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Global view of the upper level outflow patterns associated with tropical cyclone intensity changes during FGGE /
Abstract
The characteristics of the upper tropospheric outflow patterns which occur with tropical cyclone intensification and weakening over all of the global tropical cyclone basins during the year long period of the First GARP Global Experiment (FGGE) are discussed. By intensification is meant the change in the tropical cyclone's maximum wind or central pressure, not the change of the cyclone's outer 1 to 3 deg radius mean wind which we classify as cyclone strength. All the 80 tropical cyclones which existed during the FGGE year are studied. Two-hundred mb wind fields are derived from the analysis of the European Center for Medium Range Weather Forecasting (ECMWF) which makes extensive use of upper tropospheric satellite and aircraft winds. Corresponding satellite cloud pictures from the polar orbiting U.S. Defense Meteorological Satellite Program (DMSP) and other supplementary polar and geostationary satellite data are also used.
... This asymmetry in the outflow layer was recognized based on several early TC cases (e.g., Black & Anthes, 1971) and applied as a feature in outflow layer classification. For example, Chen and Gray (1985) identified four primary configurations of channelized outflow from 80 intensifying TC cases, while Li et al. (2017) classified 259 cases of TC rapid intensification (RI) over the western North Pacific (WNP) into five outflow patterns. ...
... Intensifying TCs are associated with "open" outflow patterns, whereas non-intensifying TCs are associated with "closed" outflow patterns (Merrill, 1988a, b). The double-channel outflow or single equatorward outflow in the upper levels is favorable for a high TC intensification rate (Chen & Gray, 1985), whereas Lyu et al. (2019) found that RI occurs more frequently in TCs with north-facing outflow channels. Moreover, TC intensification is associated with multiple outflow channels, and TCs usually weaken when one of the effective upper-level outflow channels is cut off (e.g., Merrill & Veldon, 1996;Sadler, 1976Sadler, , 1978Ventham & Wang, 2007). ...
... According to previous studies, the approximate height of the TC outflow layer is located at 200 hPa (Chen & Gray, 1985;Doyle et al., 2017). Here, statistical results also show that the azimuthal-averaged tangential wind weakens to zero and reverses near a 5° radius while the radial wind stabilizes near a 5° radius (not shown). ...
... Interestingly two other TCs were also 15 active in the near vicinity which the authors claim to have affected the rain event. For the rain 16 event considered here, Fitow also did not experience ET; TC Danas was located to its east; 17 and there is evidence presented in Section 7 that Danas may have acted as a source of 18 moisture and potential vorticity for the rain systems. The case of TC Fitow thus bears some 19 similarities to these previous studies, but contain some important differences and additions 20 which we will discuss. ...
... In addition, if part of the cold air wrapped into the 14 typhoon's inner core, the inner convection would be suppressed. Similarities between the 15 study here and previous work is that the heavy rain event seems to have been a result of an 16 interaction, whatever that might be, between Fitow and an evolving and favourable 17 environmental flow, with: (i) an upper-level jet entrance region very near the rain area; (ii) a 18 tropical storm to the east of Fitow; and (iii) coastal frontogenesis with its associated 19 secondary circulation. In addition we will discuss the role that Fitow played in the event, 20 even though the rain occurred well to the north of its circulation center and it was dissipating 21 as the rain developed. ...
... Wind shear can be readily 14 inferred from this diagram. Comparison between observed and forecast environmental winds 15 that were directly influencing the Fitow circulation, suggests that ACCESS-TC was 16 forecasting the near-environment of the storm quite well, although the turning of the winds 17 with height is confined to a shallower layer in the forecast. An interesting aspect in the 18 figures is the trend towards southwesterly wind shear just prior to and during landfall. ...
Typhoon Fitow made landfall south of Shanghai, China, on 6 October 2013. During the following two days, precipitation in excess of 300 mm day−1 occurred 400 km to the north of the typhoon center. The rain-producing systems included (i) outward-spiraling rainbands, which developed in the storm’s north sector in favorable environmental wind shear, and (ii) frontal cloud as a result of coastal frontogenesis.
Over the rain area, in addition to enhanced ascent, there were increases in low-level moisture, convective instability, and midlevel relative vorticity. There is evidence of a preconditioning period prior to the rain when midlevel subsidence and boundary layer moistening occurred. From analysis of low-level equivalent potential temperature the following observations were made: (i) after landfall, a cold, dry airstream wrapped into Fitow’s circulation from the north, limiting the inner-core rainfall and producing a cold-air boundary, and (ii) an extended warm, moist airstream from the east converged with the cold-air intrusion over the rain area.
The heavy rain occurred as the large-scale flow reorganized. Major anticyclones developed over China and the North Pacific. At upper levels, a large-amplitude trough relocated over central China with the entrance to a southwesterly jet positioned near Shanghai. Back trajectories from the rain area indicate that four environmental interactions developed: (i) increasing midlevel injection of moist potential vorticity (PV) from Fitow’s circulation; (ii) low-level warm, moist inflow from the east; (iii) midlevel inflow from nearby Typhoon Danas; and (iv) decreasing mid- to upper-level injection of PV from the midlatitude trough. The authors propose that the resultant PV structure change provided a very favorable environment for the development of rain systems.
