Impact of Variations in Upper-Level Shear on Simulated Supercells

It has previously been suggested, based on limited observations, that vertical wind shear in the upper troposphere is a key control on supercell morphology, with the low-precipitation, high-precipitation, and classic archetypes favored under strong, weak, and moderate shear, respectively. The idea is that, with increasing upper-level shear (ULS), hydrometeors are transported farther from the updraft by stronger storm-relative anvil-level winds, limiting their growth and thereby reducing precipitation intensity. The present study represents the first attempt to test this hypothesis, using idealized simulations of supercells performed across a range of 6–12-km shear profiles. Contrary to expectations, there is a significant increase in surface precipitation and an associated strengthening of outflow winds as ULS magnitude is increased from 0 to 20 m s−1. These changes result from an increase in storm motion, which drives stronger low-level inflow, a wider updraft, and enhanced condensation. A further increase in ULS magnitude to 30 m s−1 promotes a slight reduction in storm intensity associated with surging rear-flank outflow. However, this transition in behavior is found to be sensitive to other factors that influence cold-pool strength, such as mixed-layer depth and model microphysics. Variations in the vertical distribution and direction of ULS are also considered, but are found to have a much smaller impact on storm intensity than variations in ULS magnitude. Suggestions for the disparity between the current results and the aforementioned observations are offered and the need for further research on supercell morphology—in particular, simulations in drier environments—is emphasized.

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Observations and simulation of intense convection embedded in a warm conveyor belt – how ambient vertical wind shear determines the dynamical impact

10.5194/wcd-2020-49 ◽

2020 ◽

Author(s):

Annika Oertel ◽

Michael Sprenger ◽

Hanna Joos ◽

Maxi Boettcher ◽

Heike Konow ◽

...

Keyword(s):

Case Studies ◽

Wind Shear ◽

Vertical Wind Shear ◽

Cloud Band ◽

Radar Observations ◽

Upper Troposphere ◽

Surface Precipitation ◽

Upper Level ◽

Upper Level Jet

Abstract. Warm conveyor belts (WCBs) are dynamically important, strongly ascending and mostly stratiform cloud-forming airstreams in extratropical cyclones. Despite the predominantly stratiform character of the WCB's large-scale cloud band, convective clouds can be embedded in it. This embedded convection leads to a heterogeneously structured cloud band with locally enhanced hydrometeor content, intense surface precipitation and substantial amounts of graupel in the middle troposhere. Recent studies showed that embedded convection forms dynamically relevant quasi-horizontal potential vorticity (PV) dipoles in the upper troposphere. Thereby one pole can reach strongly negative PV values associated with inertial or symmetric instability near the upper-level PV waveguide, where it can interact with and modify the upper-level jet. This study analyses the characteristics of embedded convection in the WCB of cyclone Sanchez based on WCB online trajectories from a convection-permitting simulation and airborne radar observations during the North Atlantic Waveguide and Downstream Impact EXperiment (NAWDEX) field campaign (IOPs 10 and 11). In the first part, we present the radar reflectivity structure of the WCB and corroborate its heterogeneous cloud structure and the occurrence of embedded convection. Radar observations in three different sub-regions of the WCB cloud band reveal the differing intensity of its embedded convection, which is qualitatively confirmed by the ascent rates of the online WCB trajectories. The detailed ascent behaviour of the WCB trajectories reveals that very intense convection with ascent rates of 600 hPa in 30–60 min occurs, in addition to comparatively moderate convection with slower ascent velocities as reported in previous case studies. In the second part of this study, a systematic Lagrangian composite analysis based on online trajectories for two sub-categories of WCB-embedded convection – moderate and intense convection – is performed. Composites of the cloud and precipitation structure confirm the large influence of embedded convection: Intense convection produces locally very intense surface precipitation with peak values exceeding 6 mm in 15 minutes and large amounts of graupel of up to 2.8 g kg−1 in the middle troposphere (compared to 3.9 mm and 1.0 g kg−2 for the moderate convective WCB sub-category). In the upper troposphere, both convective WCB trajectory sub-categories form a small-scale and weak PV dipole, with one pole reaching weakly negative PV values. However, for this WCB case study – in contrast to previous case studies reporting convective PV dipoles in the WCB ascent region with the negative PV pole near the upper-level jet – the negative PV pole is located east of the convective ascent region, i.e., away from the upper-level jet. Moreover, the PV dipole formed by the intense convective WCB trajectories is weaker and has a smaller horizontal and vertical extent compared to a previous NAWDEX case study of WCB-embedded convection, despite faster ascent rates in this case. The absence of a strong upper-level jet and the weak vertical shear of the ambient wind in cyclone Sanchez are accountable for the weak diabatic PV modification in the upper troposphere. This implies that the strength of embedded convection alone is not a reliable measure for the effect of embedded convection on upper-level PV modification and its impact on the upper-level jet. Instead, the profile of vertical wind shear and the alignment of embedded convection with a strong upper-level jet play a key role for the formation of coherent negative PV features near the jet. Finally, these results highlight the large case-to-case variability of embedded convection not only in terms of frequency and intensity of embedded convection in WCBs but also in terms of its dynamical implications.

