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

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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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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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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Impact of Variations in Upper-Level Shear on Simulated Supercells

Monthly Weather Review ◽

10.1175/mwr-d-16-0412.1 ◽

2017 ◽

Vol 145 (7) ◽

pp. 2659-2681 ◽

Cited By ~ 16

Author(s):

Robert A. Warren ◽

Harald Richter ◽

Hamish A. Ramsay ◽

Steven T. Siems ◽

Michael J. Manton

Keyword(s):

Mixed Layer Depth ◽

Vertical Wind Shear ◽

Cold Pool ◽

Slight Reduction ◽

Upper Troposphere ◽

Surface Precipitation ◽

Storm Intensity ◽

Upper Level ◽

High Precipitation ◽

The Vertical Distribution

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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Atlantic Hurricanes and Climate Change. Part II: Role of Thermodynamic Changes in Decreased Hurricane Frequency

Journal of Climate ◽

10.1175/jcli-d-12-00183.1 ◽

2013 ◽

Vol 26 (21) ◽

pp. 8513-8528 ◽

Cited By ~ 8

Author(s):

Megan S. Mallard ◽

Gary M. Lackmann ◽

Anantha Aiyyer

Keyword(s):

Case Studies ◽

Wind Shear ◽

Vertical Wind Shear ◽

Vertical Wind ◽

Effect Of Temperature ◽

Physical Processes ◽

Thermodynamic Conditions ◽

Atlantic Hurricanes ◽

Reduced Activity

Abstract A method of downscaling that isolates the effect of temperature and moisture changes on tropical cyclone (TC) activity was presented in Part I of this study. By applying thermodynamic modifications to analyzed initial and boundary conditions from past TC seasons, initial disturbances and the strength of synoptic-scale vertical wind shear are preserved in future simulations. This experimental design allows comparison of TC genesis events in the same synoptic setting, but in current and future thermodynamic environments. Simulations of both an active (September 2005) and inactive (September 2009) portion of past hurricane seasons are presented. An ensemble of high-resolution simulations projects reductions in ensemble-average TC counts between 18% and 24%, consistent with previous studies. Robust decreases in TC and hurricane counts are simulated with 18- and 6-km grid lengths, for both active and inactive periods. Physical processes responsible for reduced activity are examined through comparison of monthly and spatially averaged genesis-relevant parameters, as well as case studies of development of corresponding initial disturbances in current and future thermodynamic conditions. These case studies show that reductions in TC counts are due to the presence of incipient disturbances in marginal moisture environments, where increases in the moist entropy saturation deficits in future conditions preclude genesis for some disturbances. Increased convective inhibition and reduced vertical velocity are also found in the future environment. It is concluded that a robust decrease in TC frequency can result from thermodynamic changes alone, without modification of vertical wind shear or the number of incipient disturbances.

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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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The Unexpected Rapid Intensification of Tropical Cyclones in Moderate Vertical Wind Shear. Part III: Outflow–Environment Interaction

Monthly Weather Review ◽

10.1175/mwr-d-18-0370.1 ◽

2019 ◽

Vol 147 (8) ◽

pp. 2919-2940 ◽

Cited By ~ 7

Author(s):

David R. Ryglicki ◽

James D. Doyle ◽

Daniel Hodyss ◽

Joshua H. Cossuth ◽

Yi Jin ◽

...

Keyword(s):

Wind Shear ◽

Vertical Wind Shear ◽

Environmental Flow ◽

Vertical Wind ◽

Environment Interaction ◽

Rapid Intensification ◽

Field Matching ◽

Upper Level ◽

Bernoulli Flow ◽

Dynamic High Pressure

Abstract Interactions between the upper-level outflow of a sheared, rapidly intensifying tropical cyclone (TC) and the background environmental flow in an idealized model are presented. The most important finding is that the divergent outflow from convection localized by the tilt of the vortex serves to divert the background environmental flow around the TC, thus reducing the local vertical wind shear. We show that this effect can be understood from basic theoretical arguments related to Bernoulli flow around an obstacle. In the simulation discussed, the environmental flow diversion by the outflow is limited to 2 km below the tropopause in the 12–14-km (250–150 hPa) layer. Synthetic water vapor satellite imagery confirms the presence of upshear arcs in the cloud field, matching satellite observations. These arcs, which exist in the same layer as the outflow, are caused by slow-moving wave features and serve as visual markers of the outflow–environment interface. The blocking effect where the outflow and the environmental winds meet creates a dynamic high pressure whose pressure gradient extends nearly 1000 km upwind, thus causing the environmental winds to slow down, to converge, and to sink. We discuss these results with respect to the first part of this three-part study, and apply them to another atypical rapid intensification hurricane: Matthew (2016).

