Bright band

From Glossary of Meteorology

bright band

Radar signature of the melting layer; a narrow horizontal layer of stronger radar reflectivity in precipitation at the level in the atmosphere where snow melts to form rain. The bright band is most readily observed on range–height indicator (RHI) or time–height indicator (THI) displays.

As ice crystals fall toward warmer temperatures at lower heights, they tend to aggregate and form larger snowflakes. This growth accounts for an increase in radar reflectivity as the falling particles approach the melting level. As they cross the 0°C level, the particles begin melting from the surface inward and finally collapse into raindrops. The reflectivity maximum in the melting layer is explained partly by the difference in the value of the dielectric factor,
, of water and ice (
see radar reflectivity). When a water film begins to form on a melting snowflake, its radar reflectivity may increase by as much as 6.5 dB because of the thermodynamic phase change. The reflectivity decreases below the melting level because when flakes collapse into raindrops, their fall velocities increase, causing a decrease in the number of precipitation particles per unit volume. The size of the particles also becomes smaller in the melting process, as their density increases from that of the snow and melting snow to that of liquid water. Both the reduction in size of the precipitation particles and the decrease in their concentration lead to a decrease in the strength of the radar echo at altitudes below the melting level, so that an isolated, horizontal layer of high reflectivity is established, usually centered about 100 m below the 0°C isotherm. The bright band is observed primarily in stratiform precipitation. The strong convective currents in active showers and thunderstorms tend to destroy the horizontal stratification essential for creating and sustaining the bright band.

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