One mechanism for tropical disturbance development over the South China Sea: Coupling of Lower-Upper Troposphere (CLUT) #

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1 One mechanism for tropical disturbance development over the South China Sea: Coupling of Lower-Upper Troposphere (CLUT) # WANG Lei * (State Key Laboratory of Tropical Oceanography, South China Sea Institute of Oceanology, Chinese Academy of Sciences, GuangZhou, ) Abstract: In this paper, one triggering mechanism (Coupling of Lower-Upper Troposphere (CLUT)) is proposed to explain tropical disturbance development and tropical cyclone formation over the South China Sea(SCS). The main contents of CLUT mechanism include: (1) Persistent large amount of latent heat release is the critical condition for tropical cyclogenesis and the amount of latent heat release determines one tropical disturbance to develop or not; (2) The Lower-Upper tropospheric coupling is necessary to generate large amount of latent heat release during tropical cyclogenesis; (3) The reasons why non-developing disturbances can not develop into tropical cyclones are due to the deficiency of lower-upper tropospheric coupling in dynamic conditions and/or in thermal conditions. Key words: tropical cyclone; tropical disturbance; South China Sea; cyclone genesis 0 Introduction How do tropical cyclones form? Atmospheric scientists have pondered and studied this question for many years, but the answers are still not complete. Although some aspects of the transformation of atmospheric disturbances into tropical cyclones are relatively well understood, the general problem of tropical cyclogenesis remains, in large measure, one of the great mysteries of the tropical atmosphere [1]. Both scientific and practical reasons continue to drive the search for answers. A tropical disturbance is a discrete tropical weather system with apparently organized convection (generally to 600 km in diameter) originating in the tropics or subtropics, having a non-frontal migrating character and maintaining its identity for 24 hours or more. Tropical disturbances are often the precursors of tropical cyclones. The different stages of development typically follow one another as tropical disturbance to tropical depression to tropical storm to typhoon/hurricane. Compared to the number of tropical disturbances, the number of formed tropical cyclones is much less due to the high percentage of the non-developing tropical disturbances. Therefore, a primary concern related to tropical disturbances is determining their potential for developing into tropical cyclones [2]. Although many theories proposed to try to explain the process of tropical cyclogenesis, no well-accepted and closed theory for tropical cyclone formation exists. Among these theories, the two most influential theories are Conditional Instability of the Second Kind (CISK) [3] and Wind Induced Surface Heat Exchange (WISHE) [4,5]. However, the relative rareness of tropical cyclone formation has not been easily explained by existing theories and has not been well understood. As Gray [6] once commented, It seems unlikely that the formation of tropical cyclones will be adequately understood until we more thoroughly document the physical differences between those systems which develop into tropical cyclones from those prominent tropical disturbances which have a favorable climatological and synoptic environment, look very much like they will develop Foundations: The specialized research fund for the doctoral program of Higher Education for Youths ( ); the Strategic Priority Research Program of the Chinese Academy of Sciences (No.XDA ); the National Natural Science Foundation of China (No ); the foundation for returned scholars of Ministry of Education of China Brief author introduction:wang lei(1979-),male,associate researcher,main research: Air-Sea interaction. wanglei.hkust@gmail.com - 1 -

