scholarly journals An intercomparison of tropical cyclone best-track products for the southwest Pacific

2016 ◽  
Vol 16 (6) ◽  
pp. 1431-1447 ◽  
Author(s):  
Andrew D. Magee ◽  
Danielle C. Verdon-Kidd ◽  
Anthony S. Kiem
Keyword(s):  
Tropical Cyclone ◽  
Vertical Wind Shear ◽  
Geostationary Satellite ◽  
Vertical Shear ◽  
Tropical Cyclogenesis ◽  
Southwest Pacific ◽  
Zonal Winds ◽  
Spatio Temporal ◽  
Joint Typhoon Warning Center

Abstract. Recent efforts to understand tropical cyclone (TC) activity in the southwest Pacific (SWP) have led to the development of numerous TC databases. The methods used to compile each database vary and are based on data from different meteorological centres, standalone TC databases and archived synoptic charts. Therefore the aims of this study are to (i) provide a spatio-temporal comparison of three TC best-track (BT) databases and explore any differences between them (and any associated implications) and (ii) investigate whether there are any spatial, temporal or statistical differences between pre-satellite (1945–1969), post-satellite (1970–2011) and post-geostationary satellite (1982–2011) era TC data given the changing observational technologies with time. To achieve this, we compare three best-track TC databases for the SWP region (0–35° S, 135° E–120° W) from 1945 to 2011: the Joint Typhoon Warning Center (JTWC), the International Best Track Archive for Climate Stewardship (IBTrACS) and the Southwest Pacific Enhanced Archive of Tropical Cyclones (SPEArTC). The results of this study suggest that SPEArTC is the most complete repository of TCs for the SWP region. In particular, we show that the SPEArTC database includes a number of additional TCs, not included in either the JTWC or IBTrACS database. These SPEArTC events do occur under environmental conditions conducive to tropical cyclogenesis (TC genesis), including anomalously negative 700 hPa vorticity (VORT), anomalously negative vertical shear of zonal winds (VSZW), anomalously negative 700 hPa geopotential height (GPH), cyclonic (absolute) 700 hPa winds and low values of absolute vertical wind shear (EVWS). Further, while changes in observational technologies from 1945 have undoubtedly improved our ability to detect and monitor TCs, we show that the number of TCs detected prior to the satellite era (1945–1969) are not statistically different to those in the post-satellite era (post-1970). Although data from pre-satellite and pre-geostationary satellite periods are currently inadequate for investigating TC intensity, this study suggests that SPEArTC data (from 1945) may be used to investigate long-term variability of TC counts and TC genesis locations.

scholarly journals An intercomparison of tropical cyclone best-track products for the Southwest Pacific

2016 ◽  
Author(s):  
Andrew D. Magee ◽  
Danielle C. Verdon-Kidd ◽  
Anthony S. Kiem
Keyword(s):  
Tropical Cyclone ◽  
Tropical Cyclones ◽  
Environmental Conditions ◽  
Tropical Cyclogenesis ◽  
Southwest Pacific ◽  
Temporal Differences ◽  
Joint Typhoon Warning Center ◽  
Over Time

Abstract. Recent efforts to understand tropical cyclone (TC) activity in the Southwest Pacific (SWP) have led to the development of numerous TC databases. The methods used to compile each database vary and are based on data from different meteorological centres, standalone TC databases and archived synoptic charts. Therefore the aims of this study are to (i) examine spatial and temporal differences between the TC databases, and, (ii) investigate how changes in observational technology influence the temporal quality of TC records over time. To achieve this, we compare three best-track TC databases for the SWP region (0°–35°S, 135°E–120°W) from 1945–2011: the Joint Typhoon Warning Center (JTWC), the International Best Track Archive for Climate Stewardship (IBTrACS), and the Southwest Pacific Enhanced Archive of Tropical Cyclones (SPEArTC). The results of this study suggest that SPEArTC is the most complete repository of TCs for the SWP region. In particular, we show that the SPEArTC database includes a number of additional TCs, not included in either the JTWC or IBTrACS database. These SPEArTC events do occur under environmental conditions conducive to tropical cyclogenesis (TC genesis). Further, while changes in observational technologies from 1945 have undoubtedly improved our ability to detect and monitor TCs, we show that the number of TCs detected prior to the satellite era (1945–1969) are not statistically different to those in the post-satellite era (post-1970). Although studies on TC intensity should be limited to post-satellite/post-geostationary satellite eras only, this study suggests that SPEArTC data (from 1945) may be used to investigate long-term variability of TC counts and TC genesis locations.