... What was surprising was that the neutral pattern was also a good indicator of rapid storm growth. Chen and Gray (1985) attribute this to upper-level winds that are not in contact with the storm's outflow. Therefore, the shearing effect is removed, allowing the cyclone to remove its excess mass and heat without impediment. ...
... The strongly divergent, neutral outflow pattern is easily discernible by the lateral cirrus banding around the storm's circumference. Poorly divergent storms have ragged edges and develop very slowly (Chen and Gray, 1985). Fig. 9 shows a 33 kt/6 hour change, whereas Fig. 10 shows a 27 kt/24 hour change). ...
... These differences stem from the way the tests were conducted. Chen and Gray (1985) (Atkinson and Holiday, 1977), the surface winds can be derived from this pressure reading (see Table 4 for the equation). This was the method used to determine the "calculated wind" in Table 4 Another forecasting aid is to expect the explosive development to begin 24-48 hours after tropical storm strength is attained. ...
... Without aircraft, this method of intensity forecasting is no longer used in the NWPAC. Chen and Gray (1985) and Merrill (1988) tied intensity changes to environmental outflow patterns but without any specific intensity forecast scheme. A statistical intensity forecast scheme developed for the Atlantic by Merrill (1987) was unable to demonstrate forecast skill over the SHIFOR climatology and persistence forecast model by Jarvinen and Neumann (1979). ...
... Assuming environmental conditions of low vertical wind shear near the cyclone center, the vertical updrafts in the inner core region are presumed to transport cyclonic to upward momentum transport (Chen and Gray, 1985). 107 A concentration of tangential wind shear at the top of the troposphere has important thermal consequences. ...
... The most widely accepted physical explanation for rapid intensity change has been the existence of strong asymmetrical (single or multiple) outflow channels (Sadler. 19T: Holliday and Thompson, 1979;Holland and Merrill, 1984;Chen and Gray, 1985;Merrill, 1988). A double outflow channel exporting heat and mass in opposite directions away from the upper levels of the intensifying tropical cyclone is generally considered to be the most effective way to maintain upper level buoyant instability necessary for continued deep convection. ...
... The sensitivity of TC structure and intensity to upper-level environmental forcing is generally accepted due to low inertial stability aloft within a TC vortex (Holland and Merrill 1984;Chen and Zhang 2013). The provision of ''dual-channel outflow,'' terminology introduced in Chen and Gray (1985), was supposed to be more favorable for RI occurrence due to the enhanced upper-level divergent outflow. Note that the outflow channel or outflow jet requires the principal outflow streamline crosses 500-km radius with an outward component and has a wind speed maximum within 1500 km of the hurricane center (Merrill 1988). ...
... In addition, two outflow channels exist, one northeastward to the front of the westerly trough and one southwestward, giving rise to a strong divergent circulation pattern in the upper troposphere. This upper-level flow pattern is the so-called dualchannel outflow pattern defined by Chen and Gray (1985). It is more favorable for TC intensification than a single outflow channel or no outflow pattern. ...
... It is more favorable for TC intensification than a single outflow channel or no outflow pattern. Note that the southwestward outflow channel weakens at 124 h (Fig. 5f) when the TC moves close to the ridge with the outflow channel toward the northeast remaining, leading to a slowdown of the intensification rate (Chen and Gray 1985;Merrill 1988). The final termination of intensification of Andy is attributed to its landfall over South China. ...
The typical synoptic flow patterns and environmental factors that favor the rapid intensification (RI) of tropical cyclones (TCs) in the South China Sea (SCS) have been identified based on all TCs formed in the SCS between 1981 and 2011. The quantity RI is defined as the 24-h increase in maximum sustained surface wind speed by 15 m s−1 as in previous studies, which is close to the 95th percentile of 24-h intensity change of all SCS samples excluding those after landfall. There are 4.9% (2.3%) of tropical depressions (tropical storms) that experienced RI. No typhoons satisfied the RI threshold.
Six low-level synoptic flow patterns favoring RI have been identified based on 18 RI cases. In the monsoon season very few TCs experience RI due to large vertical wind shear (VWS). Most RI cases occurred in the postmonsoon season when the midlatitude troughs often penetrated into the SCS whereas the southwesterly monsoon flow is still strong in the southern SCS. Compared with those of non-RI cases, the mean initial conditions of RI cases include weak VWS and relatively strong forcing from midlatitude troughs. Several criteria of significant environmental factors for RI are statistically identified based on all TC samples. It is found that 16 non-RI TCs fitted in the RI flow patterns but only two of them satisfy all the criteria, suggesting that a combination of the synoptic flow pattern and the environmental factors can be used to predict RI in the SCS. In addition, two RI cases involving TC–trough interaction are analyzed.
... Still, compared to the understanding of midlatitude weather systems, most of the structure and behavior of tropical cyclones are still not fully understood, mainly because of the insufficient observational data over the vast tropical ocean. Past studies (Black and Anthes 1971; Sadler 1976 Sadler , 1978; Frank 1977b; Tuleya and Kurihara 1981; Pfeffer and Challa 1981; Merrill 1984; Holland and Merrill 1984; Chen and Gray 1984; Ooyama 1987; Merrill, 1988a,b; Molinari and Vollaro 1989, 1990; Kaplan and Franklin, 1991; Shi et al. 1990; Rodgers et al. 1991; DeMaria et al. 1993 ) have demonstrated that the movement and growth of tropical cyclones are affected by the interaction between the outflow layer of tropical cyclones and its environmental flows. For example, Holland and Merrill (1984) pointed out that the strong cyclonic vorticity in the lower and middle troposphere in an intense tropical cyclone becomes more stable and likely more resistant to envi-ronmental forcing, whereas the upper-tropospheric anticyclonic , asymmetric outflow is not as stable inertially (Black and Anthes 1971), and therefore is less resistant to the environmental forcing. ...
