Climatology of Vertical Wind Shear over the Tropical Atlantic

2006 ◽  
Vol 19 (12) ◽  
pp. 2969-2983 ◽  
Author(s):  
Anantha R. Aiyyer ◽  
Chris Thorncroft
Keyword(s):  
Wind Shear ◽  
Large Scale ◽  
Southern Oscillation ◽  
Vertical Wind Shear ◽  
Vertical Wind ◽  
Vertical Shear ◽  
Spatiotemporal Variability ◽  
Tropical Atlantic ◽  
Highly Correlated ◽  
Sahel Precipitation

Abstract The spatiotemporal variability of the 200–850-hPa vertical wind shear over the tropical Atlantic is examined for a period of 46 yr. This work extends and updates past studies by considering a longer data record as well as a tropospheric-deep measure of vertical wind shear. Composite fields are constructed to illustrate the spatial pattern of the large-scale circulation associated with the mean and extreme cases of vertical shear within the tropical Atlantic. The contemporaneous relationship of vertical shear with El Niño–Southern Oscillation (ENSO) and Sahel precipitation are also examined. While the ENSO–shear correlation appears to have slightly strengthened during the past decade, the Sahel–shear correlation has become significantly degraded. A combined empirical orthogonal function (EOF) analysis of the zonal and meridional components of the vertical shear reveals interannual and multidecadal modes. The leading EOF exhibits mainly interannual variability and is highly correlated with ENSO. The second EOF is associated with a multidecadal temporal evolution and is correlated with Sahel precipitation. Both EOFs correlate at the same level with tropical cyclones in the main development region of the tropical Atlantic.

scholarly journals Influence of Mean Flow on the ENSO–Vertical Wind Shear Relationship over the Northern Tropical Atlantic

2012 ◽  
Vol 25 (3) ◽  
pp. 858-864 ◽  
Author(s):  
Xiaojie Zhu ◽  
R. Saravanan ◽  
Ping Chang
Keyword(s):  
Wind Shear ◽  
Southern Oscillation ◽  
Critical Role ◽  
Vertical Wind Shear ◽  
Vertical Wind ◽  
Vertical Shear ◽  
Tropical Atlantic ◽  
Mean Flow ◽  
Model Simulations ◽  
Enso Events

Abstract Vertical wind shear over the tropical Atlantic Ocean plays an important role in mediating hurricane activity. The vertical shear variability over the main development region for Atlantic hurricanes is affected by local factors as well as by the remote influence of the El Niño–Southern Oscillation (ENSO) phenomenon, as indicated by observational and climate modeling analyses. Climate model simulations of the ENSO–shear relationship are compared with observations. It is shown that there is a strong influence of background mean flow on the ENSO–shear relationship, because of the inherently nonlinear nature of vertical wind shear. In particular, the simulation of zonal flow over the tropical Atlantic is shown to play a critical role in how the remote influence of ENSO modulates the shear. Even with realistic simulations of the ENSO-induced westerly anomaly in the upper troposphere, overestimated easterly background flow in the model simulations can alter the relationship between ENSO and vertical wind shear, resulting in decreased vertical wind shear during warm ENSO events. This nonlinear superposition has important implications for the assessment of trends in large-scale environmental factors affecting tropical cyclogenesis in climate change simulations.

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.

Response of Atmospheric Convection to Vertical Wind Shear: Cloud-System-Resolving Simulations with Parameterized Large-Scale Circulation. Part II: Effect of Interactive Radiation

2015 ◽  
Vol 73 (1) ◽  
pp. 199-209 ◽  
Author(s):  
Usama Anber ◽  
Shuguang Wang ◽  
Adam Sobel
Keyword(s):  
Wind Shear ◽  
Large Scale ◽  
Lower Boundary ◽  
Vertical Wind Shear ◽  
Surface Fluxes ◽  
Vertical Wind ◽  
Radiative Heating ◽  
Vertical Shear ◽  
Large Scale Circulation ◽  
Lower Boundary Condition