scholarly journals A Ventilation Index for Tropical Cyclones

2012 ◽  
Vol 93 (12) ◽  
pp. 1901-1912 ◽  
Author(s):  
Brian Tang ◽  
Kerry Emanuel
Keyword(s):  
Tropical Cyclone ◽  
Tropical Cyclones ◽  
Wind Shear ◽  
Large Scale ◽  
Environmental Control ◽  
Vertical Wind Shear ◽  
Vertical Wind ◽  
Vertical Shear ◽  
Tropical Cyclogenesis ◽  
Model Errors

An important environmental control of both tropical cyclone intensity and genesis is vertical wind shear. One hypothesized pathway by which vertical shear affects tropical cyclones is midlevel ventilation—or the flux of low-entropy air into the center of the tropical cyclone. Based on a theoretical framework, a ventilation index is introduced that is equal to the environmental vertical wind shear multiplied by the nondimensional midlevel entropy deficit divided by the potential intensity. The ventilation index has a strong influence on tropical cyclone climatology. Tropical cyclogenesis preferentially occurs when and where the ventilation index is anomalously low. Both the ventilation index and the tropical cyclone's normalized intensity, or the intensity divided by the potential intensity, constrain the distribution of tropical cyclone intensification. The most rapidly intensifying storms are characterized by low ventilation indices and intermediate normalized intensities, while the most rapidly weakening storms are characterized by high ventilation indices and high normalized intensities. Since the ventilation index can be derived from large-scale fields, it can serve as a simple and useful metric for operational forecasts of tropical cyclones and diagnosis of model errors.

Examination of the Combined Effect of Deep-layer Vertical Shear Direction and Lower-Tropospheric Mean Flow on Tropical Cyclone Intensity and Size Based on the ERA5 Reanalysis

2021 ◽  
Author(s):  
Buo-Fu Chen ◽  
Christopher A. Davis ◽  
Ying-Hwa Kuo
Keyword(s):  
Tropical Cyclone ◽  
Inner Core ◽  
Vertical Wind Shear ◽  
Reanalysis Data ◽  
Vertical Shear ◽  
Shear Direction ◽  
Mean Flow ◽  
Representative Subset ◽  
Favorable Conditions ◽  
The Relationship

AbstractIdealized numerical studies have suggested that in addition to vertical wind shear (VWS) magnitude, the VWS profile also affects tropical cyclone (TC) development. A way to further understand the VWS profile’s effect is to examine the interaction between a TC and various shear-relative low-level mean flow (LMF) orientations. This study mainly uses the ERA5 reanalysis to verify that, consistent with idealized simulations, boundary-layer processes associated with different shear-relative LMF orientations affect real-world TC’s intensity and size. Based on analyses of 720 TCs from multiple basins during 2004–2016, a TC affected by an LMF directed toward downshear-left in the Northern Hemisphere favors intensification, whereas an LMF directed toward upshear-right is favorable for expansion. Furthermore, physical processes associated with shear-relative LMF orientation may also partly explain the relationship between the VWS direction and TC development, as there is a correlation between the two variables.The analysis of reanalysis data provides other new insights. The relationship between shear-relative LMF and intensification is not significantly modified by other factors [inner-core sea surface temperature (SST), VWS magnitude, and relative humidity (RH)]. However, the relationship regarding expansion is partly attributed to environmental SST and RH variations for various LMF orientations. Moreover, SST is critical to the basin-dependent variability of the relationship between the shear-relative LMF and intensification. For Atlantic TCs, the relationship between LMF orientation and intensification is inconsistent with all-basin statistics unless the analysis is restricted to a representative subset of samples associated with generally favorable conditions.

scholarly journals Use of a Genesis Potential Index to Diagnose ENSO Effects on Tropical Cyclone Genesis

2007 ◽  
Vol 20 (19) ◽  
pp. 4819-4834 ◽  
Author(s):  
Suzana J. Camargo ◽  
Kerry A. Emanuel ◽  
Adam H. Sobel
Keyword(s):  
Tropical Cyclone ◽  
Relative Humidity ◽  
El Niño ◽  
El Nino ◽  
Southern Oscillation ◽  
Vertical Wind Shear ◽  
Vertical Shear ◽  
Interannual Variations ◽  
Genesis Potential Index ◽  
Potential Index