... Even when some observational data are available, the studies still suffer from not having frequent enough data to construct a chain of events or to establish a cause–effect relationship. Much of our knowledge has come from composite studies (Black and Anthes 1971; Sadler 1976 Sadler , 1978 McBride 1981; Chen and Gray 1984; Merrill 1988a,b; DeMaria et al. 1993 ), a few case studies (Molinari and Vollaro 1989 Vollaro , 1990; Kaplan and Rodgers et al. 1991), or some idealized model studies (Tuleya and Kurihara 1981; Pfeffer and Challa 1981; Ooyama 1987; Shi et. al 1990). ...
... The outflow layer of the tropical cyclone is comparatively shallow, generally confined between 100 and 300 mb (Frank 1977a ). The asymmetric outflow of the tropical cyclone is characterized by outflow jets. Chen and Gray (1984) asserted that the outflow jets help to remove mass from the central region and transport the warm and dry air to outer regions, maintaining the convective instability in the inner core region. Merrill (1984) observed that the outflow from a hurricane concentrates into one or two outflow maxima or channels. The wind maximum in the outflo ...
), there was a thermally direct, circum-jet secondary circulation in the jet entrance region and a thermally indirect one in a reversed direction in the jet exit region. In several previous studies, it was postulated that an approaching westerly jet had modulated the convection and intensity variations of Florence. In a variational numerical experiment in this study, the approaching westerly jet was flattened out by repeatedly setting the jet-level meridional wind component and zonal temperature perturbations to zero in the normal mode initialization procedure. Compared with the control experiment, the variational experiment showed that the sudden burst of Florence's inner core convection was highly correlated with the approaching upper-tropospheric westerly jet. These experiments also suggested that the approaching upper-tropospheric westerly jet was crucial to the intensification of Florence's inner core convection between 1000 and 1500 UTC 9 September, which occurred prior to the deepening of the minimum sea level pressure (from 997 to 987 mb) between 1200 UTC 9 September and 0000 UTC 10 September.Many earlier studies have attempted an explanation for the effect on tropical cyclones of upper-tropospheric forcings from the eddy angular momentum approach. The result of this study provides an alternative but complementary mechanism of the interaction between an upper-level westerly trough and a tropical cyclone.
... However, a trough is not a necessary ingredient for an upper level environmental interaction. Any feature or flow regime, such as anticyclones, that enhances the upper level outflow without producing strong vertical shear and lower to middle level drying can be associated with intensification 38 . ...
... The overall outflow associated with a TC has been shown to be strongly influenced by mid-latitude troughs or tropical uppertropospheric troughs (TUTT) if positioned favorably [39][40][41] . In an analysis of 200 hPa composite patterns compiled from data collected during the First GARP Global Experiment (FGGE) six patterns most likely to intensify a TC were identified 38 . The most frequently observed pattern was an eastward moving mid-level trough on the poleward and westward side of the TC, enhancing poleward outflow 38 . ...
... In an analysis of 200 hPa composite patterns compiled from data collected during the First GARP Global Experiment (FGGE) six patterns most likely to intensify a TC were identified 38 . The most frequently observed pattern was an eastward moving mid-level trough on the poleward and westward side of the TC, enhancing poleward outflow 38 . Another pattern identified was the TC located at the tip or in the rear of a TUTT, enhancing a single outflow channel 38 . ...
Tropical Cyclone (TC) track forecast skill has shown a steadily increasing trend in the north Atlantic basin over the last decade in contrast to little or no improvement in intensity forecast skill. This is attributable in part to a lack of in-situ observations to measure important inner-core processes and the inability of current operational numerical models to accurately resolve the inner-core dynamics. Consequently, much is unknown about TC intensity change, and the most skillful intensity forecasting techniques still rely upon climatology and persistence. The forecasting of rapid changes in intensity has been particularly difficult. The need for improved TC intensity forecasts has never been greater due to rapidly increasing population in coastal communities. This is the motivation for the present review, which seeks to discuss our current knowledge and highlight the most fruitful areas for future work. This is accomplished through a literature review of past and present research with emphasis on current gaps in knowledge.
... Elsberry et al. (Section 2.0 [13]) provide a detailed description of the development of our ECEPS-based formation, track, and intensity forecasts for western North Pacific TCs as utilized in the pre-operational test based on the Marchok (2021) vortex tracker files [19]. In contrast to the unweighted ensemble models mentioned above, our weighted mean vector motion (WMVM) track forecast gives the greatest (smallest) weight to the ensemble member tracks that have 12 h vector motions that most (least) closely resemble the previous Atmosphere 2025, 16 12 h WMVM vector, and thus tend to "go down the middle" of the cluster of member tracks. ...
