心最大风速均小于35 kn;频数的季节变化主要受控于垂直风切变要素;阿拉伯海热带气旋发生频数和ACE近年有上升趋势,年际变化主要受控于海面温度（sea surface temperature,SST）和850 hPa相对湿度要素。     

Study on climatic characteristics of China-influencing tropical cyclones

Analysis of the climatic characteristics of the tropical cyclones that affect China yields several interesting features. The frequency of these tropical cyclones tended to decrease from 1951 to 2005, with the lowest frequency in the past ten years. The decrease in the frequency of super typhoons is particularly significant. The main season of tropical cyclone activities is from May to November, with an active period from July to September. There are three obvious sources of these tropical cyclones and they vary with seasons and decades. Their movement has also changed with seasons. On average, these tropical cyclones affect China for 5.6 months annually and the period of influence decreases in the past decades. An analysis of daily data indicates that the days of typhoon influence are shorter in winter and spring and longer in summer. The frequency of tropical cyclones is the largest over southeastern China, decreasing northwestward. Taiwan is the region that is affected by tropical cyclones most frequently. The average annual precipitation associated with tropical cyclones has also decreased gradually northwestward from southeastern China.     

Buoyancy in tropical cyclones and other rapidly rotating atmospheric vortices

Motivated primarily by its application to understanding tropical-cyclone intensification and maintenance, we re-examine the concept of buoyancy in rapidly rotating vortices, distinguishing between the buoyancy of the symmetric balanced vortex or system buoyancy, and the local buoyancy associated with cloud dynamics. The conventional definition of buoyancy is contrasted with a generalized form applicable to a vortex, which has a radial as well as a vertical component. If, for the special case of axisymmetric motions, the balanced density and pressure distribution of a rapidly rotating vortex are used as the reference state, the buoyancy field then characterizes the unbalanced density perturbations, i.e. the local buoyancy. We show how to determine such a reference state without approximation.The generation of the toroidal circulation of a vortex, which is necessary for vortex amplification, is characterized in the vorticity equation by the baroclinicity vector. This vector depends, inter-alia, on the horizontal (or radial) gradient of buoyancy evaluated along isobaric surfaces. We show that for a tropical-cyclone-scale vortex, the buoyancy so calculated is significantly different from that calculated at constant height or on surfaces of constant σ (σ = (p − p^*)/(p_s − p^*), where p is the actual pressure, p^* some reference pressure and p_s is the surface pressure). Since many tropical-cyclone models are formulated using σ-coordinates, we examine the calculation of buoyancy on σ-surfaces and derive an expression for the baroclinicity vector in σ-coordinates. The baroclinic forcing term in the azimuthal vorticity equation for an axisymmetric vortex is shown to be approximately equal to the azimuthal component of the curl of the generalized buoyancy. A scale analysis indicates that the vertical gradient of the radial component of generalized buoyancy makes a comparatively small contribution to the generation of toroidal vorticity in a tropical cyclone, but may be important in tornadoes and possibly also in dust devils.We derive also a form of the Sawyer–Eliassen equation from which the toroidal (or secondary) circulation of a balanced vortex may be determined. The equation is shown to be the time derivative of the toroidal vorticity equation in which the time rate-of-change of the material derivative of potential toroidal vorticity is set to zero. In analogy with the general case, the diabatic forcing term in the Sawyer–Eliassen equation is shown to be approximately equal to the time rate-of-change of the azimuthal component of the curl of generalized buoyancy.Finally, we discuss the generation of buoyancy in tropical cyclones and contrast the definitions of buoyancy that have been used in recent studies of tropical cyclones. We emphasize the non-uniqueness of the buoyancy force, which depends on the choice of a reference density and pressure, and note that different, but equivalent interpretations of the flow dynamics may be expected to arise if different reference quantities are chosen.     

Estimating the maximum potential intensity of tropical cyclones

Estimates of the maximum potential intensity (MPI) of tropical cyclones (TC) using different model on the base of in situ measurements are analyzed. Estimates published by other researchers and the ones obtained by the author are used. The inadequacy of model estimates of MPI and the real intensity of TC is registered in a number of cases, that is, first of all, related to the neglect of a number of peculiarities of TC structure and their environment in models, which are available nowadays.     

