Descending reflectivity core

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Descending reflectivity core
DRC visualization.png
Visualization of a descending reflectivity core during the May, 2000 Tornado Outbreak.
Effecttornadogenesis, updraft intensification, mesocyclone formation

A descending reflectivity core (DRC), sometimes referred to as a blob, is a meteorological phenomenon observed in supercell thunderstorms, characterized by a localized, small-scale area of enhanced radar reflectivity that descends from the echo overhang into the lower levels of the storm. Typically found on the right rear flank of supercells, DRCs are significant for their potential role in the development or intensification of low-level rotation within these storms. The descent of DRCs has been associated with the formation and evolution of hook echoes, a key radar signature of supercells, suggesting a complex interplay between these cores and storm dynamics.

Contents

First identified and studied through mobile Doppler radar observations, DRCs offer a higher resolution perspective than traditional operational radars, enabling a detailed examination of their structure and behavior. However, these observations often lack a broader, larger-scale view, limiting insights into the origin of DRCs and their relationship with other storm features. Advances in three-dimensional numerical simulations have furthered understanding of DRCs, shedding light on their formation mechanisms, their interaction with the storm's wind field, and the accompanying thermodynamic environment. [1]

Despite their prominence in research, DRCs present challenges in operational meteorology, particularly in forecasting tornado development. The variability in the relationship between DRC observations and changes in the storm's low-level wind field has resulted in mixed results regarding their predictive value for tornadogenesis.

Observation and Analysis

Doppler On Wheels (DOW) radar. Used for micro-scale and near-surface based analysis of the atmosphere CES 2012 - Doppler on Wheels Tornado Truck (6764011739).jpg
Doppler On Wheels (DOW) radar. Used for micro-scale and near-surface based analysis of the atmosphere

The concept of DRCs builds on the understanding of hook echoes, first documented in the 1950s. These hook echoes were initially hypothesized to form from the advection of precipitation around a supercell's rotating updraft. However, subsequent studies suggested alternative formation mechanisms, including the descent of precipitation cores from higher levels. [2] [3] DRCs have been observed using mobile doppler radar, offering higher resolution than operational radars but sometimes sacrificing larger-scale perspective. These observations have revealed the challenge in generalizing a relationship between DRCs and subsequent low-level wind field evolution. Studies using three-dimensional numerical simulations of supercells have also provided insights into DRC formation mechanisms and their interactions with three-dimensional wind fields.

Recent studies have identified different mechanisms for DRC development, not all of which lead to increases in low-level rotation. This variability might account for the difficulty in using DRC detection to aid operational forecasting of tornadogenesis. One significant study documented DRCs using Doppler on Wheels (DOW) radar data, revealing finer spatial resolution details in DRC evolution. The study presented cases where DRCs appeared as new convective cells, merging with the main echo region of storms, and influencing the formation of hook echoes. [2] [4] [5] [6]

Mechanism and Formation

The 2022 Grow, Texas tornado, with illustrations highlighting the formation of the DRC. Grow, TX Tornado with objective analysis.png
The 2022 Grow, Texas tornado, with illustrations highlighting the formation of the DRC.

The formation of descending reflectivity cores in supercell thunderstorms is a complex process influenced by various atmospheric dynamics. Research, particularly involving high-resolution radar data and numerical simulations, has identified several mechanisms through which DRCs can develop:

1. Stagnation of Midlevel Flow (Type I DRC)

One of the primary mechanisms for DRC formation involves the stagnation of midlevel flow in supercell thunderstorms. This process occurs when updrafts in the storm intensify, leading to an accumulation of precipitation at the updraft summit. As this rainwater spills down the flanks of the updraft due to its tilt and the environmental wind profile, it forms a stagnation zone on the rear side of the storm. This zone is characterized by a buildup of precipitation that begins to descend once its terminal fall speed exceeds the updraft speed. This mechanism is delicate and seems to be a rare occurrence within the lifecycle of a supercell. [5]

2. Updraft-Mesocyclone Cycling (Type II DRC)

Another mechanism for DRC formation is associated with updraft-mesocyclone cycling. In this process, a new hook echo and subsequent DRC can form as part of the cyclic nature of the supercell. This is observed when the original hook echo decays and a new hook echo forms, not from the horizontal advection of hydrometeors from the main echo region but from falling hydrometeors enhancing reflectivity at lower elevation scans. This mechanism suggests a more dynamic and recurrent formation process of DRCs in relation to the evolving structure of the supercell.

3. Discrete Propagation Processes (Type III DRC)

The third identified mechanism involves discrete propagation processes, distinct from the horizontal advection of hydrometeors. In this case, DRCs develop as reflectivity values appear within the hook echo, far from the main echo, suggesting a vertical rather than horizontal influence. This process indicates the development of DRCs due to unique atmospheric conditions that lead to the enhancement and eventual detachment of a reflectivity maximum from the main storm echo. [2] [3] [7]

Impact on tornadogenesis

The May 16, 1977 Shamrock, Texas tornado, which had an observed DRC associated with its formation. Shamrock Texas Tornado.jpg
The May 16, 1977 Shamrock, Texas tornado, which had an observed DRC associated with its formation.

The relationship between descending reflectivity cores and tornadogenesis in supercell thunderstorms is a significant area of interest for meteorologists, given the potential of DRCs to influence storm dynamics and tornado formation.

DRCs have been observed to impact the low-level wind fields in supercells, a critical factor in tornado development. Upon descent to lower levels, DRCs are often accompanied by enhanced rear-to-front flow, which can lead to the formation of counter-rotating vortices. These vortices, particularly the cyclonic member, may act as precursors or catalysts to tornadogenesis. However, the exact impact of DRCs on tornado formation is complex and not yet fully understood.