... Interestingly, the only extended outer rainband is in the northwest quadrant and is oriented southwestto-northeast. This establishes dual outflow channels to the north and toward the equator, which may be an indicator of a forthcoming rapid intensification (Chan and Gray 1985) [19]. ...
A pre-operational test started in mid-July 2024 to demonstrate the capability of the ECMWF’s ensemble (ECEPS) to predict western North Pacific Tropical Cyclones (TCs) lifecycle tracks and intensities revealed new forecasting challenges for four typhoons that started well south of 20° N. As Typhoon Gaemi (05 W) was moving poleward into an unfavorable environment north of 20° N, a sharp westward turn to cross Taiwan was a challenge to forecast. The pre-Yagi (12 W) westward turn across Luzon Island, re-formation, and then extremely rapid intensification prior to striking Hainan Island were challenges to forecast. The slow intensification of Bebinca (14 W) after moving poleward across 20° N into an unfavorable environment was better forecast by the ECEPS than by the Joint Typhoon Warning Center (JTWC), which consistently over-predicted the intensification. An early westward turn south of 20° N by Kong-Rey (23 W) leading to a long westward path along 17° N and then a poleward turn to strike Taiwan were all track forecasting challenges. Four-dimensional COAMPS-TC Dynamic Initialization analyses utilizing high-density Himawari-9 atmospheric motion vectors are proposed to better define the TC intensities, vortex structure, and unfavorable environment for diagnostic studies and as initial conditions for regional model predictions. In Part 2 study of selected 2024 season TCs that started north of 20° N, more challenging track forecasts and slow intensification rates over an unfavorable TC environment will be documented.
... There are several explanations in the literature as to how the outflow-layer dynamics affect TC development. For example, the outflow may remove the inner-core mass or warm air and thus maintain the inner-core convective instability (Holland and Merrill 1984;Shi et al. 1990;Chen and Gray 1985;Wu and Cheng 1999) or push back against the environmental winds and thus reduce the local vertical wind shear (Xu and Wang 2013;Ryglicki et al. 2019;Dai et al. 2021;Shi and Chen 2021). In addition, the eddy momentum flux in the outflow layer may enhance TC's secondary circulation by a deep balanced response and thus favors the eyewall convection and TC intensification (Challa and Pfeffer 1980;Holland and Merrill 1984;Wu and Cheng 1999;Chen et al. 2015;Ditchek et al. 2017). ...
... Finally, we should point out that although the outflow-layer structure is generally quasi axisymmetric in the idealized three-dimensional simulations with quiescent environment (Rappin et al. 2011), the outflow-layer structure of TCs in reality is often highly asymmetric due to the TC-environment interactions (Black and Anthes 1971;Chen and Gray 1985). Previous studies have shown that the upper-tropospheric troughs or cold-core vortices may facilitate TC intensification by providing asymmetric outflow channels and thus eddy FIG. 10. ...
This study revisits the issue of why tropical cyclones (TCs) develop more rapidly at lower latitudes, using ensemble axisymmetric numerical simulations and energy diagnostics based on the isentropic analysis, with the focus on the relative importance of the outflow-layer and boundary-layer inertial stabilities to TC intensification and energy cycle. Results show that although lowering the outflow-layer Coriolis parameter and thus inertial stability can slightly strengthen the outflow, it does not affect the simulated TC development, whereas lowering the boundary-layer Coriolis parameter largely enhances the secondary circulation and TC intensification as in the experiment with a reduced Coriolis parameter throughout the model atmosphere. This suggests that TC outflow is more likely a passive result of the convergent inflow in the boundary layer and convective updraft in the eyewall.
The boundary-layer inertial stability is found to control the convergent inflow in the boundary layer and depth of convection in the eyewall and thus the temperature of energy sink in the TC heat engine, which determines the efficiency and overall mechanical output of heat engine and thus TC intensification. It is also shown that the hypothesized isothermal and adiabatic compression legs at the downstream end of the outflow in the classical Carnot cycle is not supported in the thermodynamic cycle of the simulated TCs, implying that the assumed TC Carnot cycle is not closed. It is the theoretical maximum work of heat engine, not the energy expenditure following the outflow downstream, that determines the mechanical work used to intensify a TC.
... Due to the asymmetric structure [36,37], low Richardson number [38,39], and weaker inertial instability [40][41][42][43] of the TC upper-level outflow, the outflow can easily interact with the upper-level larger-scale environment and the inner-core of a TC, thereby playing a mediating role between the environment and the storm core [41]. As such, it can affect the secondary circulation and, consequently, the TC intensity [37,41,[43][44][45][46][47][48][49]. e upper-level radial outflow is much stronger [6,37,43] and usually is more concentrated close to the TC center for intensifying cases than for nonintensifying cases [43]. ...
... e upper-level radial outflow is much stronger [6,37,43] and usually is more concentrated close to the TC center for intensifying cases than for nonintensifying cases [43]. In addition, TC intensification is associated with the multiple upper-level outflow channels, and TCs usually weaken when one of the efficient upperlevel outflow channels is cut off [44,[50][51][52][53]. Kaplan and DeMaria [5] and Shu et al. [14] noted that upper-tropospheric flows for RI cases are more easterly than for non-RI cases. ...