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Observations and simulation of intense convection embedded in a warm conveyor belt – how ambient vertical wind shear determines the dynamical impact

Weather and Climate Dynamics ◽

10.5194/wcd-2-89-2021 ◽

2021 ◽

Vol 2 (1) ◽

pp. 89-110 ◽

Cited By ~ 2

Author(s):

Annika Oertel ◽

Michael Sprenger ◽

Hanna Joos ◽

Maxi Boettcher ◽

Heike Konow ◽

...

Keyword(s):

Case Studies ◽

Wind Shear ◽

Vertical Wind Shear ◽

Cloud Band ◽

Upper Troposphere ◽

Middle Troposphere ◽

Surface Precipitation ◽

Upper Level ◽

Upper Level Jet

Abstract. Warm conveyor belts (WCBs) are dynamically important, strongly ascending and mostly stratiform cloud-forming airstreams in extratropical cyclones. Despite the predominantly stratiform character of the WCB's large-scale cloud band, convective clouds can be embedded in it. This embedded convection leads to a heterogeneously structured cloud band with locally enhanced hydrometeor content, intense surface precipitation and substantial amounts of graupel in the middle troposphere. Recent studies showed that embedded convection forms dynamically relevant quasi-horizontal potential vorticity (PV) dipoles in the upper troposphere. Thereby one pole can reach strongly negative PV values associated with inertial or symmetric instability near the upper-level PV waveguide, where it can interact with and modify the upper-level jet. This study analyzes the characteristics of embedded convection in the WCB of cyclone Sanchez based on WCB online trajectories from a convection-permitting simulation and airborne radar observations during the North Atlantic Waveguide and Downstream Impact Experiment (NAWDEX) field campaign (intense observation periods, IOPs, 10 and 11). In the first part, we present the radar reflectivity structure of the WCB and corroborate its heterogeneous cloud structure and the occurrence of embedded convection. Radar observations in three different sub-regions of the WCB cloud band reveal the differing intensity of its embedded convection, which is qualitatively confirmed by the ascent rates of the online WCB trajectories. The detailed ascent behavior of the WCB trajectories reveals that very intense convection with ascent rates of 600 hPa in 30–60 min occurs, in addition to comparatively moderate convection with slower ascent velocities as reported in previous case studies. In the second part of this study, a systematic Lagrangian composite analysis based on online trajectories for two sub-categories of WCB-embedded convection – moderate and intense convection – is performed. Composites of the cloud and precipitation structure confirm the large influence of embedded convection: intense convection produces very intense local surface precipitation with peak values exceeding 6 mm in 15 min and large amounts of graupel of up to 2.8 g kg−1 in the middle troposphere (compared to 3.9 mm and 1.0 g kg−1 for the moderate convective WCB sub-category). In the upper troposphere, both convective WCB trajectory sub-categories form a small-scale and weak PV dipole, with one pole reaching weakly negative PV values. However, for this WCB case study – in contrast to previous case studies reporting convective PV dipoles in the WCB ascent region with the negative PV pole near the upper-level jet – the negative PV pole is located east of the convective ascent region, i.e., away from the upper-level jet. Moreover, the PV dipole formed by the intense convective WCB trajectories is weaker and has a smaller horizontal and vertical extent compared to a previous NAWDEX case study of WCB-embedded convection, despite faster ascent rates in this case. The absence of a strong upper-level jet and the weak vertical shear of the ambient wind in cyclone Sanchez are accountable for the weak diabatic PV modification in the upper troposphere. This implies that the strength of embedded convection alone is not a reliable measure for the effect of embedded convection on upper-level PV modification and its impact on the upper-level jet. Instead, the profile of vertical wind shear and the alignment of embedded convection with a strong upper-level jet play a key role for the formation of coherent negative PV features near the jet. Finally, these results highlight the large case-to-case variability of embedded convection not only in terms of frequency and intensity of embedded convection in WCBs but also in terms of its dynamical implications.

