Descending reflectivity core

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.

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.

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.

History

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.

Observation and Analysis

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 focused on DRCs using data from mobile, truck-borne radars and high-resolution numerical simulations. These 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.

The presence of DRCs in supercells poses both opportunities and challenges for meteorology. While they can indicate the likelihood of tornado formation better than hook echoes alone, the variability in their relationship with low-level wind changes complicates their use in tornado forecasting. Additionally, the role of DRCs in the dynamics of supercells and tornado genesis remains an area of active research.

Mechanism and Formation

The formation of Descending Reflectivity Cores (DRCs) 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 rainwater 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.

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.

References