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.

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 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 Structural and Intensity Changes of Hurricane Bret (1999). Part I: Environmental Influences

2008 ◽  
Vol 136 (11) ◽  
pp. 4320-4333 ◽  
Author(s):  
Alexander Lowag ◽  
Michael L. Black ◽  
Matthew D. Eastin
Keyword(s):  
Wind Shear ◽  
Inner Core ◽  
Critical Role ◽  
Heat Content ◽  
Vertical Wind Shear ◽  
Vertical Wind ◽  
Mean Values ◽  
The North ◽  
Intensity Changes ◽  
Environmental Prediction

Abstract Hurricane Bret underwent a rapid intensification (RI) and subsequent weakening between 1200 UTC 21 August and 1200 UTC 22 August 1999 before it made landfall on the Texas coast 12 h later. Its minimum sea level pressure fell 35 hPa from 979 to 944 hPa within 24 h. During this period, aircraft of the National Oceanic and Atmospheric Administration (NOAA) flew several research missions that sampled the environment and inner core of the storm. These datasets are combined with gridded data from the National Centers for Environmental Prediction (NCEP) Global Model and the NCEP–National Center for Atmospheric Research (NCAR) reanalyses to document Bret’s atmospheric and oceanic environment as well as their relation to the observed structural and intensity changes. Bret’s RI was linked to movement over a warm ocean eddy and high sea surface temperatures (SSTs) in the Gulf of Mexico coupled with a concurrent decrease in vertical wind shear. SSTs at the beginning of the storm’s RI were approximately 29°C and steadily increased to 30°C as it moved to the north. The vertical wind shear relaxed to less than 10 kt during this time. Mean values of oceanic heat content (OHC) beneath the storm were about 20% higher at the beginning of the RI period than 6 h prior. The subsequent weakening was linked to the cooling of near-coastal shelf waters (to between 25° and 26°C) by prestorm mixing combined with an increase in vertical wind shear. The available observations suggest no intrusion of dry air into the circulation core contributed to the intensity evolution. Sensitivity studies with the Statistical Hurricane Intensity Prediction Scheme (SHIPS) model were conducted to quantitatively describe the influence of environmental conditions on the intensity forecast. Four different cases with modified vertical wind shear and/or SSTs were studied. Differences between the four cases were relatively small because of the model design, but the greatest intensity changes resulted for much cooler prescribed SSTs. The results of this study underscore the importance of OHC and vertical wind shear as significant factors during RIs; however, internal dynamical processes appear to play a more critical role when a favorable environment is present.

scholarly journals Research Aircraft and Wind Profiler Observations in Gastineau Channel during a Taku Wind Event*

2006 ◽  
Vol 21 (4) ◽  
pp. 489-501 ◽  
Author(s):  
Nicholas A. Bond ◽  
Carl F. Dierking ◽  
James D. Doyle
Keyword(s):  
Wind Shear ◽  
Vertical Wind Shear ◽  
Vertical Wind ◽  
Naval Research ◽  
Wind Profiler ◽  
Mean Flow ◽  
Gap Flow ◽  
Wind Event ◽  
Research Aircraft ◽  
Downslope Flow

Abstract The flow in Gastineau Channel near Juneau, Alaska, during the moderate Taku wind event of 18 October 2004 is examined using observations from the University of Wyoming’s King Air research aircraft, two wind profilers, and surface weather stations. These data sources reveal low-level winds directed down the central portion of Gastineau Channel, that is, gap flow. Farther down the channel, and above this gap flow, the winds were strongly cross channel in association with the downslope flow that characterizes Taku events. The transition region between these two flows included strong vertical wind shear and severe turbulence; measurements from the King Air indicate turbulent kinetic energy locally exceeding 50 m2 s−2. A high-resolution simulation of this case using the Naval Research Laboratory’s Coupled Ocean–Atmosphere Mesoscale Prediction System reproduced the observed character of the mean flow. This case illustrates the hazard to aviation posed by even a moderate Taku wind event and shows the value of a wind profiler for monitoring the vertical wind shear responsible for the hazard.

scholarly journals Projections of the Tropical Atlantic Vertical Wind Shear and Its Relationship with ENSO in SP-CCSM4

2014 ◽  
Vol 27 (22) ◽  
pp. 8342-8356 ◽  
Author(s):  
Xiaojie Zhu ◽  
Li Xu ◽  
Cristiana Stan
Keyword(s):  
Wind Shear ◽  
Linear Trend ◽  
Vertical Wind Shear ◽  
Vertical Motion ◽  
Walker Circulation ◽  
Vertical Wind ◽  
Tropical Atlantic ◽  
Convective Activity ◽  
Equatorial Atlantic ◽  
Climate System Model

