simulating Z and R over a wide range of DSDs (see Fig. 4). The relationship between
radar reflectivity Z and rainfall intensity R relies on the actual DSD, which varies
between different types of storms [7, 13, 72] and within storms [74–76]. Therefore,
changes in the DSD introduce a time-varying bias in radar rainfall estimates, because
of the use of climatological Z – R relationships. Updrafts and downdrafts can also
cause the Z – R relationship to differ greatly from the one obtained in still air [7].
Some approaches address DSD variability with conventional radars such as
identifying different precipitation types and applying different Z – R equations. In
the US, the relationship Z ¼ 300R
1.4 is often used for convective precipitation,
whereas in the UK, the equation Z ¼ 200R
1.6 is used for stratiform precipitation.
Automatic classification of stratiform and convective precipitation can potentially
lead to better rainfall estimates.
Conventional SP weather radars can only measure the radar reflectivity
(at horizontal or vertical polarization) to derive precipitation intensity. However,
DP radars can provide more detailed information related to the characteristics of
precipitation particles such as shape, size, spatial orientation and discrimination of
thermodynamic phase [33, 77]. This is because raindrops have an oblate spheroidal
shape being their maximal dimensions horizontally oriented, and that the degree of
oblateness depends on the raindrop size [8].
DP radars can measure Z dr and K dp . Z dr is a measure of the size of the raindrops
and when combined with Z may improve the estimation of precipitation. K dp is
almost linearly related to the liquid water content and it also provides the possibility
of better estimates of rainfall rates in heavy precipitation [8]. Therefore, algorithms
of the forms R ¼ cZ
α
10
βZdr and R ¼ cK
β
dp have been proposed in the literature. The
R À K dp algorithm is useful in heavy precipitation and it has the advantage that K dp is
Fig. 4 Z – R measurements computed from DSD data measured by a disdrometer in the UK. The
red line shows the climatological Z – R equation used operationally in the UK to estimate radar
rainfall rates
Precipitation Measurement with Weather Radars
245
radar reflectivity Z and rainfall intensity R relies on the actual DSD, which varies
between different types of storms [7, 13, 72] and within storms [74–76]. Therefore,
changes in the DSD introduce a time-varying bias in radar rainfall estimates, because
of the use of climatological Z – R relationships. Updrafts and downdrafts can also
cause the Z – R relationship to differ greatly from the one obtained in still air [7].
Some approaches address DSD variability with conventional radars such as
identifying different precipitation types and applying different Z – R equations. In
the US, the relationship Z ¼ 300R
1.4 is often used for convective precipitation,
whereas in the UK, the equation Z ¼ 200R
1.6 is used for stratiform precipitation.
Automatic classification of stratiform and convective precipitation can potentially
lead to better rainfall estimates.
Conventional SP weather radars can only measure the radar reflectivity
(at horizontal or vertical polarization) to derive precipitation intensity. However,
DP radars can provide more detailed information related to the characteristics of
precipitation particles such as shape, size, spatial orientation and discrimination of
thermodynamic phase [33, 77]. This is because raindrops have an oblate spheroidal
shape being their maximal dimensions horizontally oriented, and that the degree of
oblateness depends on the raindrop size [8].
DP radars can measure Z dr and K dp . Z dr is a measure of the size of the raindrops
and when combined with Z may improve the estimation of precipitation. K dp is
almost linearly related to the liquid water content and it also provides the possibility
of better estimates of rainfall rates in heavy precipitation [8]. Therefore, algorithms
of the forms R ¼ cZ
α
10
βZdr and R ¼ cK
β
dp have been proposed in the literature. The
R À K dp algorithm is useful in heavy precipitation and it has the advantage that K dp is
Fig. 4 Z – R measurements computed from DSD data measured by a disdrometer in the UK. The
red line shows the climatological Z – R equation used operationally in the UK to estimate radar
rainfall rates
Precipitation Measurement with Weather Radars
245