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Variability of the Atlantic Meridional Mode during the Atlantic Hurricane Season

Journal of Climate ◽

10.1175/2010jcli3549.1 ◽

2011 ◽

Vol 24 (5) ◽

pp. 1409-1424 ◽

Cited By ~ 19

Author(s):

Dimitry Smirnov ◽

Daniel J. Vimont

Keyword(s):

Tropical Cyclone ◽

Wind Shear ◽

Vertical Wind Shear ◽

Ocean Model ◽

Atlantic Hurricane ◽

Hurricane Season ◽

Meridional Mode ◽

Upper Level ◽

Atlantic Meridional Mode ◽

Sst Anomalies

Abstract An observational and modeling study is conducted to investigate the structure of the Atlantic Meridional Mode (AMM) during the Atlantic hurricane season, and the relationship between AMM-related SST anomalies and environmental conditions that influence seasonal tropical cyclone activity. The observational analysis shows that during the Atlantic hurricane season the AMM exhibits a similar SST and low-level wind structure as during boreal spring (when the AMM is most active). Observed AMM SST variations are accompanied by air temperature and moisture anomalies that are limited to the boundary layer and an anomalous baroclinic circulation structure in the northern subtropical Atlantic with an anomalous lower-level cyclonic circulation residing under an anomalous upper-level anticyclone during a warm phase. This baroclinic structure contributes to a reduction in vertical wind shear over the tropical Atlantic that is dominated by changes in the upper-level flow. Two sets of model experiments were conducted, in which the NCAR Community Atmospheric Model version 3.1 (CAM3.1) was coupled to a slab ocean model or a data ocean model. In each experiment, the model was either initialized with or forced by AMM-like SST anomalies during boreal summer. The simulations yielded a similar spatial structure to that in the observations, including the baroclinic atmospheric circulation and associated reduction in vertical wind shear. The similarity between the modeled and observed AMM structures strongly suggests a causal relationship in which the AMM-like SST anomalies are responsible for generating environmental conditions that can strongly influence seasonal tropical cyclone variability.

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A Statistical Investigation of Mesoscale Precursors of Significant Tornadoes: The Italian Case Study

Atmosphere ◽

10.3390/atmos11030301 ◽

2020 ◽

Vol 11 (3) ◽

pp. 301

Author(s):

Roberto Ingrosso ◽

Piero Lionello ◽

Mario Marcello Miglietta ◽

Gianfausto Salvadori

Keyword(s):

Potential Energy ◽

Wind Shear ◽

Deep Level ◽

Convective Available Potential Energy ◽

Vertical Wind Shear ◽

Statistical Investigation ◽

Italian Case ◽

Large Wind

In this study, mesoscale environments associated with 57 significant tornadoes occurring over Italy in the period 2000–2018 are analyzed. The role of the vertical Wind Shear in the lower and middle troposphere, in terms of low-level shear (LLS) and deep-level shear (DLS), and of the convective available potential energy (CAPE) as possible precursors of significant tornadoes is statistically investigated. Wind shear and CAPE data are extracted from the ERA-5 and ERA-Interim reanalyses. Overall, the study indicates that: (a) values of these variables in the two uppermost quartiles of their statistical distribution significantly increases the probability of tornado occurrences; (b) the probability increases for increasing values of LLS and DLS, and (c) is maximum when either wind shear or CAPE are large. These conclusions hold for both the reanalysis datasets and do not depend upon the season and/or the considered area. With the possible exception of weak tornadoes, which are not included in our study, our results show that large wind shear, in the presence of medium-to-high values of CAPE, are reliable precursors of tornadoes.

