Joint Centre for Mesoscale Meteorology (JCMM)


JCMM Internal Report No.88:

Radar estimates of rainfall rates at the ground in bright band and non-bright band events

by T J Smyth & A J Illingworth

The major error in estimating precipitation rates at the ground from radar arises from the change in radar reflectivity (Z) with height. In stratiform precipitation Z rises in the "bright band" where the snow is melting, and then falls steeply in the dry ice above. In regions of vigorous convection containing graupel, the bright band is absent and the fall in Z with height is less marked. If accurate estimates of surface precipitation are to be made based upon radar observations taken within or above the bright band, it is important to use a correction scheme which uses a different Vertical Profile of Reflectivity (VPR) for the two precipitation types. Using a large radar dataset, this work addresses the issue of VPR variability by constructing average vertical profiles and sorting them into snow and graupel categories using the value of the Linear Depolarisation Ratio (LDR) at the melting layer. These observations show that convection is often embedded within stratiform precipitation and some convective showers containing snow have bright bands. As a result, the use of an average profile over the whole region scanned by radar leads to only marginal improvement in rain estimates. Instead, this work proposes that a bright band correction should always be applied unless the value of Z, 1.5km above the bright band height, exceeds 30dBZ, in which case a "convective vertical profile of -2dBkm-1 should be used. This Z threshold indicates that graupel only forms when the Ice Water Content (IWC) exceeds 0.35gm-3 which has implications for ice parametrisation schemes in mesoscale models.

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