Abstract ENSO (El Niño–Southern Oscillation) has a large influence on tropical cyclone activity. The authors examine how different environmental factors contribute to this influence, using a genesis potential index developed by Emanuel and Nolan. Four factors contribute to the genesis potential index: low-level vorticity (850 hPa), relative humidity at 600 hPa, the magnitude of vertical wind shear from 850 to 200 hPa, and potential intensity (PI). Using monthly NCEP Reanalysis data in the period of 1950–2005, the genesis potential index is calculated on a latitude strip from 60°S to 60°N. Composite anomalies of the genesis potential index are produced for El Niño and La Niña years separately. These composites qualitatively replicate the observed interannual variations of the observed frequency and location of genesis in several different basins. This justifies producing composites of modified indices in which only one of the contributing factors varies, with the others set to climatology, to determine which among the factors are most important in causing interannual variations in genesis frequency. Specific factors that have more influence than others in different regions can be identified. For example, in El Niño years, relative humidity and vertical shear are important for the reduction in genesis seen in the Atlantic basin, and relative humidity and vorticity are important for the eastward shift in the mean genesis location in the western North Pacific.

scholarly journals Attributing Tropical Cyclogenesis to Equatorial Waves in the Western North Pacific

2011 ◽  
Vol 68 (2) ◽  
pp. 195-209 ◽  
Author(s):  
Carl J. Schreck ◽  
John Molinari ◽  
Karen I. Mohr
Keyword(s):  
Tropical Cyclone ◽  
Tropical Cyclones ◽  
Wave Spectrum ◽  
Vertical Wind Shear ◽  
Warm Season ◽  
Threshold Value ◽  
Rainfall Anomaly ◽  
Tropical Cyclogenesis ◽  
Positive Anomaly ◽  
Equatorial Wave

Abstract Tropical cyclogenesis is attributed to an equatorial wave when the filtered rainfall anomaly exceeds a threshold value at the genesis location. It is argued that 0 mm day−1 (simply requiring a positive anomaly) is too small a threshold because unrelated noise can produce a positive anomaly. A threshold of 6 mm day−1 is too large because two-thirds of storms would have no precursor disturbance. Between these extremes, consistent results are found for a range of thresholds from 2 to 4 mm day−1. Roughly twice as many tropical cyclones are attributed to tropical depression (TD)-type disturbances as to equatorial Rossby waves, mixed Rossby–gravity waves, or Kelvin waves. The influence of the Madden–Julian oscillation (MJO) is even smaller. The use of variables such as vorticity and vertical wind shear in other studies gives a larger contribution for the MJO. It is suggested that its direct influence on the rainfall in forming tropical cyclones is less than for other variables. The impacts of tropical cyclone–related precipitation anomalies are also presented. Tropical cyclones can contribute more than 20% of the warm-season rainfall and 50% of its total variance. The influence of tropical cyclones on the equatorial wave spectrum is generally small. The exception occurs in shorter-wavelength westward-propagating waves, for which tropical cyclones represent up to 27% of the variance. Tropical cyclones also significantly contaminate wave-filtered rainfall anomalies in their immediate vicinity. To mitigate this effect, the tropical cyclone–related anomalies were removed before filtering in this study.

scholarly journals Prediction and Diagnosis of Tropical Cyclone Formation in an NWP System. Part III: Diagnosis of Developing and Nondeveloping Storms

2007 ◽  
Vol 64 (9) ◽  
pp. 3195-3213 ◽  
Author(s):  
K. J. Tory ◽  
N. E. Davidson ◽  
M. T. Montgomery
Keyword(s):  
Tropical Cyclone ◽  
Large Scale ◽  
Deep Convection ◽  
Weather Prediction ◽  
Vertical Wind Shear ◽  
Forecast Model ◽  
Vertical Shear ◽  
Limited Area ◽  
Vortex Interactions ◽  
Secondary Mechanism

Abstract This is the third of a three-part investigation into tropical cyclone (TC) genesis in the Australian Bureau of Meteorology’s Tropical Cyclone Limited Area Prediction System (TC-LAPS), an operational numerical weather prediction (NWP) forecast model. In Parts I and II, a primary and two secondary vortex enhancement mechanisms were illustrated, and shown to be responsible for TC genesis in a simulation of TC Chris. In this paper, five more TC-LAPS simulations are investigated: three developing and two nondeveloping. In each developing simulation the pathway to genesis was essentially the same as that reported in Part II. Potential vorticity (PV) cores developed through low- to middle-tropospheric vortex enhancement in model-resolved updraft cores (primary mechanism) and interacted to form larger cores through diabatic upscale vortex cascade (secondary mechanism). On the system scale, vortex intensification resulted from the large-scale mass redistribution forced by the upward mass flux, driven by diabatic heating, in the updraft cores (secondary mechanism). The nondeveloping cases illustrated that genesis can be hampered by (i) vertical wind shear, which may tilt and tear apart the PV cores as they develop, and (ii) an insufficient large-scale cyclonic environment, which may fail to sufficiently confine the warming and enhanced cyclonic winds, associated with the atmospheric adjustment to the convective updrafts. The exact detail of the vortex interactions was found to be unimportant for qualitative genesis forecast success. Instead the critical ingredients were found to be sufficient net deep convection in a sufficiently cyclonic environment in which vertical shear was less than some destructive limit. The often-observed TC genesis pattern of convection convergence, where the active convective regions converge into a 100-km-diameter center, prior to an intense convective burst and development to tropical storm intensity is evident in the developing TC-LAPS simulations. The simulations presented in this study and numerous other simulations not yet reported on have shown good qualitative forecast success. Assuming such success continues in a more rigorous study (currently under way) it could be argued that TC genesis is largely predictable provided the large-scale environment (vorticity, vertical shear, and convective forcing) is sufficiently resolved and initialized.