2 45 50 but still do not develop. Our understanding of the detailed physical processes associated with the early stages of tropical cyclone formation is still inadequate and operational forecast skill is not very high. The study will follow this comment from Gray to examine why some tropical disturbances could develop, while other tropical disturbances never developed over the South China Sea (SCS). These studies will improve the understandings of the physical processes associated with the early stages of tropical cyclogenesis and the ability to do operational daily forecast for tropical cyclone formation. 1 Data The record of tropical disturbances over the SCS used in this study are from topical disturbance alert messages forwarded by the University of Illinois at Urbana-Champaign (UIUC) weather server as Tropical Storm and Hurricane WX (WX-TROPL) products ( The originator of these alert messages includes the various national weather services, including the Joint Typhoon Warning Center (JTWC), the Japan Meteorological Agency (JMA), the Hong Kong Observatory (HKO), and the National Hurricane Center (NHC). The information for global tropical disturbances and tropical cyclones are included in these products. For tropical disturbances, information about the positions, intensity estimates and potential for the development to tropical cyclones are provided. If a tropical disturbance intensifies and develops into a tropical depression later, this tropical disturbance is called developing tropical disturbance in this study. Tropical depressions are defined as weak tropical cyclones having maximum sustained surface winds of less than 18 m s -1, characteristically having one or more closed isobars. Tropical depressions must have a closed surface circulation in order to be classified in this category, while tropical disturbances may or may not be associated with detectable perturbations of wind fields. If a tropical disturbance does not intensify and later dissipates, this tropical disturbance is called non-developing tropical disturbance. Since 1987, a series of SSM/I instruments were launched. The SSM/I is a passive radiometer measuring the thermal emission of the earth and atmosphere at four frequencies (19, 22, 37, and 85 GHz). The propagation of the microwave radiation through the atmosphere is influenced by the integrated amounts of water vapor and liquid water in the atmospheric column [7,8]. Therefore, the brightness temperatures carry signals from all these geophysical parameters and can then be converted into geophysical parameters (e.g., columnar water vapor (CWV), columnar liquid water (CLW) and rain rate) using retrieval algorithms. The SSM/I data from three satellites (F13, F14 and F15) are used in this analysis. With a swath width of about 1400 km for each of the satellites, high-resolution coverage is now available almost globally on a daily basis. The SeaWinds scatterometer on the Quick Scatterometer (QuikSCAT) satellite is a microwave radar launched and operated by the U.S. National Aeronautics and Space Administration (NASA). Since July 1999, the QuikSCAT satellite scatterometer has provided spatially extensive measurements of near-surface wind speeds and directions over the world s ocean. With a 1600-km swath, QuikSCAT samples more than 90% of the global ocean every 24 hours. We use the wind speed and direction at a given location obtained from the radar backscatter measurements of sea surface roughness at 25 km resolution and multiple azimuth looks. The relative humidity and vertical velocity at 500 hpa, the divergence at and the vertical wind shear are from the National Center for the Environmental Prediction/National Center for Atmospheric Research (NCEP/NCAR) reanalysis data set [9]. The horizontal resolution of the - 2 -

3 90 95 reanalysis data is 2.5ºlatitude 2.5ºlongitude. The NCEP/NCAR Reanalysis project uses a state-of-the-art analysis/forecast system to perform data assimilation using past data. Six-hourly, daily, and monthly averages of the NCEP/NCAR reanalysis are available from the National Oceanic and Atmospheric Administration (NOAA) Climate Diagnostics Center (CDC) in Boulder, Colorado. The outgoing longwave radiation (OLR) is from the NOAA Interpolated OLR data set with 2.5ºlatitude 2.5ºlongitude resolution which is provided by CDC. 2 Results The SCS is one region where tropical disturbances and tropical cyclones occur frequently [10-19]. The environmental conditions due to synoptic-scale waves/disturbances prior to tropical cyclogenesis are examined for 36 tropical cyclones over the SCS from July 1999 to June 6. For the dynamic environmental conditions, strong low-level cyclonic vorticity, upper-level divergence and mid-tropospheric ascending motion occur before tropical cyclogenesis. For the thermodynamic environmental conditions, low columnar water vapor (CWV), columnar liquid water (CLW) and mid-tropospheric relative humidity increase significantly before tropical cyclone formation. The OLR decreases significantly before cyclogenesis, which represents lower temperatures and higher altitudes of cloud tops. The SST and vertical wind shear (VWS) do not change significantly before tropical cyclone formation, comparing with the climatological values. u v The divergence term ( ( f )( ) ) in the vorticity equation is calculated quantitatively x y based upon the QuikSCAT ocean surface wind data. The calculated mean divergence term is about 10.3 times the mean relative vorticity increase rate around the genesis position one day prior to tropical cyclone genesis, which shows the important contributions of the divergence term to the vorticity increase prior to tropical cyclone formation [20]. It is suggested that criteria related with the divergence and the divergence term be applied in early detections of tropical cyclogenesis using the QuikSCAT satellite data. 30 non-developing and 13 developing tropical disturbances over the SCS in 0 and 1 are compared using satellite and reanalysis data sets, in order to understand why some tropical disturbances developed into tropical cyclones, while others did not. Direct satellite observation from SSM/I reveal that persistent large amount of latent heat release is necessary for cyclogenesis, and the amount of latent heat release is one critical factor to determine one tropical disturbance to develop or not [21,22]. The amount of latent heat release is determined by five following factors: (1) SST; (2) the difference of divergence between and 850 ( D D850 ), which determines the strength of ascending motion; (3) 500 relative humidity; (4) 850 relative vorticity; and (5) OLR. One index for latent heat release combining these five factors is defined, and this index can represent the trend of the amount of total latent heat release. Among environmental factors examined, factors which are significant different in the means of parameters for non-developing and developing tropical disturbances with 95% confidence level include 500 vertical velocity, 850 divergence, D D850 and OLR. SST and divergence are significant different with 90% confidence level. These results suggest that the reasons why the amount of total latent heat release is low for non-developing tropical disturbances include: (1) insufficient convergent flow at 850 and divergent flow at, under which conditions the upward motion is not strong enough; (2) high OLR, which indicates higher - 3 -

