《热带气象学》全套教学课件

1、正确答案： b1.1 热带地区范围的限定 一般： 将南北纬30以内的赤道区域称作低纬度热带区域 ，包括热带tropics、副热带subtropics。 地理学： 2327S 2327N 热带 2327N6633N 温带 气象学观点：将赤道两边东风带区定为热带；将副热带高压区定为副热带，其高压脊线为东西风分界线，冬季平均在15N左右，夏季北移至35N附近。在地球的这一部分，大气的运动跟中高纬度的情况不同，有其明显的地区特点。从动力学角度而言，由于低纬度地区柯氏参数很小甚至趋于零，那里的大气必然会强烈地反映出 f 趋于零时的运动特征。从大气的热状况而言，大气的实测温度分布也有其纬度特点：中高纬地区

2、存在着较强的经向温度梯度，大气是斜压的；在低纬度地区温度分布均匀，其径向梯度很小，大气近于正压。因此：中高纬度大尺度天气扰动的能源:斜压位能的释放。热带低纬度地区，潜热释放则成了驱动热带地区环流系统的主要能源。 故：控制或影响低纬度大气动力学的基本因子中，有两个较为重要的因子： 效应 凝结潜热释放 1.4 参考书目 Dynamics of Tropical Cyclones, Yuqing Wang, Nanjing University, 2010.Atmospheric Science, the second edition, John M. Wallcae Academic press,

3、 University of Washington, USA, 2005大气动力学，北京大学出版社；刘式适、刘式达主编，2000年版。动力气象学教程，气象出版社，吕美仲、彭永清主编，1999年版。热带气旋动力学，气象出版社，钮学新主编，1999年版 Research interests are in the following areas: high resolution hurricane forecast (tracks, landfall, and intensity), monsoon forecasts on short, medium range, and monthly time

4、 scale and studies of interseasonal and interannual variability of the tropical atmosphere. Associate Professor, MeteorologyvmisraResearch interests are in climate variability and predictability; to understand climate variations from diurnal, intra-seasonal to interannual time scales; regional atmos

5、pheric models, atmospheric general circulation models and coupled ocean-atmosphere models.;ENSO, the South American Monsoons, Tropical Atlantic Variability, US hydroclimate and some aspects of equatorial African climate;Diagnosing the role of air-sea and land-atmosphere interactions in climate and w

6、eather variations Lydia StefanovaAssociate Research ScientistlstefanovaResearch interests in the assessment and modeling of global and regional climate variability, and the interactions between climate variability and hydrology, ecology and society.Ch2 热带地区的基本特点及热带大气运动的特征王咏青 南京信息工程大学Email：yongqing2.

7、1 低纬及热带地区的环境特点2.1.1 下垫面的主要特征大部分地区为海洋，是大气中的水汽源地赤道两侧海陆分布不对称，造成不同加热梯度，大陆、海洋冬季夏季冷热不同，是形成季风的重要原因青藏高原跨越中低纬（25N-40N，70E-105E),对中低纬的大气环流有重要热力和动力影响 动力：爬坡、绕流形成兰州小高压、西南涡 热力：高原夏季为热源，在高原上空（100300hPa）形成一个稳定的南亚高压。 1 低纬地区的环境特点2.1.2 地气系统的净辐射特征大气能量的源地35S-35N 主要是太阳短波辐射，净吸收35S以南，35N以北 主要是地气长波辐射，净辐射The tropics defined b

8、y zonal winds, predominantly easterly winds between 30S and 30N (dark blue and purple shades); the mean zonal winds for JanuaryDecember 1979 to 2009. 2.1.2 热带的纬向风2.1.3 温度廓线Fig 1.16. Standard atmospheric temperature profile (left) and height of the tropopause in the tropics, midlatitudes, and poles (

9、right). The troposphere is heated from below by latent heat, longwave radiation, and sensible heat. The tropics experience surplus heating and vertical expansion of the troposphere in response to that heating. In addition, deep tropical clouds transfer latent heat high in the atmosphere. The result