Some studies suggest that the presence of a DRC may be a more reliable indicator of tornado likelihood than the hook echo alone. While hook echoes are a well-known radar signature of supercells, their presence does not consistently indicate tornado formation. In contrast, the appearance of a DRC, especially when observed in conjunction with other favorable conditions, might more accurately signify the potential for tornadogenesis. [4]

Despite these observations, the predictive value of DRCs in forecasting tornadoes remains a challenge due to their variability. Not all DRCs lead to increases in low-level rotation, and their impact on the wind field can vary significantly from one storm to another. This variability makes it difficult to generalize the role of DRCs [2] [5] [7]

Notable instances

Multiple long-lived and violent tornadoes have been observed to have their formation associated with a DRC.

See also

Related Research Articles

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<span class="mw-page-title-main">Supercell</span> Thunderstorm that is characterized by the presence of a mesocyclone

A supercell is a thunderstorm characterized by the presence of a mesocyclone, a deep, persistently rotating updraft. Due to this, these storms are sometimes referred to as rotating thunderstorms. Of the four classifications of thunderstorms, supercells are the overall least common and have the potential to be the most severe. Supercells are often isolated from other thunderstorms, and can dominate the local weather up to 32 kilometres (20 mi) away. They tend to last 2–4 hours.

<span class="mw-page-title-main">Mesocyclone</span> Region of rotation within a powerful thunderstorm

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<span class="mw-page-title-main">1990 Plainfield tornado</span> Deadly tornado in Illinois

The 1990 Plainfield tornado was a devastating tornado that occurred on the afternoon of Tuesday, August 28, 1990. The violent tornado killed 29 people and injured 353. It is the only F5/EF5 rated tornado ever recorded in August in the United States, and the only F5 tornado to strike the Chicago area. There are no known videos or photographs of the tornado itself; however, in 2011, a video surfaced online showing the supercell that spawned the tornado. The Plainfield tornado was part of a small outbreak that produced several tornadoes in the Northern United States, specifically Kansas, and the Canadian province of Ontario.

<span class="mw-page-title-main">Squall line</span> Line of thunderstorms along or ahead of a cold front

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<span class="mw-page-title-main">Wall cloud</span> Cloud formation occurring at the base of a thunderstorm

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<span class="mw-page-title-main">Hook echo</span> Weather radar signature indicating tornadic circulation in a supercell thunderstorm

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<span class="mw-page-title-main">Funnel cloud</span> Funnel-shaped cloud not touching the ground

A funnel cloud is a funnel-shaped cloud of condensed water droplets, associated with a rotating column of wind and extending from the base of a cloud but not reaching the ground or a water surface. A funnel cloud is usually visible as a cone-shaped or needle like protuberance from the main cloud base. Funnel clouds form most frequently in association with supercell thunderstorms, and are often, but not always, a visual precursor to tornadoes. Funnel clouds are visual phenomena, but these are not the vortex of wind itself.

The National Severe Storms Laboratory (NSSL) is a National Oceanic and Atmospheric Administration (NOAA) weather research laboratory under the Office of Oceanic and Atmospheric Research. It is one of seven NOAA Research Laboratories (RLs).

<span class="mw-page-title-main">Lemon technique</span> Meteorological method to determine relative strength of thunderstorm cells

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<span class="mw-page-title-main">Bounded weak echo region</span> Weather feature

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<span class="mw-page-title-main">Tornadogenesis</span> Process by which a tornado forms

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<span class="mw-page-title-main">Rear flank downdraft</span> Type of region

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<span class="mw-page-title-main">Doppler on Wheels</span> Fleet of X-band radar trucks maintained by the Center for Severe Weather Research (CSWR)

Doppler on Wheels is a fleet of X-band and C-band mobile and quickly-deployable truck-borne radars which are the core instrumentation of the Flexible Array of Radars and Mesonets affiliated with the University of Illinois and led by Joshua Wurman, with the funding partially provided by the National Science Foundation (NSF), as part of the "Community Instruments and Facilities," (CIF) program. The DOW fleet and its associated Mobile Mesonets and deployable weather stations have been used throughout the United States since 1995, as well as occasionally in Europe and Southern America. The Doppler on Wheels network has deployed itself through hazardous and challenging weather to gather data and information that may be missed by conventional stationary radar systems.

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<span class="mw-page-title-main">Tornado vortex signature</span> Weather radar pattern

A tornadic vortex signature, abbreviated TVS, is a Pulse-Doppler radar weather radar detected rotation algorithm that indicates the likely presence of a strong mesocyclone that is in some stage of tornadogenesis. It may give meteorologists the ability to pinpoint and track the location of tornadic rotation within a larger storm, and is one component of the National Weather Service's warning operations.

<span class="mw-page-title-main">VORTEX projects</span> Field experiments that study tornadoes

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A mesovortex is a small-scale rotational feature found in a convective storm, such as a quasi-linear convective system, a supercell, or the eyewall of a tropical cyclone. Mesovortices range in diameter from tens of miles to a mile or less and can be immensely intense.

<span class="mw-page-title-main">TWISTEX</span> Tornado research experiment

TWISTEX was a tornado research experiment that was founded and led by Tim Samaras of Bennett, Colorado, US, that ended in the deaths of three researchers in the 2013 El Reno tornado. The experiment announced in 2015 that there were some plans for future operations, but no additional information has been announced since.

The following is a glossary of tornado terms. It includes scientific as well as selected informal terminology.

References

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