Radar Signatures for Severe Convective Weather

Detection

A Bounded Weak Echo Region (BWER) is a nearly vertical channel of weak radar echo, surrounded on the sides and top by significantly stronger echo. The BWER, sometimes called a vault, is related to the strong updraft in a severe convective storm that carries newly formed hydrometeors to high levels before they can grow to radar-detectable sizes. BWERs are typically found at midlevels of convective storms, 3-10 km above the ground, and are up to a few kilometres in diameter.

 

To detect a BWER signature use the following techniques:

Reflectivity: PPI/Plan View

Determining the updraft location:

1. Step up in PPI elevation scans until you run out of >50 dBZ echoes.
2. Step down one elevation scan.
3. Centre cursor on the upshear side of >50 dBZ core (this is to account for storm movement and a slight storm top displacement during the time that elapsed between the lowest scan and storm top-level scan in the presence of strong winds aloft).
4. Set a reference point.
5. Step down through the elevations.
6. Is there evidence of a doughnut or ring-shaped weak reflectivity echo at any elevation, particularly the midlevels (approximately 2-6 km above the ground) in the vicinity of the reference point?
   Note: The doughnut or ring shape does not have to be a "closed loop".
   If yes, a BWER is present. Otherwise, it might be easier to view the BWER with a cross section.
   Note also that storms containing a BWER are almost always supercells, so confirmatory evidence can be obtained by looking for a mesocyclone signature (or at least cyclonic shear) surrounding the midlevel BWER.

Reflectivity: RHI/Cross-Section

Determining the updraft location:

1. Step up in elevation scans until you run out of >50 dBZ echoes.
2. Step down one elevation scan.
3. Centre cursor on the upshear side of >50 dBZ core. (this is to account for storm movement and a slight storm top displacement during the time that elapsed between the lowest scan and storm top-level scan in the presence of strong winds aloft).
4. Step down to the lowest elevation scan.
5. Choose the most direct cross section from the reflectivity core at the lowest tilt (which should exhibit a strong reflectivity gradient) to the reference point you set in the upper levels, representing the storm top (see figure below).
   Note: You may need to use an "arbitrary" (rather than radial) cross section if the storm top marker and the nearest low-level core are not oriented along a radial that originates at the radar location.

 

6. Is there an area of weak echoes bounded by strong echoes above and on both sides? If yes, a BWER is present.

 

Potential Difficulties in Detection

• Low-topped storms – If low equilibrium level (this is often the tropopause) is present, the thunderstorm is low-topped and vertically compressing the BWER signature making it difficult to detect. This may be a particularly serious issue far away from the radar.
• Radar sampling:
  • Radar not aligned well with the storm. The radial from the radar does not allow a direct line from the storm top to the low-level core. An "arbitrary" (non-radial) cross-section needs to be used, which is an interpolated RHI with smoothed data.

• • Resolution degradation as range increases – radar sampling degrades as averaging takes place across a broader and broader beam. This process smoothes out extreme reflectivity or velocity values, weakening or washing out the signature.
  • The thunderstorm is too far away from the radar – therefore the radar beam overshoots the BWER.
  • The thunderstorm is too close to the radar – therefore the radar undershoots the BWER (BWER "hides" inside "cone of silence").

 

Examples of Bounded Weak Echo Regions

 

 

Bounded Weak Echo Region Look-a-Likes

The following signatures look like BWERs, but aren’t.

• Apparent signature is not on the updraft/inflow side of the storm. A BWER is the centre of the intense updraft. A BWER-like signature must be a “look-a-like” if no updraft echo on radar can be found co-located with the signature. This problem can be avoided if you use the detection method explained above.

 

• Elevated core from a separate updraft – more often than not storms are multicellular in nature with several updrafts located in close proximity to each other. Sometimes this can lead to confusion while interrogating storms, connecting elevated cores that are slightly displaced from the low-level core and assuming it forms one core with a BWER.

 

• Overspreading anvil – this should rarely be confused with a BWER, as anvils tend to have weaker reflectivities, than those often found within potentially severe thunderstorm cores.

Conceptual Model

A Bounded Weak Echo Region (BWER) is used as one of many signatures suggesting a severe thunderstorm. The presence of a BWER indicates the thunderstorm possesses an intense updraft. A deep, persistent BWER is often associated with the presence of a mesocyclone, as BWERs are almost always associated with supercells. Note, however, that the BWER is not a product of storm-scale rotation centred on the weak echo "hole" that marks the centre of the BWER. It is the intense updraft within the BWER that prevents echo descent from above or echo development within. The BWER itself outlines the most intense core of the updraft. On the flanks of this intense updraft, large amounts of supercooled liquid water and growing hail form the sheath of high reflectivity surrounding the BWER itself. The hail, graupel, and rain also descend both downshear and upshear of the updraft, creating a bounded weak echo region within the sloping echo overhang. It is important to note that precipitation has been forced upshear, often into strong upper-level flow, creating the surrounding echo overhang, and weak echoes below indicating the intense updraft.