The Northwest Pacific tropical cyclone (TC) intensification is classified into rapid intensification (RI), normal intensification (NI), and slow intensification (SI) categories. The initial location and intensity, the preceding intensity change, the motion direction, the occurrence month, and the intensification duration time are all found to differ for RI cases compared with NI and SI cases. The dependence of RI, NI, and SI on environmental conditions is further examined statistically by using the intensification rates of named TCs, for the 21-year period 1995–2015, obtained from JTWC best track data, and the environmental conditions derived from the ERA-Interim reanalysis data and GODAS high-resolution global ocean analysis data. It was found that deep-layer and upper-mid vertical wind shear (VWS), upper-level outflow, sea surface temperature (SST), and ocean heat content (OHC) are statistically different among RI, NI, and SI both before and during intensification. RI is enhanced by weaker and decreasing VWS, warmer oceans, and stronger and increasing outflow. In contrast, SI typically occurs with larger and increasing VWS, cooler oceans, and weaker, static outflow. The impacts of low-level VWS and net moisture inflow are only significantly different between RI and SI and between NI and SI, but not between RI and NI. Another key finding is that increased upper-level outflow and decreased VWS are important precursors and hence are possible predictors, of RI onset. The direction of upper-level outflow affects TC intensification, with NW and NE outflow being more favorable for TC RI than SE and SW outflow.
... On the other hand, another larger, synoptic scale anticyclone is often found that pre-existed within the vicinity of the intensifying TC. The location of this larger anticyclone can vary depending upon many environmental factors, including the lowerlevel forcing mechanisms that are helping to create the cyclone.Chen and Gray, 1985) 5.1.2 Dual-channel Outflow. Tropical cyclones with a dual-channel outflow pattern (Fig. 6.5) generally intensify at an average maximum rate of 35 kt/6 hr. ...
... Anomalously high SST can cause more heat and moisture flux from the ocean to the atmosphere. This condition favors further development of the TC (Holliday and Thompson, 1979; Merrill, 1987).Table 6Chen and Gray, 1985) 3 NOVEMBER OOZ (6) Land, coast, and mountain effects. These effects can be quite complex (Merrill, 1987). ...
One of the keys to safe and successful naval operations in the tropics is an understanding of tropical meteorology. The Tropical Cyclone Forecasters Reference Guide is designed primarily as a ready reference for weather forecasters required to provide tropical meteorology support to staff commanders. This report provides an overview of tropical cyclone intensity forecast support and is Chapter 6 of the reference guide. Subjects discussed include tropical cyclone intensity forecast procedures, major warning center operations, and tropical cyclone intensity terminology.
... Ez a korábbi fejlődési szakaszban átmenetileg kapcsolatba került vele, de később eltávolodott tőle, ugyanakkor idővel a ciklonközpontban is megnövekedett kissé az örvényesség mértéke, mely az egyre magasabb szinteken és egyre jobban meleggé váló magra utalhatott. A többi ciklonnal ellentétben határozott szétáramlás alakult ki a ciklon felett a magasban, különösen annak északi és nyugati oldalán, ahol két, a trópusi ciklonok esetében jellegzetes és általában kedvező hatású pólusirányú szétáramlási csatorna is létrejött, tovább erősítve a divergenciát (Chen & Gray, 1985). A geopotenciál mezőben is határozott különbség mutatkozott meg, ugyanis jóval szimmetrikusabb és egyben kompaktabb volt e paraméteren a ciklon megjelenése, mint az (1)es és (2)-es esetben, amikor a mérsékelt övi előzmény miatt nagyobb területre kiterjedtek az alacsonyabb értékek. ...
... Compared with the middle and lower layers, outflow layers are more asymmetric (Black and Anthes, 1971). Chen and Gray (1985) proposed three outflow patterns for intensifying TCs, namely single channel, double channel and no channel, and the change of outflow patterns is closely related to the uppertropospheric environmental flow (Merrill, 1988a, b). Recently, with the improvement of detection methods (Komaromi and Doyle, 2017;Ohigashi et al., 2020;Ohigashi et al., 2021) and numerical simulation techniques (Dai et al., 2017;Montgomery et al., 2019), many advances have been achieved in the studies on the characteristics of TC outflow layers and their relationships with TC intensity variation. ...
In this study, we investigate the structural characteristics of the upper-level outflow and its impact on the rapid intensification (RI) of Typhoon Roke (2011), which experienced an evident outflow transformation from equatorward to poleward during its RI period. The simulations by the Weather Research and Forecasting Model suggest that the upper-level outflow extends from 100 hPa to 150 hPa, with an upper-level warm core at around 150 hPa. The upper-level outflow is enhanced ahead of the typhoon intensification, which is closely related to the outflow-environment interaction. Further analyses indicate that at the early stage of Roke (2011) before the RI, the strong equatorward outflow and the updraft south of the typhoon center are enhanced, favoring the onset of RI. During the RI period, the strong divergent flow near the entrance of the southwesterly jet in front of the upper-level trough, induces the poleward outflow. The eddy flux convergence of angular momentum inward propagated to the typhoon center from a 1000-km radius further enhances the poleward outflow and leads to the development of the vertical motion north of the typhoon center. Then Roke (2011) intensifies rapidly. Simultaneously, the shallow weak positive potential vorticity (PV) anomaly south of the southwesterly jet increases the inner-core PV, favoring the sustained intensification of Roke (2011). After Roke (2011) reaches its peak intensity, its intensity decreases due to the increase of vertical wind shear and the approaching of the southwesterly jet. It is indicated that the interaction between the upper-level outflow and the upper-tropospheric trough has significant influence on the RI of TC.