10、is that the tropopause is highest in the tropics. The average height of the tropical tropopause is almost 7 km higher than the average tropopause height at the poles (Fig.1.16).2.1.4 信风带逆温Fig. 1.18. (a) Schematic of mean sea level pressure and air flow and its relationship to stability in the tropos

11、phere for the tropical north Atlantic and (b) conceptual model of the vertical profile of the trade wind inversion, from west to east across the equatorial oceans. 2.1.5 热带湿度Fig. 1.19. (a) The distribution of surface water vapor percentage by latitude and (b) annual mean water vapor content (specifi

12、c humidity) profile. Data in (b) from Oort (1983). 2.1.6 热带海平面气压范围热带气压梯度力很小；海平面气压在副热带高压最高中可达：about 1024 hPa；最低在热带气旋中可达 870 hPa思考题：从左图的温度曲线中，你能发现温度随纬度的变化的趋势有什么不一致的地方，为什么？2.1.6 降水Simplified classification of precipitation patterns by latitude 2.1.7 大气角动量的源地2.2 低纬大气运动的频谱特征 2.2.1 空间尺度：大气湍流（1m1km） 积云对流（1

13、km10km） 中小尺度 (中- 220km 中- 20200km 中- 2001000km) 长波天气尺度（20006000km) 超长波行星尺度104km 2.2.2 时间尺度： 高频变化： 1小时1天， 重力波 天气变化： 1天10天， 大气长波 低频变化： 20天 ， 准双周振荡 30天50天， 季节内变化，3060天低频振荡 季节变化： 50天1年， 干季、湿季 甚低频变化：1年2年， 准两年周期振荡 2年9年， El-Nino 11年, 太阳黑子 年代际变化：10年100年，干旱、增温2.3 低纬和中高纬大气过程的主要区别中高纬地区低纬地区500hPa长波槽脊副高地面气旋（反气旋）

14、地面涡旋能量源斜压不稳定潜热释放.2.3.1、大气运动方程的变形 平面近似的运动方程 ： 大气运动学方程：在低纬度，由于 f 很小，所以需要将f 做变换：若： 则：故有：2.3.2 低纬度地区气象要素分布均匀 特征水平尺度：L106m 特征厚度： D 120 kt.) TCs have multiple eyewalls at some point in their lifecycleConcentric Eyewalls (will discuss more later)Aircraft Study of Eyewalls (Jorgensen,1984)EyewallConvectionL

15、ight precipimmediatelyoutside ofeyewallOutward slopewith height (30 - 45)Maximum tangentialwind 2 kmoutside of maximumupdraft core“Bright Band”(maximum inradar reflectivitydue to meltingice particles)Stratiformprecip (90%of area, 60%of total rain)RainbandsLocalized bands of heavy precipitation outsi

16、de of the eyewallSome have localized wind maxima, some do notJorgensen suggests that rainbands with wind maxima are actually nascent eyewallsRita (2005) 4 Zonally Asymmetric Features of the TropicsTropospheric Winds at 850 and 200 mb LevelsTropospheric Winds at 850 and 200 mb LevelsTropospheric Wind

17、s at 850 and 200 mb LevelsThe Motion Field in the Upper TroposphereThe Motion Field in the Upper TroposphereThe Temperature FieldThe Temperature FieldThe Temperature FieldThe East/West Circulations in the Tropics The time-averaged east/west circulations are essentially divergent. Here we decompose a

18、 horizontal velocity vector V into a rotational part, V , and a divergent part, V , i.e., We define the intensity of the Hadley and east/west circulations by the respective relations:velocity potentialThe Moisture FieldThe Sea Level Pressure FieldPrecipitation FieldQuestions ?Zonally Asymmetric Feat

19、ures ? The time-averaged east/west circulations are essentially divergent motions. velocity potential? Hadley and east/west circulations?Tropical Cyclone LifecycleOutlineTropical CyclogenesisEnvironmental ConditionsTheoriesCISKWISHEMCSTropical Depression - Storm - HurricaneDissipationCharacterizatio