Abstract The authors investigate the effects of cloud–radiation interaction and vertical wind shear on convective ensembles interacting with large-scale dynamics in cloud-resolving model simulations, with the large-scale circulation parameterized using the weak temperature gradient approximation. Numerical experiments with interactive radiation are conducted with imposed surface heat fluxes constant in space and time, an idealized lower boundary condition that prevents wind–evaporation feedback. Each simulation with interactive radiation is compared to a simulation in which the radiative heating profile is held constant in the horizontal and in time and is equal to the horizontal-mean profile from the interactive-radiation simulation with the same vertical shear profile and surface fluxes. Interactive radiation is found to reduce mean precipitation in all cases. The magnitude of the reduction is nearly independent of the vertical wind shear but increases with surface fluxes. Deep shear also reduces precipitation, though by approximately the same amount with or without interactive radiation. The reductions in precipitation due to either interactive radiation or deep shear are associated with strong large-scale ascent in the upper troposphere, which more strongly exports moist static energy and is quantified by a larger normalized gross moist stability.

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.

Tropical and Subtropical North Atlantic Vertical Wind Shear and Seasonal Tropical Cyclone Activity

2020 ◽  
Vol 33 (13) ◽  
pp. 5413-5426
Author(s):  
Jhordanne J. Jones ◽  
Michael M. Bell ◽  
Philip J. Klotzbach
Keyword(s):  
Tropical Cyclone ◽  
North Atlantic ◽  
Wind Shear ◽  
Large Scale ◽  
Southern Oscillation ◽  
Function Analysis ◽  
Vertical Wind Shear ◽  
Vertical Wind ◽  
Boreal Summer ◽  
Leading Mode

AbstractGiven recent insights into the role of anticyclonic Rossby wave breaking (AWB) in driving subseasonal and seasonal North Atlantic tropical cyclone (TC) activity, this study further examines tropical versus subtropical impacts on TC activity by considering large-scale influences on boreal summer tropical zonal vertical wind shear (VWS) variability, a key predictor of seasonal TC activity. Through an empirical orthogonal function analysis, it is shown that subtropical AWB activity drives the second mode of variability in tropical zonal VWS, while El Niño–Southern Oscillation (ENSO) primarily drives the leading mode of variability. Linear regressions of the four leading principal components against tropical North Atlantic zonal VWS and accumulated cyclone energy show that while the leading mode holds much of the regression strength, some improvement can be achieved with the addition of the second and third modes. Furthermore, an index of AWB-associated VWS anomalies, a proxy for AWB impacts on the large-scale environment, may be a better indicator of summertime VWS anomalies. The utilization of this index may be used to better understand AWB’s contribution to seasonal TC activity.

scholarly journals The Extremely Active 2017 North Atlantic Hurricane Season

2018 ◽  
Vol 146 (10) ◽  
pp. 3425-3443 ◽  
Author(s):  
Philip J. Klotzbach ◽  
Carl J. Schreck III ◽  
Jennifer M. Collins ◽  
Michael M. Bell ◽  
Eric S. Blake ◽  
...  
Keyword(s):  
North Atlantic ◽  
Wind Shear ◽  
Large Scale ◽  
Vertical Wind Shear ◽  
Vertical Wind ◽  
Tropical Atlantic ◽  
Seasonal Forecasts ◽  
Atlantic Hurricane ◽  
Steering Flow ◽  
Hurricane Season

Abstract The 2017 North Atlantic hurricane season was extremely active, with 17 named storms (1981–2010 median is 12.0), 10 hurricanes (median is 6.5), 6 major hurricanes (median is 2.0), and 245% of median accumulated cyclone energy (ACE) occurring. September 2017 generated more Atlantic named storm days, hurricane days, major hurricane days, and ACE than any other calendar month on record. The season was destructive, with Harvey and Irma devastating portions of the continental United States, while Irma and Maria brought catastrophic damage to Puerto Rico, Cuba, and many other Caribbean islands. Seasonal forecasts increased from calling for a slightly below-normal season in April to an above-normal season in August as large-scale environmental conditions became more favorable for an active hurricane season. During that time, the tropical Atlantic warmed anomalously while a potential El Niño decayed in the Pacific. Anomalously high SSTs prevailed across the tropical Atlantic, and vertical wind shear was anomalously weak, especially in the central tropical Atlantic, from late August to late September when several major hurricanes formed. Late-season hurricane activity was likely reduced by a convectively suppressed phase of the Madden–Julian oscillation. The large-scale steering flow was different from the average over the past decade with a strong subtropical high guiding hurricanes farther west across the Atlantic. The anomalously high tropical Atlantic SSTs and low vertical wind shear were comparable to other very active seasons since 1982.