... Environmental factors favorable for RI have been revealed, including weaker vertical wind shear (VWS), warmer sea surface temperature (SST), higher oceanic heat content, higher relative humidity in the low-to mid-troposphere, and the external forcing from upper-level systems (Gray 1968;Holliday and Thompson 1979;Chen and Gray 1985;Kaplan and DeMaria 2003;Kaplan et al., 2010;Shieh et al., 2013;Chen et al., 2015). The environmental factors also do impact on TC size changes. ...
Tropical cyclone (TC) rapid intensification (RI) is usually accompanied by a rapid eyewall contraction, followed by a slow contraction, and then a nearly steady eyewall. However, this study shows that Hurricane Helene (2006) exhibited an eyewall expansion during its 30-h rapid intensification period. The possible environmental influence on the eyewall expansion during the RI of Helene is examined. It is found that the synoptic-scale circulations led to additional low-level inflows and upper-level outflows that may play an important role in the eyewall expansion during the RI of Helene. Examination of the divergence of the absolute angular momentum flux (AAMF) associated with the environmental circulation suggests that the synoptic-scale atmospheric circulation played an important role in the eyewall expansion during the RI of Helene. In the lower and middle troposphere, the synoptic-scale cross-equatorial flow, which was enhanced by the Helene-induced wave train, led to the horizontal convergence of absolute angular momentum flux, while the TC-trough interaction and the related outflow in the upper troposphere resulted in the divergence of AAMF. The environment-induced low-level convergence and upper-level divergence of AAMF were superimposed on the secondary circulation of Helene and may be important to the eyewall expansion during the RI by accelerating the tangential wind outside of the eyewall. This study suggests that RI can occur with an eyewall expansion.
... In particular, we ask whether the TC outflow plays an important role in the asymmetric structural evolution, and if so, how. As the upper part of the TC secondary circulation, the outflow always organizes as a concentrated channel or several channels (Chen and Gray 1985;Ooyama 1987). In the upper-level outflow region, the inertial stability is very low (or even slightly negative) because of the TC anticyclonic wind. ...
This study investigates the role of the asymmetric interaction between the tropical cyclone (TC) and the environmental flow in governing the TC inner-core asymmetric structure. Motivated by the limitations of bulk measures of vertical wind shear in representing the complete environmental flow, the TC outflow is used as a focus for the asymmetric interaction. By analyzing an idealized numerical simulation, it is demonstrated that parcels can go directly from the asymmetric rainband to the upper-level outflow. The relatively large vertical mass flux in the rainband region also suggests that the asymmetric rainband is an important source of the outflow. In a simulation that suppresses convection by reducing the water vapor within the rainband region, the upper-level outflow is weakened, further supporting the hypothesis that the rainband and outflow are directly connected. Finally, it is demonstrated that the asymmetric outflow and the outer rainband are coupled through the descending inflow below the outflow. Some of the main characteristics of the outflow–rainband relationship are also supported by a real-case numerical simulation of Hurricane Bill (2009). The relationship is potentially useful for understanding and predicting the evolution of the TC inner-core structure during the interaction with the large-scale environmental flow.
... Outflow magnitude was largest during periods of intensification and smallest during periods of weakening, a pattern that was found in both storms. This relationship between increase in outflow magnitude and increase in TC intensity is in good agreement with many studies that previously identified such a relationship [e.g., Sadler, 1976;Chen and Gray, 1986;Merrill, 1988;Molinari and Vollaro, 1989;Pfeffer and Challa, 1992;Wu and Emanuel, 1994;Frank and Ritchie, 1999;Hanley et al., 2001;Kimball and Evans, 2002;Rappin et al., 2011]. The inward radial progression during intensification noted in both TCs is a result that has not received as much attention, but that agrees with the enhanced divergence (and subsequent mass evacuation from the core) arguments of Molinari et al. [1995] and Grimes and Mercer [2015]. ...
Upper tropospheric outflow was examined during the life cycles of two hurricanes in the eastern and central Pacific Ocean. The outflow from Hurricanes Iselle and Julio was evaluated by using analyses from the Navy Global Environmental Model, which were very highly correlated with satellite atmospheric motion vector and NOAA G-IV dropsonde observations. A synoptic overview provided the environmental context for the life cycles of both tropical cyclones (TCs). Then, the outflow magnitude and direction within 6 radial degrees of each TC center were analyzed in relation to TC intensity, the synoptic environment, and inertial stability, with the following results. In both Iselle and Julio, the azimuthally averaged outflow magnitude was maximized initially more than 4 radial degrees from the center, and that maximum moved steadily inward during a 4 day intensification period and reached a position radially inward of 2° within 6 h of the time of maximum surface winds. Furthermore, the direction of the outflow in both TCs was related to the evolution of the large-scale upper tropospheric flow pattern, particularly the phasing of subtropical jet ridges and troughs moving eastward north of both TCs. Finally, outflow channels were consistently bounded by regions of lowest (highest) values of inertial stability counterclockwise (clockwise) from the maximum outflow azimuth, a pattern that persisted throughout the life cycles of both storms regardless of intensity, environmental flow, and the number and direction of outflow channels present.