Persistent BWERs, as reasonably reliable proxies for supercells, suggest that the storm is capable of producing damaging winds, large hail and, on occasion, tornadoes. If some additional evidence confirms the presence of a supercell, destructive winds and giant hail should be considered. Flash flooding is not an automatic hazard associated with supercells, in particular for smaller or faster-moving storms. Supercells tend to have a comparatively low precipitation efficiency, but also process inordinate amounts of water vapour. The result of these two opposing drivers for heavy rainfall is that flash flooding is more likely with larger and/or slower-moving supercells. A climatology performed in the US in 2007 confirmed that supercells produce severe weather more frequently than other storm types, and also produce more intense severe weather (Duda & Gallus, 2010). An assessment of the likely tornado threat requires an inspection of the near storm environment (NSE) for suitable low-level and LCL values (see "Low-level Mesocyclone" signature) and the presence of low-level convergence/rotation on radar.

 

Determining Thunderstorm Classification

A persistent Bounded Weak Echo Region is almost always associated with supercell thunderstorms. For further confirmation, look for the existence of a mesocyclone or at least cyclonic shear with the BWER. To help determine or confirm the classification of the thunderstorm you are observing, use the following flow chart to help diagnose which thunderstorm conceptual model you should consider more closely.

Diagnosis

Once you have confidently identified a Bounded Weak Echo Region signature, this section will help you estimate the storm severity associated with it. Generally, the spatial and temporal scales of a signature are related to the updraft strength. In other words, the larger and/or more long–lived the signature, the stronger the updraft that produced it. In velocity-based signatures, updraft severity can usually also be gauged by the magnitude of the measured radial velocities. Examining a storm's overall temporal evolution will suggest whether the storm is becoming more or less severe. Radar signatures and associated storm developments can also be time-shifted relative to each other, as is the case in supercell tornadoes that occur during the collapse of the parent storm.

When comparing signatures to diagnose relative severity, keep in mind that it is assumed that signatures are sampled at equal ranges from the radar. Otherwise, a storm sampled at greater range (with a wider beam) can appear to be weak and/or weakening, while a storm sampled at a closer range (with a narrower beam) can appear to be strong and/or strengthening.

 

Degree of Severity

Longevity of the BWER

• A longer-lived BWER indicates a steady, strong updraft core and is pointing towards (but is not caused by) steady storm-scale rotation.

Average dBZ of BWER

• The higher the average dBZ echo outlining the BWER, the stronger the associated updraft as it is able to create very large radar targets (most likely hail). For an updraft to be deemed strong, the suspended reflectivity values need to be at least 50 dBZ.

Vertical Depth of the BWER

• The larger the height, H, the distance between the ground and the lower level of the overhang echo aloft, the more likely the updraft resides in the ideal hail growth layer (-10ºC to -30ºC) with supercooled liquid water droplets required for the production of large hail. This should be confirmed against the -10°C to -30°C layer of a proximity sounding. A "taller" BWER also indicates that echo development or descent is prevented in a taller column of air, a property related to updraft intensity.

Considering all these aspects will help to determine overall whether you are dealing with a significant signature. In the context of a severe thunderstorm warning decision, a significant BWER could single-handedly provide sufficient evidence for a severe thunderstorm, based on its tight connection to storm-scale rotation. Generally, radar information should never be used in isolation and should always be used in conjunction with the near storm environment and any reports.

 

Most Likely Convective Hazards

If a thunderstorm has been determined to be severe and possesses a BWER of significance, the following convective hazards should be considered to be included in the severe thunderstorm warning:

• Damaging winds – A BWER is a representation of an intense updraft, with potential to produce a strong downdraft as well as intense inflow to the updraft. Destructive winds should be expected given some additional evidence confirming that you are dealing with a supercell (unless the storm is elevated above a strong inversion and above a cold air mass).
• Large hail – A strong updraft has the potential to produce large hail, providing the updraft extends into the hail growth layer, -10º to -30ºC. Warnings for giant hail may be warranted due to the supercellular classification of the thunderstorm.
• Heavy rainfall resulting in flash flooding – A particularly strong updraft has the potential to produce large amounts of precipitation accumulating and falling as heavy rainfall resulting in flash flooding. Melting hail could also contribute to heavy rainfall. However, even when dealing with a supercell, flash flooding is not an automatic hazard, in particular for smaller or faster-moving storms. Supercells tend to have a comparatively low precipitation efficiency, but also process inordinate amounts of water vapour. The result of these two opposing drivers for heavy rainfall is that flash flooding is more likely with larger and/or slower-moving supercells.
• Tornado – Tornadoes are possible and therefore should be at least considered, subject to evidence that the near-storm environment supports tornadoes (strong 0- to 1-km AGL shear, and a low LCL) and the presence of low-level rotation or at least strong low-level convergence in the radial velocities.

See Conceptual Models for more details on why particular severe weather should be included.