Radar Signatures for Severe Convective Weather

Detection

An S-band Three-Body Scatter Spike (TBSS), also known as a "hail flare" or "flare echo", is a 10-30 km long, low reflectivity (< 20 dBZ), mid-level echo "spike" aligned radially downrange from a high reflectivity (usually 63 dBZ+) core (Lemon, 1998). This echo spike is strictly an artifact of the electromagnetic radar beam being subject to "Mie scattering" instead of the usual "Rayleigh scattering" process.

 

To determine if the signature you are seeing is a TBSS use the following technique:

Reflectivity: PPI/Plan View

1. For S-band radars, step up and down in elevation, looking closely at the maximum reflectivity core, mainly in the midlevels (often at or colder than the freezing level), keeping a look out for a "flare" or "spike" downrange of the core. Note: It is best to have a good handle on the environmental temperature structure, as this signature should be strongest around 0 to -30°C.
2. If the flare has weak reflectivities (<20 dBZ), is positioned downrange and along the same radar radial as the most intense part of the actual core, is almost exclusively aloft, and begins the same distance from the core as the core is above the ground, then a TBSS is present. Note that if the reflectivity increases at longer ranges along the potential TBSS, it is not a TBSS.

Note: TBSS signature is even more common with a C-band radar, but due to shorter radar wavelengths, we cannot deduce that large hail caused the TBSS signature.

Reflectivity: RHI/Cross-Section

S-band radars can also detect a TBSS in the cross section.

1. Place a vertical cross section through the reflectivity core at the location where a local maximum in the reflectivity is present within the 0 to -30ºC layer.
2. If the flare has weak reflectivities (<20 dBZ), is positioned downrange and  along the same radar radial as the most intense part of the actual core, is almost exclusively aloft and begins the same distance from the core as the core is from the ground, then a TBSS is present.

Note: vertical cross sections must be precisely placed to reveal the critical storm features such as a TBSS, therefore they are less useful for real-time detection.

 

Velocity PPI/Plan View

This method is probably the most difficult for the detection of a TBSS. It might be best used to confirm a TBSS determination utilising one of the previous methods.

1. For S-band radars, step up and down in elevation, looking closely at the maximum reflectivity core, keeping a look out for a "flare" or "spike" in the reflectivity channel downrange from the core.
2. If the flare has near zero or weak velocities, is positioned along the same radar radial, is almost exclusively aloft and is located the same distance from the core as the core is from the ground, then a TBSS is present.
   Note: the most effective detection of a TBSS is via spectrum width.

The strength and length of the TBSS is related to the intensity and vertical extent of the reflectivity core. Therefore a TBSS should be easier to detect with a more intense and elevated reflectivity core. Also the larger the highly reflective core area, the more extensive the TBSS (Lemon, 1998).

Keep in mind this signature is an artifact artifact of Mie scattering and must not be construed as hail actually reaching the surface beneath the echo spike itself.

 

Potential Difficulties in Detection

• Obscured echoes – TBSS can be obscured by either separate downrange storms or by downrange precipitation from the same storm falling from a large anvil.

 

 

• Hail core is close to ground – There are two ways this affects the detection of a TBSS: one, the TBSS will begin immediately behind the echo core, and therefore may be obscured (as mentioned above) and two, the TBSS will be short in length, and therefore not stand out as much as a more pronounced TBSS from an elevated core.

 

• Radar sampling:
  • The thunderstorm is too far away from the radar – Therefore theradar beam overshoots the main hail core.
  • The thunderstorm is too close to the radar – Therefore the radar cannot scan the storm in the mid-levels where the signature might have been detected.
  • There is a large potential for second or third trip echoes to interfere with the detection of this signature.

 

Examples of Three-Body Scatter Spikes

 

TBSS Look-a-Likes

• TBSS located in the wrong position – The TBSS should be located radially downrange from a high reflectivity core of approximately >63dBZ reflectivities.
• Actual precipitation/topography returns – Light showers could be mistaken for a TBSS if they were in the correct position.
• Sun spike