Ocean feedback to tropical cyclones: climatology and processes

This study presents the first multidecadal and coupled regional simulation of cyclonic activity in the South Pacific. The long-term integration of state-of the art models provides reliable statistics, missing in usual event studies, of air–sea coupling processes controlling tropical cyclone (TC) intensity. The coupling effect is analyzed through comparison of the coupled model with a companion forced experiment. Cyclogenesis patterns in the coupled model are closer to observations with reduced cyclogenesis in the Coral Sea. This provides novel evidence of air–sea coupling impacting not only intensity but also spatial cyclogenesis distribution. Storm-induced cooling and consequent negative feedback is stronger for regions of shallow mixed layers and thin or absent barrier layers as in the Coral Sea. The statistical effect of oceanic mesoscale eddies on TC intensity (crossing over them 20 % of the time) is also evidenced. Anticyclonic eddies provide an insulating effect against storm-induced upwelling and mixing and appear to reduce sea surface temperature (SST) cooling. Cyclonic eddies on the contrary tend to promote strong cooling, particularly through storm-induced upwelling. Air–sea coupling is shown to have a significant role on the intensification process but the sensitivity of TCs to SST cooling is nonlinear and generally lower than predicted by thermodynamic theories: about 15 rather than over 30 hPa °C^?1 and only for strong cooling. The reason is that the cooling effect is not instantaneous but accumulated over time within the TC inner-core. These results thus contradict the classical evaporation-wind feedback process as being essential to intensification and rather emphasize the role of macro-scale dynamics.     

Mesoscale structure of tropical cyclones in the surface wind field

The mesoscale structure of tropical cyclones in the northwest Pacific in 2002–2007 at different stages of evolution (from the genesis to the maximum intensity) is studied using the QuikSCAT satellite data and JRA-25 reanalysis data. It is demonstrated that the genesis of the tropical cyclone was preceded by the formation of the stable disturbance that was observed in the vorticity field on the average 47 hours before the first report. The variability is noted of the mesoscale structure of the cyclone during the process of its formation and evolution: the increase in the intensity of mesovortices, the decrease in their number as a result of the merging, the narrowing of the area occupied by them, and localization of this area near the center at the stage of maximum development. It is shown that the relationship between the mean intensity of mesovortices at the initial disturbance and the tropical cyclone intensity is close to linear and has high correlation coefficients.     

Feedback between convective heating and dynamics and movements of tropical cyclones

Summary It is shown that there exists a mechanism that can cause north-northwest movement of tropical cyclones in addition to already recognised mechanisms such as steering current and beta drift. This mechanism depends on the interaction between organised convection and dynamics. In the initial stages of formation of a cyclone, it is assumed that the hydrodynamic instabilities result in an incipient disturbance that organises some convection giving rise to a heat source. The atmospheric response to a localized heat source located off the equator in the northern hemisphere produces a low level vorticity field with a maximum in the northwest sector of the original heat source. If the Ekman-CISK which depends on the low level vorticity, was the dominating mechanism for moisture convergence, the location of the heat source would move to the new location of vorticity maximum. A repetition of this process would result in a northwest movement of the heat source and hence that of the cyclone. The movement of a tropical vortex under the influence of this mechanism which depends on asymmetries created by linear dispersion of Rossby waves is first illustrated using a linear model. It is then demonstrated that this process also enhances the motion of a tropical vortex in a nonlinear model. Importance of this feedback and the resulting movements of a tropical vortex in determining the actual track of a cyclone and in bogusing an initial vortex for prediction models are illustrated.With 6 Figures     

我国热带气旋研究十年进展

利用中国气象局上海台风研究所（CMA-STI）1949-2015年热带气旋（TC）最佳路径数据集和NCEP/NCAR再分析资料,对西北太平洋（含南海海域）1949-2015年TC快速增强（RI）集中区位置变化和影响因素等进行统计分析。结果表明：（1）RI的发生频数及伴有RI发生的TC频数均呈减少趋势,RI持续时间占TC生命史的比例及伴有RI发生的TC占TC总频数的比例呈震荡减小趋势。（2）RI集中区北界南移、南界北移,总体收缩南移,东界西移、西界东移,总体收缩西移。（3）西北太平洋环境风垂直切变（VWS）的弱切变区向西向南的气候漂移和海表面温度正距平区域的向南扩展是导致发生RI的TC最北纬度显著向南漂移的可能原因,发生RI的TC最南纬度向北的漂移则可能与高海表面温度（SST）向北扩展密切相关。（4）RI集中区的200 hPa高空辐散变强、850 hPa水汽输送加强等有利环境场条件的叠加,也对RI集中区的气候漂移有重要影响。

     

The behavior of total lightning activity in severe Florida thunderstorms

The development of a new observational system called LISDAD (Lightning Imaging Sensor Demonstration and Display) has enabled a study of severe weather in central Florida. The total flash rates for storms verified to be severe are found to exceed 60 fpm, with some values reaching 500 fpm. Similar to earlier results for thunderstorm microbursts, the peak flash rate precedes the severe weather at the ground by 5–20 min. A distinguishing feature of severe storms is the presence of lightning ‘jumps' — abrupt increases in flash rate in advance of the maximum rate for the storm. The systematic total lightning precursor to severe weather of all kinds — wind, hail, tornadoes — is interpreted in terms of the updraft that sows the seeds aloft for severe weather at the surface and simultaneously stimulates the ice microphysics that drives the intracloud lightning activity.