20、n of TCs (Phase Space)Tropical Cyclogenesis“Transformation of a group of disorganized thunderstorms into a self-sustaining synoptic-scale vortex”Necessary (but not sufficient) conditions for genesis:Pre-existing convectionSignificant planetary vorticityFavorable wind shear patternMoist mid-troposphe

21、reWarm ocean, deep mixed layerConditionally unstable atmosphereTropical CyclogenesisPre-existing convectionSource for latent heat in atmosphereMust become concentrated in one area to draw in low-level airTropical CyclogenesisSignificant planetary vorticityConvection near the Equator results in littl

22、e if any rotation in the low-level inflowGenerally, systems must be at least 2-3 away from the Equator in order to have a chance to developTropical CyclogenesisFavorable wind shear patternWind shear is defined as the wind vector difference between the 850 and 200 mb level (arbitrary)High westerly sh

23、earLow easterly shearIn general, low values ( 20 kt) of vertical windshear are desired. Bad convectiontorn apartGood latentheat can concentrate inone areaSatellite derived vertical wind shear 1-August 2006Tropical StormChrisTropical CyclogenesisMoist mid-troposphereA dry troposphere encourages evapo

24、ration, which leads to cooling and suppression of further convective activityGOES-12 Water Vapor 1-August 2006Red Areas = DryGray/Blue Areas = MoistTropical CyclogenesisWarm Ocean and deep mixed layerSea surface temperatures (SSTs) must exceed 26.5C (as a rule)Allows for enough latent heat flux from

25、 ocean to maintain secondary circulation when energy is released near the TC core in the eyewallFlux Magnitudes/divs/mpo/About_MPO/Seminars/2006/0506_Desflots_Abstract.pdfObservationsModeled150010005000Importance of the Deep Mixed LayerMixed Layer Layer of the ocean from the surface to the depth whe

26、re water temperatures do not cool significantly with depth High winds churn (搅拌) up the ocean surface and mix up cooler water from belowDeeper mixed layers prevent much cooler water from surfacingCool surface water will weaken or kill the incipient systemMixed LayerTropical CyclogenesisConditionally

27、 Unstable AtmosphereLapse rate between the dry adiabatic lapse rate (DALR) and the saturated adiabatic lapse rate (SALR)Parcels become unstable when saturatedLevel of Free Convection (LFC)LFC reached by convective plumes in the tropical atmosphereParadox （讨论）: How can there be large-scale convection

28、 (leading to genesis) in the tropics if the LFC can only be reached by individual convective plumes, which would result in individual cumulonimbus clouds?Theories of Tropical CyclogenesisConditional Instability of the Second Kind (CISK)Theory developed by Charney and Eliassen (1964)3 basic assumptio

29、ns:Initial perturbation is a synoptic-scale waveFrictional convergence results in latent heat release above center of low-level cyclonic vorticityMagnitude of latent heat release proportional to Ekman pumpingJule CharneyCISKIn addition, atmosphere must be conditionally unstableFriction with surface

30、causes causes inflow intothe disturbance to be “deflected” inward towardthe surface center. Mass continuity dictatesupward motion must result. This process iscalled “Ekman Pumping”1Upward motion causes saturation and thuslatent heat release. If conditionally unstable,upward motion will continue and

31、enhance secondary circulation. Vortex will stretch, whichwill intensify low-level cyclonic vorticity (throughconservation of angular momentum)2LLHRCISKCharney and Eliassen showed that development took about 2.5 days over a 100 km region in their modelSimilar to tropical cyclogenesis time and space s

32、calesThough attractive and intuitive, CISK has been discredited in recent timesWind Induced Surface Heat Exchange (WISHE)Problems with CISK theory (Xu and Emanuel 1989)Tropical atmosphere generally not conditionally unstable, but rather near neutral to moist convectionLatent heating does not directl

