Bright Banding & Hail

Hail and other frozen hydrometeors can cause increased reflectivity.  Two of the most common terms or phenomena associated with this increased reflectivity are bright banding and hail [contamination].  While both of these are associated with increased reflectivity, they are also generally associated with sufficiently different circumstances to warrant separate discussions. 

Bright banding is a result of enhanced reflectivity and is generally a cool season phenomena, occurring over a wide-area, when the radar beam goes above the zero degree isotherm, above which most of the precipitation is frozen.  This is illustrated in Figure 3.  From an operational perspective, bright banding is generally more obvious than hail.  Bright banding is particularly obvious when a series of radar images are added together – for example 6 hours of precipitation.

Figure 3 – Illustration of bright banding.

Hail is predominantly a warm season phenomena and is more localized.  Hail can cause very high reflectivities of 60 dBZ or more, while the accompanying rainfall by itself would have a lower reflectivity.  This is addressed in individual WSR-88D units with an upper limit or “hail cap.”  The hail cap is set at various levels across the country according to the typical reflectivity or rainfall rate above which hail is assumed to be occurring.  This has proven to be an effective filtering mechanism; however, this may also cause underestimation of atypical heavy precipitation events.  In cases of hail, both the incremental and accumulated estimates may be significantly in error.   Similarly, melting snow may also reflect like large raindrops and again imply enhanced rainfall rates.  Depending on the location of the RFC (and radars), the hail may be predominant at various times throughout the precipitation event.  For example, in some areas, the hail is seen most often at the front end of the event and the RFCs rely heavily on the hail cap to assist in removing this data.  Other areas may see hail occur during the event.  In some areas, of particular concern is the presence of embedded convective cells within a widespread stratiform event.  The high reflectivities then greatly impact the bias estimates and the overall precipitation fields and MAPX.  Figure 4 illustrates typical hail contamination.

 

 

NEED HAIL CONTAMINATION FIGURE

Figure 4 – Illustration of hail contamination.

 

 

Beam Blockage

Beam blockage is a problem when an object such as a structure or mountain interferes with the beam.  While it is generally very obvious that a portion of the radar circle is blocked, correcting this issue can be quite difficult.  If the radar is tilted to shoot over the object, there is the risk of overshooting actual precipitation.  Hybrid scans, using a combination of tilts, can be employed to look at multiple elevations and then combine the resulting reflectivities.  Figure 5 illustrates the concept of beam blockage and multiple scan elevations.

Figure 5 – Beam Blockage and multiple tilt illustration.

 

Distance Effects

The effects of distance - both near and far from an individual radar - need to be considered when examining a multisensor precipitation field.  Above each radar, a “cone of silence” exists where even the steepest tilt cannot reach.  Moving away from the radar, the increasing elevation of even the lowest beam tilt can overshoot low-level precipitation.  In addition, signal range degradation effects begin to occur, which leads to poor representation of the precipitation beyond certain distances.  Bin size also increases with distance, which leads to incomplete beam filling and poor representation of small scale storm structures (e.g. convective events).  Multisensor precipitation estimation programs used at RFCs such as RFC-Wide MPE are intended to resolve these issues by estimating biases based on multiple radars and radar climatologies, as opposed to single radar coverages.