scholarly journals Reevaluating the Role of the Saharan Air Layer in Atlantic Tropical Cyclogenesis and Evolution

2010 ◽  
Vol 138 (6) ◽  
pp. 2007-2037 ◽  
Author(s):  
Scott A. Braun
Keyword(s):  
Tropical Cyclones ◽  
Wind Shear ◽  
Large Scale ◽  
Vertical Wind Shear ◽  
Negative Influence ◽  
Vertical Wind ◽  
Vertical Shear ◽  
Tropical Cyclogenesis ◽  
Saharan Air Layer ◽  
The Individual

Abstract The existence of the Saharan air layer (SAL), a layer of warm, dry, dusty air frequently present over the tropical Atlantic Ocean, has long been appreciated. The nature of its impacts on hurricanes remains unclear, with some researchers arguing that the SAL amplifies hurricane development and with others arguing that it inhibits it. The potential negative impacts of the SAL include 1) vertical wind shear associated with the African easterly jet; 2) warm air aloft, which increases thermodynamic stability at the base of the SAL; and 3) dry air, which produces cold downdrafts. Multiple NASA satellite datasets and NCEP global analyses are used to characterize the SAL’s properties and evolution in relation to tropical cyclones and to evaluate these potential negative influences. The SAL is shown to occur in a large-scale environment that is already characteristically dry as a result of large-scale subsidence. Strong surface heating and deep dry convective mixing enhance the dryness at low levels (primarily below ∼700 hPa), but moisten the air at midlevels. Therefore, mid- to-upper-level dryness is not generally a defining characteristic of the SAL, but is instead often a signature of subsidence. The results further show that storms generally form on the southern side of the jet, where the background cyclonic vorticity is high. Based upon its depiction in NCEP Global Forecast System meteorological analyses, the jet often helps to form the northern side of the storms and is present to equal extents for both strengthening and weakening storms, suggesting that jet-induced vertical wind shear may not be a frequent negative influence. Warm SAL air is confined to regions north of the jet and generally does not impact the tropical cyclone precipitation south of the jet. Composite analyses of the early stages of tropical cyclones occurring in association with the SAL support the inferences from the individual cases noted above. Furthermore, separate composites for strongly strengthening and for weakening storms show few substantial differences in the SAL characteristics between these two groups, suggesting that the SAL is not a determinant of whether a storm will intensify or weaken in the days after formation. Key differences between these cases are found mainly at upper levels where the flow over strengthening storms allows for an expansive outflow and produces little vertical shear, while for weakening storms, the shear is stronger and the outflow is significantly constrained.

scholarly journals A Genesis Index for Monsoon Disturbances

2016 ◽  
Vol 29 (14) ◽  
pp. 5189-5203 ◽  
Author(s):  
Sarah D. Ditchek ◽  
William R. Boos ◽  
Suzana J. Camargo ◽  
Michael K. Tippett
Keyword(s):  
Tropical Cyclone ◽  
Convective Available Potential Energy ◽  
Vertical Wind Shear ◽  
Vertical Shear ◽  
Tropical Cyclone Genesis ◽  
Monsoon Region ◽  
Australian Monsoon ◽  
Cyclone Genesis ◽  
Monsoon Disturbance ◽  
Monsoon Disturbances