33、y transfer to an increase in kinetic (运动) energy (e.g. secondary circulation)Offset (抵消，补偿) by adiabatic cooling and some radiative heat lossKerry EmanuelWISHEEkman pumping creates upwardmotion. This initiates downdrafts at some distance from updrafts. Troposphere moistened by stratiform precipitati

34、on hereb) Mid-level mesocyclone extends downto boundary layer. Warm ocean providesheat flux. As air moistens, dry air is cycledout and incoming flow remains moist to intensify secondary circulationc) Downdrafts disappear and intense convection erupts inthe unstable column. Surfacevortex intensifiesR

35、ole of Mesoscale Convective Vortices (MCVs)WISHE theory contains a mid- (and then low) level MCVMCVs have been theorized to play an important role in tropical cyclogenesisStretching of MCV to surface by downdraftsMerger of one or more MCVs to create a single, stronger MCV (Simpson et al. 1997)/gcg/A

36、tlas/atlas/IMG.26.jpegForecasting Tropical CyclogenesisDynamical modelsNot too good; many false alarmsProblems: Data initialization (little data over oceans), scale (too small), microphysics (not correctly represented)Statistical modelsPredict genesis based on larger-scale environmental forcingsHave

37、 shown some skill Forecasting Tropical Cyclogenesis/projects/gparm/data/current/xyrfpr.gifDeMaria et al.Forecasting Tropical CyclogenesisHennonTC Development StagesTropical disturbancePre-existing disturbanceTropical depressionTropical stormHurricane (Atlantic)DissipationTropical DepressionClosed ci

38、rculationMaximum sustained winds 73 mphStrength classified by Saffir-Simpson scale Also called “Severe tropical cyclone” (Australia), “Typhoon” (WPAC), “Very Severe Cyclonic Storm (India)”Hurricane Bonnie (1998)DissipationFactors:Center moves over landCirculation moves into or draws in dry airKatrin

39、a (2005), Chris (2006)Storm moves over cooler waterLower latent/sensible heat fluxStorm remains stationary for too longMixing up of cooler waterUnfavorable atmospheric conditionsIncrease vertical wind shearKatrina (2005)TS Chris (2006) VisTS Chris (2006) Water VaporExtratropical Transition (ET)As TC

40、s move poleward, they typically encounter cooler SSTsLimits oceanic heat fluxes, convection diminishesWarm core begins to dissipateFrontal features begin to emergeThis process is called extratropical transitionHart Phase SpaceA convenient method of classifying synoptic-scale storms as either warm co

41、re (“tropical”), cold core (“extratropical”), or a hybrid （混合物）combination (“subtropical”)Variables:900-600 mb thickness gradient across the cycloneWarm core has higher thickness over center900-600 mb vertical geopotential height gradientWarm core has a gradient that decreases with height600-300 mb

42、vertical geopotential height gradientDistinguishes between deep and shallow warm core systemsCyclone Parameter -VT: Thermal WindWarm-core example: Hurricane Floyd 14 Sep 1999Two layers of interest:-VTU 0-VTL 0Tropospheric warm core Cyclone Parameter -VT: Thermal WindCold-core example: Cleveland Supe

43、rbomb 26 Jan 1978-VTU 0-VTL 33 m s-1) are plotted in blue and red respectively. In all cases, data span the period 1949-1997; from Maloney and Hartmann (2000).The MJO has also been hypothesized to have broader atmospheric impacts: a Rossby wave train emanates （传出，表现出） into the Northern Hemisphere mi

44、dlatitudes from the Maritime Continent during passage of the active phase of the MJO through this region.This leads to MJO modulation of midlatitude weather, especially in the boreal （北方的）winter. For example, plumes of moisture (the “Pineapple Express”), flowing from MJO rainfall maxima over the cen

45、tral Pacific, lead to heavy precipitation and floods along the west coast of the United States and Canada (Fig. 5.9). In addition to the enhanced precipitation, winter cold air outbreaks over southern California and the southwestern deserts of North America are frequently timed with particular phase

46、s of the MJO. Summary of MJO impacts globally The impacts of the MJO on global weather patterns are summarized in Figs. 5.10a and 5.10b, which show the MJO impact on weather out to 13 weeks. Forecasting the MJO Forecasts of the MJO passage became available in the first few years of the 21st century.

