What is the Trigger for Tropical Cyclogenesis? Yan LIU Nolan, D. S., 2007: What is the trigger for tropical cyclogenesis? Aust. Meter. Mag., 56, 241-266. Professor and Chair, Department of Atmospheric Sciences RSMAS/MPO University of Miami dnolan@rsmas.miami.edu

Genesis The Greek Γένεσις (origin, birth); the Hebrewבְּרֵאשִׁית (in the beginning). The same stock as words gene, generation, genealogy. A 'beginning' in the Bible is usually some point on a complexity scale that is preceded by a period of convergence of certain pre- beginning elements, and succeeded by a period of divergence of post-beginning elements.

Tropical Cyclone Larry (2006) 0000 UTC 16 March first monitored (25 knot) 1200 UTC 18 March hurricane intensity (65 knot) 1200 UTC 19 March peak intensity (110 knot) The inability to accurately forecast TC genesis can be a critical barrier to achieving sufficiently long warning times for landfalling cyclones

Understanding Tropical Cyclogenesis deep moist convective updraughts overcoming the inhibition caused by the static stability (Malkus & Riehl 1960; Riehl & Malkus 1961; Gray 1968) the feedback between deep convection and the surrounding large-scale motions (Charney & Eliassen 1964; Ooyama 1964) a finite-amplitude instability: a pre-existing disturbance of some strength is required for genesis (McBride 1981; Frank 1987; Rotunno & Emanuel 1987; Emanuel 1989) the merger and/or axisymmetrisation of pre-existing vortices (Ritchie & Holland 1997; Hendricks et al. 2004; Montgomery et al. 2006) the moist thermodynamics of the precursor, mesoscale vortex (Bister & Emanuel 1997)

Model Design and Motivation WRF (Version 2.1.1) Domain ID 1 2 ΔX（km） 6 Grid Number 240*240 180*180 Radiation / MCP Microphysics WSM6 PBL YSU PBL What’s the trigger? SST fixed at 29°C, no mean wind or wind shear the similar resolution as Hendricks et al. (2004) and Montgomery et al. (2006) but focus on broader precipitation and embedded convection similar evolution as Bister & Emanuel (1997) 前人模拟也有类似Genesis的延迟，但没有如此剧烈的突然增强 高分辨率模式使得可以分析细微对流结构发展

Initial Conditions 涡度为半径的高斯函数，R使边界风速为0 初始涡旋为平衡态，加入小扰动后，前12小时为不稳定发展

Zonal Velocity & Relative Humidity 南北向剖面，地面最大风速减弱，最大风速在1.5km，中上层依然较干 最大风速增强、上升、加深，无收缩

Inner-Core Relative Humidity 2 km 5 km 8 km 40h中高层湿度达到80%，至此涡旋开始增强抬升（两者先后不一定） Inner-core relative humidity averaged over a 160 km x 160 km box around the vortex centre, on model levels of z = 2, 5 and 8 km.

Azimuthal Wind 48 h 54 h 60 h 66 h 78 h 96 h

Mean Inner-Core Variables（36-72 h） RH Vorticity Moist Heating 热成风，cold-core 加热中心与上升速度中心重合，下方产生正涡度 cold-core θ Perturbation W Divergence

CFADs for W, LH, ζ W LH ζ 24 h W LH ζ 60 h W与LH的pattern类似，暖上升冷下沉，极值中心 对流加热发展加深，冷下沉变化较少 60 h

A Mid-level Vortex Initial Condition 10 hours later 风廓线与地面涡旋相同，最大值高度升高 72 h

Mean Inner-Core Variables（42-78 h） RH 12 hours later Vorticity Moist Heating 中层涡度发展甚至早于RH的发展 θ Perturbation W Divergence

Deeper Surface Vortex Cases 12 hours earlier 6 hours earlier

Shallower Surface Vortex Cases 36 hours later 高层加湿并没有延迟太晚，RH的脉冲 中层涡旋增强则延迟较多，Genesis延迟可能与此有关

Changes in Upper-Level Humidity +6 +34 saturated (90% RH) from 0 to 12 km dry (5% RH) above 7 km dry (5% RH) above 4 km RH脉冲

Mid-Level Vortex with Very Weak Surface Winds Vsurf =1m/s RH=90%

artifact Convective Pulse total condensate W 并没有大范围上升，仍是零星的 高湿度，双周期边界条件，小的domain diabatic heating wind vectors 54h, 6.5km 61h, 6.5km

CFAD Diagrams of Vertical Vorticity 70 h mid-level vortex case the rapid appearance of much higher vorticity values at low levels 72 h 74 h

Vertical Vorticity & Moist Heating

Low-Level Vorticity & Mid-Level Diabatic Heating the trigger 0.5 km 70 h 71 h 72 h 涡度中心与加热中心基本一致 涡度中心与TC中心重合，Genesis触发（加热在中心效率最高） 5.0 km

Summary TC genesis was marked by the very rapid appearance of a small-scale, low-level vortex near the centre of the surrounding mesoscale vortex. Genesis did not occur until the inner core had achieved deep near-saturation and the mid-level vortex had elevated, contracted, and intensified. The smaller-scale, surface vortex was created by a single, long-lived updraught that erupts very near the centre of the larger mid-level vortex, which is the trigger for tropical cyclogenesis.

Kinetic Energy Efficiency 36 h 60 h surface vortex 0 ≤ r ≤ 40 km, 4 km ≤ z ≤ 10 km surface surface relevant genesis KEE relevant KEE mid-level mid-level

Initial Conditions