Diagnosis

Once you have confidently identified a Low-Level Mesocyclone signature, this section will help you estimate the storm severity associated with it. Generally, the spatial and temporal scales of a signature are loosely 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

Mesocyclone severity has been studied extensively since the 1970s. A nomogram was developed by David Andra (1997) and others within the WSR-88D Operational Support Facility (OSF) which normalises the observed rotational velocity for range from the radar assuming the following characteristics of the radar and mesocyclone:

  1. The mesocyclone is approximated by a Rankine combined vortex
  2. The signature has a diameter of about ~6.5 km
  3. The radar has an effective half power beamwidth of 0.9 degrees
Subjective strength classification of a mesocyclone as a function of range from a radar origin (km) and mesocyclone rotational velocity (m/s). This nomogram is intended as a rough guide only.

Subjective strength classification of a mesocyclone as a function of range from a radar origin (km) and mesocyclone rotational velocity (m/s). This nomogram is intended as a rough guide only.

To determine the mesocyclone strength, use Doppler radar to:

  1. Read off the maximum inbound and outbound velocities (e.g., inbound value of -25 ms-1 and outbound value of +15 ms-1). In the event that the velocity couplet has no sign reversal (i.e., both velocity extrema in the couplet are either inbounds or outbounds), read off both "extreme" values (e.g., outbounds of 35 ms-1 and 5 ms-1).
  2. Add the magnitudes of the two values together and divide by 2 to obtain the rotational velocity strength. The first example in (1) above yields a rotational velocity:
    Vr = (|-25|+ 15)/2 ms-1 = 20 ms-1
    For the second case above (no sign reversal):
    Vr = (35 + 5)/2 ms-1 = 20 ms-1
  3. Determine the range the mesocyclone centre from the radar (in km).
  4. Using the rotational velocity and the range from radar, read off the strength of the mesocyclone in the nomogram.

A limitation to the mesocyclone nomogram is that it is specific to a 3.5 nm (~6.5 km) diameter mesocyclone. It is important to be aware of this limitation as the nomogram could underestimate/overestimate the intensity of the mesocyclone in those cases where the mesocyclone has a diameter significantly smaller/larger than 6.5 km. Further, the classification is subjective and depends on the radar beam width.

After classifying the strength of the mesocyclone, make sure the radar-detected rotation meets the remainder of the mesocyclone criteria:

  • Diameter < 10 km
  • Vertical extent > 3 km
  • Persistence > 10 min

The mesocyclone strength nomogram is a subjective classification tool for assessing the strength of the mesocyclone. A Low-Level Mesocyclone is one of the few radar signatures that suggest directly that you should issue a severe thunderstorm warning based on the direct reading of the mesocyclone strength and the tight connection to a supercellular thunderstorm. In general, however, radar information should never be used in isolation and should always be considered 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 Low-Level Mesocyclone, the following convective hazards should be considered to be included in the severe thunderstorm warning:

  • Destructive winds – a low-level mesocyclone is associated with a supercell thunderstorm with potential to produce a very strong downdraft. Destructive winds should be considered over damaging winds due to the supercellular classification (except: if you can confidently diagnose an elevated supercell the downdrafts of which are well insulated from the surface by a deep layer of potentially cold air).
  • Large hail – a particularly strong updraft has the potential to produce large hail, providing the updraft extends into the hail growth layer, -10ē to -30ēC. Some consideration to giant hail should be taken due to the supercellular classification of the thunderstorm.
  • Heavy rainfall resulting in flash flooding – 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.
  • Tornado – around 20% to 40% of low-level mesocyclones are associated with tornadoes. Stronger low-level mesocyclones, especially when embedded in tornadogenesis-supporting storm environments, warrant consideration of a tornado warning.

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