47、These forecasts provide guidance weeks in advance on the local wind, pressure and convection anomalies to be expected at a given location.This information contributes to an increase in general predictability of tropical weather and gives insight into （洞察）the likely danger of tropical cyclone activit

48、y in regions with large MJO signatures; even where the convective signature is weak, the MJO may modulate wind shear and low-level vorticity. Observational analysis of theoretical tropical wave structures and the MJO using a combination of available satellite data and global model analyses identifie

49、d the evolution of the MJO structure through a complete cycle (Fig. 5.11). While the MJO can be seen in many fields without filtering, by filtering the data correctly, we can identify the MJO signal more easily. Fig. 5.11. Boreal winter (DJF) composite OLR and 850 hPa vector wind anomalies. Hatching

50、 (shading) levels denote OLR anomalies greater (less) than positive (negative) 7.5, 15, 22.5 and 30W m-2 respectively. Statistically significant vector wind anomaliesare plotted. The magnitude of the largest vector varies between panels and is shown on he bottom right of each panel. The phases label

51、ed here correspond to the phases pictured in Fig. 5.13; from Wheeler and Hendon (2004).To construct a forecast of a system with a fairly regular cycle, it is helpful to reference where you are in this cycle. For example, we refer to the “peak” and “trough” of a wave. Since the MJO has a complicated

52、spatial structure encompassing the global tropics, it is helpful to split the MJO cycle into more than two phases. A number of approaches have been used, but typically the cycle is split into either four or eight phases. One example using eight phases is plotted in Fig. 5.13. The Index: RMM1 and RMM

53、2Based on the first two Empirical Orthogonal Functions (EOFs) of the combined fields of near-equatorially-averaged 850 hPa zonal wind, 200 hPa zonal wind, and satellite-observed outgoing longwave radiation (OLR) data. Projection of the daily observed data onto such multiple-variable EOFs, with the a

54、nnual cycle and components of interannual variability removed, yields principal component (PC) time series that vary mostly on the intraseasonal time scale of the MJO only.This projection thus serves as an effective filter for the MJO without the need for time filtering, making the PC time series an

55、 effective index for real time use. We call the two PC time series that form the index the Real-time Multivariate MJO series 1 (RMM1), and RMM2. For the observations, the OLR data is that measured by the NOAA polar-orbitting satellites, while for the winds we use the NCEP/NCAR Reanalyses and the GAS

56、P analyses. The index is usually available in near real time about 12 hours after the end of each Greenwich day (i.e. at about 1200 UTC). For more details, see Wheeler and Hendon (2004). Approaches for Diagnosing the MJO Following Madden and Julians initial studies, MJO patterns have been identified

57、 using：lower and upper tropospheric winds, surface pressure, and proxies for deep convection (such as OLR, cloud top temperature, or precipitable water). Two approaches are frequently used for diagnosing the structure and evolution of the MJO: selective filtering of the data analysis of the data usi

58、ng Empirical Orthogonal Functions (EOFs). Filtering of the data to identify the MJO (or any other regular oscillation) requires first identifying spatial and temporal scales of the system.Two common choices for filtering are to identify the typical period of the oscillation, or its signature in wave

59、number-frequency space.Figure 5.6 has an example with this approach. While a number of MJO forecasts exist (Table 5.1), they all use the same basic approach of filtering observed data (including operational analyses and satellite data) based upon previously diagnosed structures for the different pha

60、ses of the MJO, then constructing a statistically based forecast of the MJO evolution. The differences between the forecasts come from the choice of input data and reference structures. Referring back to the phase diagram example in Fig. 5.11, we examine how RMM1 and RMM2 are constructed. Fig. 5.11T