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Effect of TC–Trough Interaction on the Intensity Change of Two Typhoons

Weather and Forecasting ◽

10.1175/waf836.1 ◽

2005 ◽

Vol 20 (2) ◽

pp. 199-211 ◽

Cited By ~ 16

Author(s):

Hui Yu ◽

H. Joe Kwon

Keyword(s):

Tropical Cyclone ◽

Wind Shear ◽

Large Scale ◽

Vertical Wind Shear ◽

Vertical Wind ◽

Intensity Change ◽

Eddy Flux ◽

Upper Troposphere ◽

Shallow Layer ◽

Upper Level

Abstract Using large-scale analyses, the effect of tropical cyclone–trough interaction on tropical cyclone (TC) intensity change is readdressed by studying the evolution of upper-level eddy flux convergence (EFC) of angular momentum and vertical wind shear for two TCs in the western North Pacific [Typhoons Prapiroon (2000) and Olga (1999)]. Major findings include the following: 1) In spite of decreasing SST, the cyclonic inflow associated with a midlatitude trough should have played an important role in Prapiroon’s intensification to its maximum intensity and the maintenance after recurvature through an increase in EFC. The accompanied large vertical wind shear is concentrated in a shallow layer in the upper troposphere. 2) Although Olga also recurved downstream of a midlatitude trough, its development and maintenance were not strongly influenced by the trough. A TC could maintain itself in an environment with or without upper-level eddy momentum forcing. 3) Both TCs started to decay over cold SST in a large EFC and vertical wind shear environment imposed by the trough. 4) Uncertainty of input adds difficulties in quantitative TC intensity forecasting.

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Synergetic Use of the WSR-88D Radars, GOES-R Satellites, and Lightning Networks to Study Microphysical Characteristics of Hurricanes

Journal of Applied Meteorology and Climatology ◽

10.1175/jamc-d-19-0122.1 ◽

2020 ◽

Vol 59 (6) ◽

pp. 1051-1068 ◽

Cited By ~ 1

Author(s):

Jiaxi Hu ◽

Daniel Rosenfeld ◽

Alexander Ryzhkov ◽

Pengfei Zhang

Keyword(s):

Vertical Wind Shear ◽

Vertical Motion ◽

Precipitation Pattern ◽

The Novel ◽

Surface Precipitation ◽

Wind Speeds ◽

Warm Rain ◽

Mapping Array ◽

Upper Level ◽

Observational Techniques

AbstractThis study analyzes the microphysics and precipitation pattern of Hurricanes Harvey (2017) and Florence (2018) in both the eyewall and outer rainband regions. From the retrievals by a satellite red–green–blue scheme, the outer rainbands show a strong convective structure while the inner eyewall has less convective vigor (i.e., weaker upper-level reflectivities and electrification), which may be related to stronger vertical wind shear that hinders fast vertical motions. The WSR-88D column-vertical profiles further confirm that the outer rainband clouds have strong vertical motion and large ice-phase hydrometeor formation aloft, which correlates well with 3D Lightning Mapping Array source counts in height and time. From the results from this study, it is determined that the inner eyewall region is dominated by warm rain, whereas the external rainband region contains intense mixed-phase precipitation. External rainbands are defined here as those that reside outside of the main hurricane circulation, associated with surface tropical storm wind speeds. The synergy of satellite and radar dual-polarization parameters is instrumental in distinguishing between the key microphysical features of intense convective rainbands and the warm-rain-dominated eyewall regions within the hurricanes. Substantial amounts of ice aloft and intense updrafts in the external rainbands are indicative of heavy surface precipitation, which can have important implications for severe weather warnings and quantitative precipitation forecasts. The novel part of this study is to combine ground-based radar measurement with satellite observations to study hurricane microphysical structure from surface to cloud top so as to fill in the gaps between the two observational techniques.