Abstract The vertical wind shear over the tropical Atlantic Ocean and its relationship with ENSO are analyzed in the superparameterized Community Climate System Model, version 4 (SP-CCSM4) and in the conventional CCSM4. The climatology of vertical wind shear over the tropical Atlantic and the ENSO–shear relationship are well simulated in the control runs of SP-CCSM4 and CCSM4. However, because of different representations of cloud processes, in a warmer climate such as the representative concentration pathway 8.5 (RCP8.5) scenario, SP-CCSM4 projects increased mean westerlies at 200 hPa during July through October (JASO), whereas CCSM4 projects decreased mean westerlies at 200 hPa over the equatorial Atlantic. The different changes in the upper-level wind further contribute to different projection of JASO mean vertical wind shear over the equatorial Atlantic. In the RCP8.5 scenario, when excluding the linear trend, projection of the ENSO–shear relationships by SP-CCSM4 retains similar features as in the observed current climate, whereas the ENSO–shear relationship projected by CCSM4 indicates an increase in the vertical wind shear dominating the tropical Atlantic during El Niño events. The difference in projection of ENSO–shear relationship is, to a certain extent, related to the different response of the tropical Atlantic SST to ENSO. Analysis of the climate change projection of Walker circulation, cloud cover, and convective activity illustrates that superparameterization simulates a stronger suppression of African convection than the conventional parameterization of moist processes. The weak convective activity diminishes the divergent wind associated with the vertical motion, which contributes to increased westerlies projected in SP-CCSM4.

scholarly journals Testing Vertical Wind Shear and Nonlinear MJO/ENSO Interactions as Predictors for Subseasonal Atlantic Tropical Cyclone Forecasts

2022 ◽  
Author(s):  
Matthew A. Janiga
Keyword(s):  
Wave Breaking ◽  
Wind Shear ◽  
Southern Oscillation ◽  
Vertical Wind Shear ◽  
Vertical Wind ◽  
Lead Times ◽  
Madden Julian Oscillation ◽  
Rossby Wave Breaking ◽  
Non Linear ◽  
Linear Interactions

Abstract Hansen et al. (2020) found patterns of vertical wind shear, relative humidity (RH) and non-linear interactions between the Madden-Julian Oscillation and El Niño-Southern Oscillation that impact subseasonal Atlantic TC activity. We test whether these patterns can be used to improve subseasonal predictions. To do this we build a statistical-dynamical hybrid model using Navy-ESPC reforecasts as a part of the SUBX project. By adding and removing Navy-ESPC reforecasted values of predictors from a logistic regression model, we assess the contribution of skill from each predictor. We find that Atlantic SSTs and the MJO are the most important factors governing subseasonal Atlantic TC activity. RH contributes little to subseasonal TC predictions, however, shear predictors improve forecast skill at 5-10 day lead times, before forecast shear errors become too large. Non-linear MJO/ENSO interactions did not improve skill compared to separate linear considerations of these factors but did improve the reliability of predictions for high-probability active TC periods. Both non-linear MJO/ENSO interactions and the subseasonal shear signal appear linked to PV streamer activity. This study suggests that correcting model shear biases and improving representation of Rossby wave-breaking is the most efficient way to improve subseasonal Atlantic TC forecasts.

scholarly journals A new paradigm for intensity modification of tropical cyclones: thermodynamic impact of vertical wind shear on the inflow layer

2009 ◽  
Vol 9 (3) ◽  
pp. 10711-10775 ◽  
Author(s):  
M. Riemer ◽  
M. T. Montgomery ◽  
M. E. Nicholls
Keyword(s):  
Boundary Layer ◽  
Tropical Cyclones ◽  
Wind Shear ◽  
Vertical Wind Shear ◽  
Vertical Wind ◽  
Vertical Shear ◽  
Carnot Cycle ◽  
Close Link ◽  
Storm Intensity ◽  
Intensity Evolution