... There are several documented instances of upper level influence on the intensification of mature storms. Observational studies (Chen, 1985;Molinari, 1989) feature asymmetries where interactions with mid-latitude disturbances allow for low eddy momentum flux values as an outflow channel is established from the inner radii to the outer radii (Molinari, 1990;1995). Such outflow channels are typically found in the northeast sector of storms in association with trough interactions (Hanley, 2002), although she also found that identifying such signatures using satellite data was a formidable challenge. ...
... Chen과Gray(1984) 그리고 Merrill(1988a)에 의해 강조되었다. 이들 연구자들의 대부분은 강화된 태풍 이 태풍 중심의 몇 백 km 서쪽 그리고 북쪽 상층 대류 권에서 열대․아열대 기압골이 존재함을 보였다. ...
In this study, the cause of rapid intensity change of typhoon Nakri(0208) in view of point of a trough-typhoon interaction using diagnostic methods was examined based on 6-hourly GDAPS data from 10 to 13 July, 2002. At 0000 UTC 13 July, high PV(Potential Vorticity) region moved southeastward, reaching to the western edge of the Korean peninsula and near typhoon center at surface and there shows an increasing value of EFC(Eddy Momentum Flux Convergence). Also, as the trough and typhoon approach one another at the same time, the vertical shear(850-200 hPa) increases to more than 15 m/s. Thus, it might be concluded that the trough-typhoon interaction made intensified significantly, providing the fact that typhoon Nakri(0208) underwent substantial weakening while moving northward to around Jeju island.
... These upper level outflows can be seen in hand analyses with the benefit of geostationary data, but are not always present in model data. A good composite study of equatorial outflow interaction with a Southern Hemisphere anticyclone is included in Chen and Gray (1985). Similarly, the link between TC activity in SCS and fire activity in the MC has been noticed by in situ forecasters with the Malaysian Meteorological Department. ...
Much research and speculation exists about the meteorological and climatological impacts of biomass burning in the Maritime Continent (MC) of Indonesia and Malaysia, particularly during El Niño events. However, the MC hosts some of the world's most complicated meteorology, and we wish to understand how tropical phenomena at a range of scales influence observed burning activity. Using Moderate Resolution Imaging Spectroradiometer (MODIS) derived active fire hotspot patterns coupled with aerosol data assimilation products, satellite based precipitation, and meteorological indices, the meteorological context of observed fire prevalence and smoke optical depth in the MC are examined. Relationships of burning and smoke transport to such meteorological and climatic factors as the interannual El Niño-Southern Oscillation (ENSO), El Niño Modoki, Indian Ocean Dipole (IOP), the seasonal migration of the Intertropical Convergence Zone, the 30-90 day Madden Julian Oscillation (MJO), tropical waves, tropical cyclone activity, and diurnal convection were investigated. A conceptual model of how all of the differing meteorological scales affect fire activity is presented. Each island and its internal geography have different sensitivities to these factors which are likely relatable to precipitation patterns and land use practices. At the broadest scales as previously reported, we confirm ENSO is indeed the largest factor. However, burning is also enhanced by periods of El Niño Modoki. Conversely IOD influences are unclear. While interannual phenomena correlate to total seasonal burning, the MJO largely controls when visible burning occurs. High frequency phenomena which are poorly constrained in models such as diurnal convection and tropical cyclone activity also have an impact which cannot be ignored. Finally, we emphasize that these phenomena not only influence burning, but also the observability of burning, further complicating our ability to assign reasonable emissions.
... These upper level outflows can be seen in hand analyses with the benefit of geostationary data, but are not always present in model data. A good composite study of equatorial outflow interaction with a Southern Hemisphere anticyclone is included in Chen and Gray (1985). Similarly, the link between TC activity in SCS and fire activity in the MC has been noticed by in situ forecasters with the Malaysian Meteorological Department. ...
Much research and speculation exists about the meteorological and climatological impacts of biomass burning in the Maritime Continent (MC) of Indonesia and Malaysia, particularly during El Nino events. However, the MC hosts some of the world's most complicated meteorology, and we wish to understand how tropical phenomena at a range of scales influence observed burning activity. Using Moderate Resolution Imaging Spectroradiometer (MODIS) derived active fire hotspot patterns coupled with aerosol data assimilation products, satellite based precipitation, and meteorological indices, the meteorological context of observed fire prevalence and smoke optical depth in the MC are examined. Relationships of burning and smoke transport to such meteorological and climatic factors as the interannual El Nino-Southern Oscillation (ENSO), El Nino Modoki, Indian Ocean Dipole (IOD), the seasonal migration of the Intertropical Convergence Zone, the 30-90 day Madden Julian Oscillation (MJO), tropical waves, tropical cyclone activity, and diurnal convection were investigated. A conceptual model of how all of the differing meteorological scales affect fire activity is presented. Each island and its internal geography have different sensitivities to these factors which are likely relatable to precipitation patterns and land use practices. At the broadest scales as previously reported, we corroborate ENSO is indeed the largest factor. However, burning is also enhanced by periods of El Nino Modoki. Conversely, IOD influences are unclear. While interannual phenomena correlate to total seasonal burning, the MJO largely controls when visible burning occurs. High frequency phenomena which are poorly constrained in models such as diurnal convection and tropical cyclone activity also have an impact which cannot be ignored. Finally, we emphasize that these phenomena not only influence burning, but also the observability of burning, further complicating our ability to assign reasonable emissions.