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Environmental Factors in the Upscale Growth and Longevity of MCSs Derived from Rapid Update Cycle Analyses

Monthly Weather Review ◽

10.1175/2010mwr3233.1 ◽

2010 ◽

Vol 138 (9) ◽

pp. 3514-3539 ◽

Cited By ~ 56

Author(s):

Michael C. Coniglio ◽

Jason Y. Hwang ◽

David J. Stensrud

Keyword(s):

Potential Vorticity ◽

Wind Shear ◽

Vertical Wind Shear ◽

Cold Pool ◽

Inertial Oscillation ◽

Low Level ◽

Convective Systems ◽

Mesoscale Convective ◽

Rapid Transition ◽

Upper Level

Abstract Composite environments of mesoscale convective systems (MCSs) are produced from Rapid Update Cycle (RUC) analyses to explore the differences between rapidly and slowly developing MCSs as well as the differences ahead of long- and short-lived MCSs. The composite analyses capture the synoptic-scale features known to be associated with MCSs and depict the inertial oscillation of the nocturnal low-level jet (LLJ), which remains strong but tends to veer away from decaying MCSs. The composite first storms environment for the rapidly developing MCSs contains a stronger LLJ located closer to the first storms region, much more conditional instability, potential instability, and energy available for downdrafts, smaller 3–10-km vertical wind shear, and smaller geostrophic potential vorticity in the upper troposphere, when compared to the environment for the slowly developing MCSs. The weaker shear above 3 km for the rapidly developing MCSs is consistent with supercell or discrete cell modes being less likely in weaker deep-layer shear and the greater potential for a cold pool to trigger convection when the shear is confined to lower levels. Furthermore, these results suggest that low values of upper-level potential vorticity may signal a rapid transition to an MCS. The composite environment ahead of the genesis of long-lived MCSs contains a broader LLJ, a better-defined frontal zone, stronger low-level frontogenesis, deeper moisture, and stronger wind shear above 2 km, when compared to short-lived MCSs. The larger shear above 2 km for the long-lived MCSs is consistent with the importance of shear elevated above the ground to help organize and maintain convection that feeds on the elevated unstable parcels after dark and is indicative of the enhanced baroclinicity ahead of the MCSs.

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Effects of Upper-Level Shear on the Structure and Maintenance of Strong Quasi-Linear Mesoscale Convective Systems

Journal of the Atmospheric Sciences ◽

10.1175/jas3681.1 ◽

2006 ◽

Vol 63 (4) ◽

pp. 1231-1252 ◽

Cited By ~ 83

Author(s):

Michael C. Coniglio ◽

David J. Stensrud ◽

Louis J. Wicker

Keyword(s):

Three Dimensional ◽

Vertical Wind Shear ◽

Vertical Displacement ◽

Mesoscale Convective Systems ◽

Cold Pool ◽

Density Currents ◽

3D Simulations ◽

Convective Systems ◽

Mesoscale Convective ◽

Upper Level

Abstract Recent observational studies have shown that strong midlatitude mesoscale convective systems (MCSs) tend to decay as they move into environments with less instability and smaller deep-layer vertical wind shear. These observed shear profiles that contain significant upper-level shear are often different from the shear profiles considered to be the most favorable for the maintenance of strong, long-lived convective systems in some past idealized simulations. Thus, to explore the role of upper-level shear in strong MCS environments, a set of two-dimensional (2D) simulations of density currents within a dry, statically neutral environment is used to quantify the dependence of lifting along an idealized cold pool on the upper-level shear. A set of three-dimensional (3D) simulations of MCSs is produced to gauge the effects of the upper-level shear in a more realistic framework. Results from the 2D experiments show that the addition of upper-level shear to a wind profile with weak to moderate low-level shear increases the vertical displacement of parcels despite a decrease in the vertical velocity along the cold pool interface. Parcels that are elevated above the surface (1–2 km) overturn and are responsible for the deep lifting in the deep-shear environments, while the surface-based parcels typically are lifted through the cold pool region in a rearward-sloping path. This deep overturning helps to maintain the leading convection and greatly increases the size and total precipitation output of the convective systems in more complex 3D simulations, even in the presence of 3D structures. These results show that the shear profile throughout the entire troposphere must be considered to gain a more complete understanding of the structure and maintenance of strong midlatitude MCSs.