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A Numerical Study of Aviation Turbulence Encountered on 13 February 2013 over the Yellow Sea between China and the Korean Peninsula

Journal of Applied Meteorology and Climatology ◽

10.1175/jamc-d-17-0247.1 ◽

2018 ◽

Vol 57 (4) ◽

pp. 1043-1060 ◽

Cited By ~ 4

Author(s):

Dan-Bi Lee ◽

Hye-Yeong Chun

Keyword(s):

Yellow Sea ◽

Wind Shear ◽

Vertical Wind Shear ◽

Vertical Wind ◽

The Yellow Sea ◽

Small Scale ◽

Mountain Waves ◽

Frontal System ◽

Upper Level ◽

Tropopause Folding

AbstractAt 0247 UTC 13 February 2013, a South Korean commercial aircraft encountered moderate-level clear-air turbulence at ~24 000 ft (~7.3 km) over the Yellow Sea (121.25°E, 38.55°N) en route from Incheon, South Korea, to Tianjin, China. Two crew members were severely injured by this event. To investigate the possible mechanisms of this event, a high-resolution numerical simulation using the Weather Research and Forecasting Model was conducted. In the synoptic-scale flow pattern, one of two bifurcated jet streams passed over the Yellow Sea, and strong horizontal and vertical gradients of the wind occurred on the northern edge of the jet stream near the flight route. An upper-level frontal system on the cyclonic shear side of the jet intensified as it moved northward toward a strengthening upper-level trough in northeastern China. The developed jet–frontal system induced strong vertical wind shear and tropopause folding, which extended down to about z = 5 km, near the observed turbulence region. Despite a relatively high stability with an intrusion of stratospheric air with tropopause folding, the strong vertical wind shear led to a small Richardson number in the incident region, which in turn induced the aviation turbulence through the Kelvin–Helmholtz instability. Although small-scale mountain waves were evident during the passage of flight before the incident time, breaking of these waves was not likely the key factor for the observed turbulence, given that the wave amplitudes were weak and that the strong zonal wind on the upstream of the mountain waves prohibited wave saturation and breakdown.

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Comments on “Multiscale Structure and Evolution of Hurricane Earl (2010) during Rapid Intensification”

Monthly Weather Review ◽

10.1175/mwr-d-16-0301.1 ◽

2017 ◽

Vol 145 (4) ◽

pp. 1565-1571 ◽

Cited By ~ 5

Author(s):

Russell L. Elsberry ◽

Myung-Sook Park

Keyword(s):

Vortex Structure ◽

Wind Shear ◽

Environmental Control ◽

Vertical Wind Shear ◽

Special Kind ◽

Vertical Wind ◽

Rapid Intensification ◽

Structure Changes ◽

Maximum Period ◽

Upper Level

Abstract This comment addresses the Tropical Storm (TS) Earl upper-level vortex structure changes during a critical stage leading to the onset of rapid intensification as described by Rogers et al. Whereas the first NOAA WP-3D mission in TS Earl provided evidence of a shallow, broad vortex structure, the second WP-3D mission just 12 h later documented a deep, vertically stacked vortex undergoing rapid intensification. The authors attribute this vortex structure change to vertical alignment processes between the low-level Earl vortex and an upper-tropospheric mesoscale vortex about 50 km to the east in the mission 1 analyses. An alternate environmental control explanation is proposed in which a special kind of upper-tropospheric vertical wind shear (VWS) associated with the outflow of Hurricane Danielle to the northwest of TS Earl is the primary factor. Two estimates of the vertical wind shear changes are interpreted relative to the diurnal convective maximum/minimum to explain how the shallow vortex during mission 1 may have been created. It is proposed that the vigorous convection over sea surface temperatures of about 30°C during the diurnal convective maximum period between mission 1 and mission 2 was able to offset the moderate VWS as Hurricane Danielle had moved farther away from Earl. Thus, an explanation for the vertically stacked TS Earl vortex observed during mission 2 in terms of an environmental VWS modulation of the diurnally varying convective processes is proposed as an alternative to a vortex realignment.

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