Abstract Synoptic-scale monsoon disturbances produce the majority of continental rainfall in the monsoon regions of South Asia and Australia, yet there is little understanding of the conditions that foster development of these low pressure systems. Here a genesis index is used to associate monsoon disturbance genesis in a global domain with monthly mean, climatological environmental variables. This monsoon disturbance genesis index (MDGI) is based on four objectively selected variables: total column water vapor, low-level absolute vorticity, an approximate measure of convective available potential energy, and midtropospheric relative humidity. A Poisson regression is used to estimate the index coefficients. Unlike existing tropical cyclone genesis indices, the MDGI is defined over both land and ocean, consistent with the fact that monsoon disturbance genesis can occur over land. The index coefficients change little from their global values when estimated separately for the Asian–Australian monsoon region or the Indian monsoon region, suggesting that the conditions favorable for monsoon disturbance genesis, and perhaps the dynamics of genesis itself, are common across multiple monsoon regions. Vertical wind shear is found to be a useful predictor in some regional subdomains; although previous studies suggested that baroclinicity may foster monsoon disturbance genesis, here genesis frequency is shown to be reduced in regions of strong climatological vertical shear. The coefficients of the MDGI suggest that monsoon disturbance genesis is fostered by humid, convectively unstable environments that are rich in vorticity. Similarities with indices used to describe the distribution of tropical cyclone genesis are discussed.

The Role of Tropical Waves in Tropical Cyclogenesis

2006 ◽  
Vol 134 (9) ◽  
pp. 2397-2417 ◽  
Author(s):  
William M. Frank ◽  
Paul E. Roundy
Keyword(s):  
Tropical Cyclone ◽  
Phase Relationship ◽  
Vertical Motion ◽  
Longwave Radiation ◽  
Wave Activity ◽  
Vertical Shear ◽  
Tropical Cyclogenesis ◽  
Strong Impact ◽  
Tropical Wave ◽  
The Waves

Abstract This paper analyzes relationships between tropical wave activity and tropical cyclogenesis in all of the earth’s major tropical cyclone basins. Twenty-nine years of outgoing longwave radiation data and global reanalysis winds are filtered and analyzed to determine statistical relationships between wave activity in each basin and the corresponding cyclogenesis. Composite analyses relative to the storm genesis locations show the structures of the waves and their preferred phase relationships with genesis. Five wave types are examined in this study, including mixed Rossby–gravity waves, tropical-depression-type or easterly waves, equatorial Rossby waves, Kelvin waves, and the Madden–Julian oscillation. The latter is not one of the classical tropical wave types, but is a wavelike phenomenon known to have a strong impact on tropical cyclogenesis. Tropical cyclone formation is strongly related to enhanced activity in all of the wave filter bands except for the Kelvin band. In each basin the structure of each composite wave and the phase relationship between the wave and cyclogenesis are similar, suggesting consistent forcing mechanisms. The waves appear to enhance the local circulations by increasing the forced upward vertical motion, increasing the low-level vorticity at the genesis location, and by modulating the vertical shear. Convective anomalies of waves associated with genesis are detectable in the analyses as long as 1 month prior to genesis. This opens up the possibility of developing statistically based genesis forecasts.

scholarly journals Reexamining the Near-Core Radial Structure of the Tropical Cyclone Primary Circulation: Implications for Vortex Resiliency

2005 ◽  
Vol 62 (2) ◽  
pp. 408-425 ◽  
Author(s):  
Kevin J. Mallen ◽  
Michael T. Montgomery ◽  
Bin Wang
Keyword(s):  
Tropical Cyclone ◽  
Vertical Wind Shear ◽  
Relative Vorticity ◽  
Core Region ◽  
Vertical Shear ◽  
Theoretical Studies ◽  
True Nature ◽  
Radial Profiles ◽  
Tangential Wind ◽  
Radial Structure

Abstract Recent theoretical studies, based on vortex Rossby wave (VRW) dynamics, have established the importance of the radial structure of the primary circulation in the response of tropical cyclone (TC)–like vortices to ambient vertical wind shear. Linear VRW theory suggests, in particular, that the degree of broadness of the primary circulation in the near-core region beyond the radius of maximum wind strongly influences whether a tilted TC vortex will realign and resist vertical shear or tilt over and shear apart. Fully nonlinear numerical simulations have verified that the vortex resiliency is indeed sensitive to the initial radial structure of the idealized vortex. This raises the question of how well the “true” nature of a TC’s primary circulation is represented by idealized vortices that are commonly used in some theoretical studies. In this paper the swirling wind structure of TCs is reexamined by utilizing flight-level observations collected from Atlantic and eastern Pacific storms during 1977–2001. Hundreds of radial profiles of azimuthal-mean tangential wind and relative vorticity are constructed from over 5000 radial flight leg segments and compared with some standard idealized vortex profiles. This analysis reaffirms that real TC structure in the near-core region is characterized by relatively slow tangential wind decay in conjunction with a skirt of significant cyclonic relative vorticity possessing a negative radial gradient. This broadness of the primary circulation is conspicuously absent in some idealized vortices used in theoretical studies of TC evolution in vertical shear. The relationship of the current findings to the problem of TC resiliency is discussed.

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