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Dynamical and Physical Processes Leading to Tropical Cyclone Intensification under Upper-Level Trough Forcing

Journal of the Atmospheric Sciences ◽

10.1175/jas-d-12-0293.1 ◽

2013 ◽

Vol 70 (8) ◽

pp. 2547-2565 ◽

Cited By ~ 22

Author(s):

Marie-Dominique Leroux ◽

Matthieu Plu ◽

David Barbary ◽

Frank Roux ◽

Philippe Arbogast

Keyword(s):

Angular Momentum ◽

Tropical Cyclone ◽

Potential Vorticity ◽

Weather Prediction ◽

Three Dimensional ◽

Vertical Wind Shear ◽

Thick Layer ◽

Limited Area ◽

Southwest Indian Ocean ◽

Upper Level

Abstract The rapid intensification of Tropical Cyclone (TC) Dora (2007, southwest Indian Ocean) under upper-level trough forcing is investigated. TC–trough interaction is simulated using a limited-area operational numerical weather prediction model. The interaction between the storm and the trough involves a coupled evolution of vertical wind shear and binary vortex interaction in the horizontal and vertical dimensions. The three-dimensional potential vorticity structure associated with the trough undergoes strong deformation as it approaches the storm. Potential vorticity (PV) is advected toward the tropical cyclone core over a thick layer from 200 to 500 hPa while the TC upper-level flow turns cyclonic from the continuous import of angular momentum. It is found that vortex intensification first occurs inside the eyewall and results from PV superposition in the thick aforementioned layer. The main pathway to further storm intensification is associated with secondary eyewall formation triggered by external forcing. Eddy angular momentum convergence and eddy PV fluxes are responsible for spinning up an outer eyewall over the entire troposphere, while spindown is observed within the primary eyewall. The 8-km-resolution model is able to reproduce the main features of the eyewall replacement cycle observed for TC Dora. The outer eyewall intensifies further through mean vertical advection under dynamically forced upward motion. The processes are illustrated and quantified using various diagnostics.

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The Unexpected Rapid Intensification of Tropical Cyclones in Moderate Vertical Wind Shear. Part I: Overview and Observations

Monthly Weather Review ◽

10.1175/mwr-d-18-0020.1 ◽

2018 ◽

Vol 146 (11) ◽

pp. 3773-3800 ◽

Cited By ~ 13

Author(s):

David R. Ryglicki ◽

Joshua H. Cossuth ◽

Daniel Hodyss ◽

James D. Doyle

Keyword(s):

Tropical Cyclones ◽

Wind Shear ◽

Vertical Wind Shear ◽

Convective Cloud ◽

Vertical Wind ◽

Rapid Intensification ◽

Brightness Temperatures ◽

Idealized Modeling ◽

Hurricane Prediction ◽

Upper Level

Abstract A satellite-based investigation is performed of a class of tropical cyclones (TCs) that unexpectedly undergo rapid intensification (RI) in moderate vertical wind shear between 5 and 10 m s−1 calculated as 200–850-hPa shear. This study makes use of both infrared (IR; 11 μm) and water vapor (WV; 6.5 μm) geostationary satellite data, the Statistical Hurricane Prediction Intensity System (SHIPS), and model reanalyses to highlight commonalities of the six TCs. The commonalities serve as predictive guides for forecasters and common features that can be used to constrain and verify idealized modeling studies. Each of the TCs exhibits a convective cloud structure that is identified as a tilt-modulated convective asymmetry (TCA). These TCAs share similar shapes, upshear-relative positions, and IR cloud-top temperatures (below −70°C). They pulse over the core of the TC with a periodicity of between 4 and 8 h. Using WV satellite imagery, two additional features identified are asymmetric warming/drying upshear of the TC relative to downshear, as well as radially thin arc-shaped clouds on the upshear side. The WV brightness temperatures of these arcs are between −40° and −60°C. All of the TCs are sheared by upper-level anticyclones, which limits the strongest environmental winds to near the tropopause.