Response of Atmospheric Convection to Vertical Wind Shear: Cloud-System-Resolving Simulations with Parameterized Large-Scale Circulation. Part I: Specified Radiative Cooling

2014 ◽  
Vol 71 (8) ◽  
pp. 2976-2993 ◽  
Author(s):  
Usama Anber ◽  
Shuguang Wang ◽  
Adam Sobel
Keyword(s):  
Wind Shear ◽  
Large Scale ◽  
Vertical Wind Shear ◽  
Surface Fluxes ◽  
Vertical Wind ◽  
Vertical Shear ◽  
Horizontal Wind ◽  
Nonmonotonic Dependence ◽  
Convective Systems ◽  
Mass Fluxes

Abstract It is well known that vertical wind shear can organize deep convective systems and greatly extend their lifetimes. Much less is known about the influence of shear on the bulk properties of tropical convection in statistical equilibrium. To address the latter question, the authors present a series of cloud-resolving simulations on a doubly periodic domain with parameterized large-scale dynamics based on the weak temperature gradient (WTG) approximation. The horizontal-mean horizontal wind is relaxed strongly in these simulations toward a simple unidirectional linear vertical shear profile in the troposphere. The strength and depth of the shear layer are varied as control parameters. Surface enthalpy fluxes are prescribed. The results fall in two distinct regimes. For weak wind shear, time-averaged rainfall decreases with shear and convection remains disorganized. For larger wind shear, rainfall increases with shear, as convection becomes organized into linear mesoscale systems. This nonmonotonic dependence of rainfall on shear is observed when the imposed surface fluxes are moderate. For larger surface fluxes, convection in the unsheared basic state is already strongly organized, but increasing wind shear still leads to increasing rainfall. In addition to surface rainfall, the impacts of shear on the parameterized large-scale vertical velocity, convective mass fluxes, cloud fraction, and momentum transport are also discussed.

scholarly journals Tropical Cyclone Climatology from Satellite Passive Microwave Measurements

2020 ◽  
Vol 12 (21) ◽  
pp. 3610
Author(s):  
Song Yang ◽  
Richard Bankert ◽  
Joshua Cossuth
Keyword(s):  
Southern Hemisphere ◽  
Northern Hemisphere ◽  
Wind Shear ◽  
Vertical Wind Shear ◽  
Passive Microwave ◽  
Vertical Wind ◽  
Vertical Shear ◽  
Shear Direction ◽  
Motion Direction ◽  
Horizontal Structure

The satellite passive microwave (PMW) sensor brightness temperatures (TBs) of all tropical cyclones (TCs) from 1987–2012 have been carefully calibrated for inter-sensor frequency differences, center position fixing using the Automated Rotational Center Hurricane Eye Retrieval (ARCHER) scheme, and application of the Backus–Gilbert interpolation scheme for better presentation of the TC horizontal structure. With additional storm motion direction and the 200–850 hPa wind shear direction, a unique and comprehensive TC database is created for this study. A reliable and detailed climatology for each TC category is analyzed and discussed. There is significant annual variability of the number of storms at hurricane intensity, but the annual number of all storms is relatively stable. Results based on the analysis of the 89 GHz horizontal polarization TBs over oceans are presented in this study. An eyewall contraction is clearly displayed with an increase in TC intensity. Three composition schemes are applied to present a reliable and detailed TC climatology at each intensity category and its geographic characteristics. The global composition relative to the North direction is not able to lead a realistic structure for an individual TC. Enhanced convection in the down-motion quadrants relative to direction of TC motion is obvious for Cat 1–3 TCs, while Cat 4–5 TCs still have a concentric pattern of convection within 200 km radius. Regional differences are evident for weak storms. Results indicate the direction of TC movement has more impact on weak storms than on Cat 4–5 TCs. A striking feature is that all TCs have a consistent pattern of minimum TBs at 89 GHz in the downshear left quadrant (DSLQ) for the northern hemisphere basins and in the downshear right quadrant (DSRQ) for the southern hemisphere basin, regarding the direction of the 200–850 hPa wind shear. Tropical depression and tropical storm have the minimum TBs in the downshear quadrants. The axis of the minimum TBs is slightly shifted toward the vertical shear direction. There is no geographic variation of storm structure relative to the vertical wind shear direction except over the southern hemisphere which shows a mirror image of the storm structure over the northern hemisphere. This study indicates that regional variation of storm structure relative to storm motion direction is mainly due to differences of the vertical wind shear direction among these basins. Results demonstrate the direction of the 200–850 hPa wind shear plays a critical role in TC structure.