The shallow water, linear model in cylindrical coordinates is used to investigate barotropic and inertial instabilities in the tropical cyclone outflow layer. When the mean state wind satisfies the condition |ζ|«|f 0 the instabilities found in our model are equivalent to the barotropic instabilities calculated with the quasi-geostrophic potential vorticity equation. The maximum instability is internal and appears at azimuthal wavenumber one.When an observed (composite) hurricane outflow layer is assumed as a basic state, the largest barotropic growth rate (e-folding time scale ≈7 days) appears at azimuthal wavenumber s = 1 and equivalent depth about 30 m. The maximum growth for s = 2 is about four times smaller than for s = 1 and occurs with the external mode. Inertial instability dominates at the very small equivalent depths and is likely to be stabilized by friction.
NCEP-NCAR reanalysis data are used to identify large-scale environmental flow patterns around west- ern North Pacific tropical storms with the goal of finding a signal for those most favorable for rapid intensification, based on the hypothesis that aspects of the horizontal flow influence tropical cyclone intensification at an early stage of development. Based on the finding that intensification rate is a strong function of initial intensity (Joint Typhoon Warning Center best track), very rapid, rapid, and slow 24-h intensification periods from a weak tropical storm stage (35 kt) are defined. By using composite analysis and scalar EOF analysis of the zonal wind around these subsets, a form of the lower-level (850 mb) combined monsoon confluence-shearline pattern is found to occur dominantly for the very rapid cases. Based on the strength of the signal, it may provide a new rapid intensification predictor for operational use. At 200 mb the importance of the location of the tropical storm under a region of flow splitting into the midlatitude westerlies to the north and the subequatorial trough to the south is identified as a common criterion for the onset of rapid intensification. Cases in which interactions with upper-level troughs occurred, prior to and during slow and rapid intensification, are studied and strong similarities to prior Atlantic studies are found.
Tropical cyclone intensity techniques developed by Dvorak have thus far been regarded by tropical meteorologists as the best identification and forecast schemes available using satellite imagery. However, in recent years, several ideologies have arisen which discuss alternative means of determining typhoon rapid intensification or weakening in the Pacific. These theories include examining channel outflow patterns, potential vorticity superposition and anomalies, tropical upper tropospheric trough interactions, environmental influences, and upper tropospheric flow transitions. It is now possible to data mine these atmospheric parameters thought partly responsible for typhoon rapid intensification and weakening to validate their usefulness in the forecast process. Using the latest data mining software tools, this study used components of NOGAPS analyses along with selected atmospheric and climatological predictors in classification analyses to create conditional forecast decision trees. The results of the classification model show an approximate R2 of 0.68 with percent error misclassifications of 13.5% for rapidly weakening typhoon events and 21.8% for rapidly intensifying typhoon events. In addition, a merged set of suggested forecast splitting rules was developed. By using the three most accurate predictors from both intensifying and weakening storms, the results validate the notion that multiple parameters are responsible for rapid changes in typhoon development.
Evidence is presented of upper‐tropospheric flow transitions during rapid tropical cyclone (TC) intensification. Transitions occur when a mid‐latitude upper trough, with a wind maximum on its eastern flank, located well to the west of a storm, relaxes as anticyclogenesis occurs near to, and east of, its equatorward extremity. During these episodes, the flow is characterized by weak inertial stability. It is proposed that this allows extremely rapid and large‐scale changes to occur in the upper‐level environment of the storm. The new environment provides seemingly favourable conditions of reduced wind shear, development of a downstream trough very near the storm, and access for the storm outflow to the tropical easterlies and mid‐latitude westerlies.
A global shallow‐water model, initialized with objective analyses at the 200 hPa level, is used to study the phenomenon, and the interaction between the environmental flow and local sources of mass and momentum. The sources are used to represent the effects of inner‐core deep convection in the outflow layer. Using the technique it seems possible, as a first approximation, to isolate the environment from the vortex development. It is shown that flow transitions are mostly independent of the presence of the TC. Short term, local enhancement of divergence over the TC occurs during superposition of environmentally‐induced divergence with the mass source. However, formation of the upper‐level vortex is the distinguishing feature of the intensification, and this occurs during an upper‐tropospheric flow transition. It is shown: (i) that the reduction in ventilation is associated with flow transitions, (ii) that these transitions directly influence the development of the upper vortex of the TC via downstream development of a weak environmental trough, and (iii) that the transitions indirectly influence the vortex development by allowing the momentum and mass sources to operate more efficiently to assist in both the development of the vortex and the outflow channels at small radii.
Several examples of flow transitions during intensification are presented to support the proposed hypothesis. Copyright © 2002 Royal Meteorological Society.
Typescript (photocopy) Thesis (M.S.)--Colorado State University, 1987. Bibliography: leaves [89]-95.
Includes bibliographical references (p. 93-99). Sponsored by AF Sponsored by NSF FACFATMS100158BLUE
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