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Practical and Intrinsic Predictability of Severe and Convective Weather at the Mesoscales

Journal of the Atmospheric Sciences ◽

10.1175/jas-d-11-0315.1 ◽

2012 ◽

Vol 69 (11) ◽

pp. 3350-3371 ◽

Cited By ~ 72

Author(s):

Christopher Melhauser ◽

Fuqing Zhang

Keyword(s):

Weather Prediction ◽

Squall Line ◽

Cold Pool ◽

Initial Condition ◽

Large Variability ◽

Forecast Performance ◽

Sensitivity Experiments ◽

Bow Echo ◽

Upper Level ◽

Convective Weather

Abstract This study explores both the practical and intrinsic predictability of severe convective weather at the mesoscales using convection-permitting ensemble simulations of a squall line and bow echo event during the Bow Echo and Mesoscale Convective Vortex (MCV) Experiment (BAMEX) on 9–10 June 2003. Although most ensemble members—initialized with realistic initial condition uncertainties smaller than the NCEP Global Forecast System Final Analysis (GFS FNL) using an ensemble Kalman filter—forecast broad areas of severe convection, there is a large variability of forecast performance among different members, highlighting the limit of practical predictability. In general, the best-performing members tend to have a stronger upper-level trough and associated surface low, producing a more conducive environment for strong long-lived squall lines and bow echoes, once triggered. The divergence in development is a combination of a dislocation of the upper-level trough, surface low with corresponding marginal environmental differences between developing and nondeveloping members, and cold pool evolution by deep convection prior to squall line formation. To further explore the intrinsic predictability of the storm, a sequence of sensitivity experiments was performed with the initial condition differences decreased to nearly an order of magnitude smaller than typical analysis and observation errors. The ensemble forecast and additional sensitivity experiments demonstrate that this storm has a limited practical predictability, which may be further improved with more accurate initial conditions. However, it is possible that the true storm could be near the point of bifurcation, where predictability is intrinsically limited. The limits of both practical and intrinsic predictability highlight the need for probabilistic and ensemble forecasts for severe weather prediction.

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Comparison of Two-Moment Bulk Microphysics Schemes in Idealized Supercell Thunderstorm Simulations

Monthly Weather Review ◽

10.1175/2010mwr3433.1 ◽

2011 ◽

Vol 139 (4) ◽

pp. 1103-1130 ◽

Cited By ~ 126

Author(s):

Hugh Morrison ◽

Jason Milbrandt

Keyword(s):

Three Dimensional ◽

Wrf Model ◽

Weather Research And Forecasting ◽

Cold Pool ◽

Fall Velocity ◽

Surface Precipitation ◽

Single Category ◽

Supercell Thunderstorm ◽

Drop Size Distributions ◽

Reduced Surface

Idealized three-dimensional supercell simulations were performed using the two-moment bulk microphysics schemes of Morrison and Milbrandt–Yau in the Weather Research and Forecasting (WRF) model. Despite general similarities in these schemes, the simulations were found to produce distinct differences in storm structure, precipitation, and cold pool strength. In particular, the Morrison scheme produced much higher surface precipitation rates and a stronger cold pool, especially in the early stages of storm development. A series of sensitivity experiments was conducted to identify the primary differences between the two schemes that resulted in the large discrepancies in the simulations. Different approaches in treating graupel and hail were found to be responsible for many of the key differences between the baseline simulations. The inclusion of hail in the baseline simulation using the Milbrant–Yau scheme with two rimed-ice categories (graupel and hail) had little impact, and therefore resulted in a much different storm than the baseline run with the single-category (hail) Morrison scheme. With graupel as the choice of the single rimed-ice category, the simulated storms had considerably more frozen condensate in the anvil region, a weaker cold pool, and reduced surface precipitation compared to the runs with only hail, whose higher terminal fall velocity inhibited lofting. The cold pool strength was also found to be sensitive to the parameterization of raindrop breakup, particularly for the Morrison scheme, because of the effects on the drop size distributions and the corresponding evaporative cooling rates. The use of a more aggressive implicit treatment of drop breakup in the baseline Morrison scheme, by limiting the mean–mass raindrop diameter to a maximum of 0.9 mm, opposed the tendency of this scheme to otherwise produce large mean drop sizes and a weaker cold pool compared to the hail-only run using the Milbrandt–Yau scheme.

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