4 cloud-top temperature; (3) cold SST. These three factors are main factors inhibiting the latent heat release and the development of non-developing tropical disturbances over the SCS. Based on these results, one triggering mechanism is proposed to explain tropical cyclone genesis----coupling of Lower-Upper Troposphere (CLUT). The main contents of CLUT mechanism include: (1) Persistent large amount of latent heat release is the critical condition for tropical cyclogenesis and the amount of latent heat release determines one tropical disturbance to develop or not; (2) The Lower-Upper tropospheric coupling is necessary to generate large amount of latent heat release during tropical cyclogenesis; (3) The reasons why non-developing disturbances cannot develop into tropical cyclones are due to the deficiency of lower-upper tropospheric coupling in dynamic conditions and/or in thermal conditions. The process of tropical cyclone genesis can be described as follows in the CLUT mechanism: Step 1: Vorticity increase due to the shear in monsoon trough generates closed cyclonic u v vorticity at the lower troposphere. The divergence term ( ( f )( ) ) in the vorticity x y equation is calculated quantitatively based upon the QuikSCAT ocean surface wind data. The calculated mean divergence term is about 10.3 times the mean relative vorticity increase rate around the tropical cyclone genesis position one-day prior to tropical cyclone formation, which shows the important contributions of the divergence term to the vorticity increase prior to tropical cyclone formation [12]. Step 2: Upper-level divergence associated with Tropical Upper Tropospheric trough (TUTT) occurs above the cyclonic circulation at the lower troposphere. Divergent flow is found to be strengthened before tropical cyclogenesis in 36 tropical cyclogenesis events examined in this study. Previous studies based on case studies also revealed the roles of upper-tropospheric synoptic systems in the initiation of the formation process [23-27]. Step 3: Strong upward motion occurs under the effects of both convergence at the lower troposphere and divergence at the upper troposphere. The magnitude of upward motion at 500 is found to be closely related with the magnitude of the difference of mean divergence between and 850, which reveals that strong upward motion during cyclogenesis is induced by strong convergent flows at the lower troposphere and divergent flows at the upper troposphere together [28]. Step 4: Water vapors are brought upward by strong ascending motion, and then are condensed to liquid water; large amount of latent heat is released. The amount of latent heat release is determined by five following factors: (1) SST; (2) D, which determines the strength of ascending motion; (3) 500 relative humidity; (4) 850 relative vorticity; and (5) OLR. Persistent large amount of latent heat release is the critical condition for tropical cyclogenesis. The amount of latent heat release determines one tropical disturbance to develop or not. Step 5: Cyclonic circulation is generated at the mid-troposphere due to the following two reasons: (1) Latent heat lease warms the air at the mid- troposphere and low pressure occurs. The balance of pressure gradient forces and Coriolis force generates cyclonic circulation. (2) Upward motion transports positive vorticity of lower troposphere upward to mid-troposphere, adding cyclonic vorticity at the mid-troposphere. The tropical disturbance develops into one tropical depression and tropical cyclogenesis occurs