Some Effects of Vertical Wind Shear on Thunderstorm Structure *

1949 ◽  
Vol 30 (5) ◽  
pp. 168-175 ◽  
Author(s):  
Horace R. Byers ◽  
Louis J. Battan
Keyword(s):  
Cross Sections ◽  
Wind Shear ◽  
Vertical Wind Shear ◽  
Vertical Wind ◽  
Vertical Shear ◽  
Radar Cross Sections ◽  
Almost All

Observations of thunderclouds obtained with a 3-cm height-finding radar set are used to obtain a description of the vertical shear of thunderclouds. Several photographs are given which show the shearing of the radar clouds. A scattergram of wind shear plotted against echo shear is presented and shows that the two variables are related, with the former exceeding the latter in almost all cases. Scatter-diagrams are given which verify that strong vertical wind shear tends to restrict the growth of thunderstorms. A series of radar cross sections illustrates the displacement of the upper part of a thundercloud which is subjected to wind shear, and the growth of another cloud column from the lower part of the thundercloud.

scholarly journals Rainfall Mechanisms for One of the Wettest Tropical Cyclones on Record in Australia—Oswald (2013)

2020 ◽  
Vol 148 (6) ◽  
pp. 2503-2525
Author(s):  
Difei Deng ◽  
Elizabeth A. Ritchie
Keyword(s):  
Tropical Cyclones ◽  
Wind Shear ◽  
Heavy Rainfall ◽  
Large Scale ◽  
Vertical Wind Shear ◽  
Monsoon Trough ◽  
Vertical Wind ◽  
Lower Latitude ◽  
Cape York ◽  
Frictional Convergence

Abstract Tropical Cyclone Oswald (2013) is considered to be one of the highest-impact storms to make landfall in northern Australia even though it only reached a maximum category 1 intensity on the Australian category scale. After making landfall on the west coast of Cape York Peninsula, Oswald turned southward, and persisted for more than 7 days moving parallel to the coastline as far south as 30°S. As one of the wettest tropical cyclones (TCs) in Australian history, the favorable configurations of a lower-latitude active monsoon trough and two consecutive midlatitude trough–jet systems generally contributed to the maintenance of the Oswald circulation over land and prolonged rainfall. As a result, Oswald produced widespread heavy rainfall along the east coast with three maximum centers near Weipa, Townsville, and Rockhampton, respectively. Using high-resolution WRF simulations, the mechanisms associated with TC Oswald’s rainfall are analyzed. The results show that the rainfall involved different rainfall mechanisms at each stage. The land–sea surface friction contrast, the vertical wind shear, and monsoon trough were mostly responsible for the intensity and location for the first heavy rainfall center on the Cape York Peninsula. The second torrential rainfall near Townsville was primarily a result of the local topography and land–sea frictional convergence in a conditionally unstable monsoonal environment with frictional convergence due to TC motion modulating some offshore rainfall. The third rainfall area was largely dominated by persistent high vertical wind shear forcing, favorable large-scale quasigeostrophic dynamic lifting from two midlatitude trough–jet systems, and mesoscale frontogenesis lifting.

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