Abstract. An important roadblock to improved intensity forecasts for tropical cyclones (TCs) is our incomplete understanding of the interaction of a TC with the environmental flow. In this paper we re-visit the classical idealised numerical experiment of tropical cyclones (TCs) in vertical wind shear on an f-plane. We employ a set of simplified model physics – a simple bulk aerodynamic boundary layer scheme and "warm rain" microphysics – to foster better understanding of the dynamics and thermodynamics that govern the modification of TC intensity. A suite of experiments is performed with intense TCs in moderate to strong vertical shear. In all experiments the TC is resilient to shear but significant differences in the intensity evolution occur. The ventilation of the TC core with dry environmental air at mid-levels and the dilution of the upper-level warm core are two prevailing hypotheses for the adverse effect of vertical shear on storm intensity. Here we propose an alternative and arguably more effective mechanism how cooler and drier (lower θe) air – "anti-fuel" for the TC power machine – can enter the core region of the TC. Strong and persistent downdrafts flux low θe air from the lower and middle troposphere into the boundary layer, significantly depressing the θe values in the storm's inflow layer. Air with lower θe values enters the eyewall updrafts, considerably reducing eyewall θe values in the azimuthal mean. When viewed from the perspective of an idealised Carnot-cycle heat engine a decrease of storm intensity can thus be expected. Although the Carnot cycle model is – if at all – only valid for stationary and axisymmetric TCs, a strong correlation between the downward transport of low θe into the boundary layer and the intensity evolution offers further evidence in support of our hypothesis. The downdrafts that flush the inflow layer with low θe air are associated with a quasi-stationary region of convective activity outside the TC's eyewall. We show evidence that, to zero order, the formation of the convective asymmetry is driven by the balanced dynamical response of the TC vortex to the vertical shear forcing. Thus a close link is provided between the thermodynamic impact in the near-core boundary layer and the balanced dynamics governing the TC vortex evolution.

scholarly journals Polar Lows – Moist Baroclinic Cyclones Developing in Four Different Vertical Wind Shear Environments

2020 ◽  
Author(s):  
Patrick Johannes Stoll ◽  
Thomas Spengler ◽  
Annick Terpstra ◽  
Rune Grand Graversen
Keyword(s):  
Wind Shear ◽  
Vertical Wind Shear ◽  
Vertical Wind ◽  
Vertical Shear ◽  
Polar Lows ◽  
North East ◽  
The North ◽  
Strong Shear ◽  
Polar Low ◽  
Mesoscale Cyclones

Abstract. Polar lows are intense mesoscale cyclones that develop in polar marine air masses. Motivated by the large variety in their proposed intensification mechanisms, cloud structure, and ambient sub-synoptic environment, we use self-organising maps to classify polar lows. The method is applied to 370 polar lows in the North-East Atlantic, which were obtained by matching mesoscale cyclones from the ERA-5 reanalysis to polar lows registered by the Norwegian Meteorological Institute in the STARS dataset. ERA-5 reproduces 93 % of the STARS polar lows. We identify five different polar-low configurations, which are characterised by the vertical wind shear vector relative to the propagation direction. Four categories feature a strong shear with different orientations of the shear vector, whereas the fifth category contains conditions with weak shear. The orientation of the vertical-shear vector for the strong shear categories determines the dynamics of the systems, confirming the relevance of the previously identified categorisation into forward and reverse-shear polar lows. In addition, we expand the categorisation with right and left-shear polar lows that propagate towards colder and warmer environments, respectively. Polar lows in the four strong shear categories feature an up-shear tilt in the vertical, typical for the intensification through moist baroclinic processes. As weak-shear conditions mainly occur at the mature or lysis stage of polar lows, we find no evidence for hurricane-like development and propose that spirali-form PLs are most likely associated with a warm seclusion process.

scholarly journals The Unexpected Rapid Intensification of Tropical Cyclones in Moderate Vertical Wind Shear. Part II: Vortex Tilt

2018 ◽  
Vol 146 (11) ◽  
pp. 3801-3825 ◽  
Author(s):  
David R. Ryglicki ◽  
James D. Doyle ◽  
Yi Jin ◽  
Daniel Hodyss ◽  
Joshua H. Cossuth
Keyword(s):  
Tropical Cyclones ◽  
Wind Shear ◽  
Deep Convection ◽  
Vertical Wind Shear ◽  
Vertical Wind ◽  
Rapid Intensification ◽  
Model Simulations ◽  
Observational Analysis ◽  
Idealized Model ◽  
Cold Anomaly

Abstract We investigate a class of tropical cyclones (TCs) that undergo rapid intensification (RI) in moderate vertical wind shear through analysis of a series of idealized model simulations. Two key findings derived from observational analysis are that the average 200–850-hPa shear value is 7.5 m s−1 and that the TCs displayed coherent cloud structures, deemed tilt-modulated convective asymmetries (TCA), which feature pulses of deep convection with periods of between 4 and 8 h. Additionally, all of the TCs are embedded in an environment that is characterized by shear associated with anticyclones, a factor that limits depth of the strongest environmental winds in the vertical. The idealized TC develops in the presence of relatively shallow environmental wind shear of an anticyclone. An analysis of the TC tilt in the vertical demonstrates that the source of the observed 4–8-h periodicity of the TCAs can be explained by smaller-scale nutations of the tilt on the longer, slower upshear precession. When the environmental wind shear occurs over a deeper layer similar to that of a trough, the TC does not develop. The TCAs are characterized as collections of updrafts that are buoyant throughout the depth of the TC since they rise into a cold anomaly caused by the tilting vortex. At 90 h into the simulation, RI occurs, and the tilt nutations (and hence the TCAs) cease to occur.

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