5 Fig. 1 Process of tropical cyclogenesis in the CLUT mechanism. (a) Step 1: Vorticity increase due to the shear in monsoon trough generates closed cyclonic vorticity at the lower troposphere; (b) Step 2: Upper-level divergence associated with Tropical Upper Tropospheric trough (TUTT) occurs above the cyclonic circulation at the lower troposphere; (c) Step 3: Strong upward motion occurs under the effects of both convergence at the lower troposphere and divergence at the upper troposphere; (d) Step 4: Water vapors are brought upward by strong ascending motion, and then are condensed to liquid water and large amount of latent heat is released; (e) Step 5: Cyclonic circulation is generated at the mid-troposphere, and then the tropical disturbance develops into one tropical depression and tropical cyclogenesis occurs. 3 Conclusion 190 In this paper, one triggering mechanism, Coupling of Lower-Upper Troposphere (CLUT), is proposed to explain tropical disturbance development and tropical cyclone genesis over the SCS. The main contents of CLUT mechanism include: (1) Persistent large amount of latent heat release is the critical condition for tropical cyclogenesis. The amount of latent heat release determines one tropical disturbance to develop or - 5 -

6 not. The amount of latent heat release is determined by five following factors: (a) SST; (b) the difference of divergence between and 850 ( D D850 ), which determines the strength of ascending motion; (c) 500 relative humidity; (d) 850 relative vorticity; and (e) OLR. (2) The Lower-Upper tropospheric coupling is necessary to generate large amount of latent heat release during tropical cyclogenesis. For the dynamic condition, strong convergent flow at lower troposphere and divergent flow at upper troposphere need occur together. Strong upward motion is generated under the effects of both convergence at the lower troposphere and divergence at the upper troposphere, which is necessary for large amount of latent heat release. For the thermal condition, warm SST and cold top-cloud temperature (low OLR) are necessary thermal conditions for large amount of latent heat release, in which conditions large temperature difference between the lower troposphere and upper troposphere exist during cyclogenesis. (3) The reasons why non-developing disturbances can not develop into tropical cyclones are due to the deficiency of lower-upper tropospheric coupling in dynamic conditions (The convergent flow at lower troposphere and divergent flow at upper troposphere are insufficient for non-developing tropical disturbances, under which conditions the upward motion is not strong enough) and/or in thermal conditions (The top-cloud temperature is not cold enough or SST is not warm for non-developing tropical disturbances). The CLUT mechanism we propose here is one triggering mechanism for tropical cyclogenesis, focusing on the triggering environmental conditions for initial tropical cyclone formation from tropical disturbances to tropical cyclones. In CISK and WISHE theory, they implicitly assume that a transformation from a tropical disturbance to a finite amplitude surface concentrated rotary system has already taken place; and how a weak-amplitude tropical disturbance is transformed into a surface closed vortex is not well solved. The CISK and WISHE theories are more suitable for the development and intensification of tropical cyclones, including positive feedback processes capable of sustaining development. Another difference between the CLUT mechanism with previous CISK and WISHE theory is the process which governing the latent heat during cyclogenesis. In CLUT mechanism, the amount of latent heat release is determined by the lower-upper tropospheric coupling, while it is governed by the frictional convergence in CISK theory and by the surface heat flux in WISHE theory. Combining the CLUT mechanism with CISK or WISHE theory together may provide one more complete and closed explanation for the whole lifecycle of tropical cyclones. The CLUT mechanism we proposed in this study is mainly based on observational analysis. The CLUT mechanism is planned to be tested in the numerical models in the future. To show the effects of upper tropospheric circulation, one numerical experiment in which the wave patterns at the upper troposphere are removed is planned to be done. The simulation results need to be examined further in the future to see whether the process of tropical cyclogenesis proposed in the CLUT mechanism occurs or not in the model results. References 235 [1] Emanuel, K.A.Divine wind: the history and science of hurricanes [M]. New York:Oxford University Press,5. [2] Simpson R.H., N. Frank, D. Shideler, and H.M. Johnson.Atlantic tropical disturbances,1967[j].mon. Wea. Rev.,1968, 96: [3] Charney, J.G., and A. Eliassen.On the growth of the hurricane depression [J]. J. Atmos. Sci.,1964,21: [4] Emanuel, K.A. An air-sea interaction theory for tropical cyclones. Part I: Steady-state maintenance[j]. J. Atmos. Sci.,1986,43:

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