308
Multiscale Hydrologic Remote Sensing: Perspectives and Applications
melting layer about 3 km thick. These are plan position indicator (PPI) images of
Z H , Z DR , ρ hv , K DP , and radar echo classification results. The data for these images are
measured by the S-band National Severe Storm Laboratory (NSSL) Polarimetric
WSR-88D (KOUN) radar, which has a beam width of 1°. As shown in the figure, Z H
values within the storm core are large. Because there exists a rain/hail mixture, some
Z H values are close to 60 dBZ. Accordingly, Z DR values within the storm are mostly
greater than 3.5 dB. The “big drop” region generally has a relatively large Z DR and
small Z H compared to normal values for stratiform rain. The image of ρ hv indicates
a large region of rain, where the value is generally greater than 0.95. For rain signals
within the range of 100 km, most ρ hv values are larger than 0.98. For rain signals at
a far range (e.g., >100 km north), the ρ hv value is around 0.95, which is a little lower
than the normal value of rain signals. It is likely that the partial signals come from
the melting layer and that the mixture of melting snow/hail has contaminated the
rain signal. The region around the radar site has a ρ hv around 0.8. The rain signal in
this region has been contaminated by ground clutter. A line of low ρ hv (about 0.4),
which is in front of the squall line, is clearly shown on the image. This is the signature of the gust front, which consists mainly of nonhydrometeor scatters. As the K DP
image shows, K DP is a little noisy in the light-rain region, where there may be a few
small negative values. This is because that the K DP value depends on the Φ DP measurement. However, the Φ DP measurement is noisy when the SNR is low. In this case,
most of the stratiform rain has a K DP below 0.5° km –1 . In the region of convective
rain, K DP is generally larger and sometimes might be greater than 1° km –1 . Generally
speaking, the K DP measurement is more erroneous than the other three variables.
The measurement ρ hv can be used as an indicator of signal type or signal quality.
The measurements Z H , Z DR , and K DP can be applied quantitatively for rain estimation.
13.3 POLARIMETRIC RADAR–RAIN ESTIMATION
In general, the rain variables of interest are rainfall rate (R), total raindrop concentration (N T ), rainwater content (W), and various characteristic sizes (e.g., median
volume diameter D 0 ). All of these variables are closely related to DSD, which can be
expressed based on the following equations:
N
N D dD
D
D
T
3
, [m ]
=
−
∫
( )
min
max
(13.5)
W
D N D dD
D
D
= ×
⋅
−
∫
π
6
10
3
3 ( )
min
max
, [g m ]
3
(13.6)
R
D v D N D dD
D
D
= ×
⋅
−
∫
6 10
3
3
π
( ) ( )
min
max
, [mm h ]
1
(13.7)
D N D dD
D N D dD
D
D
D
D
3
3
0
0
( )
( )
min
max
∫
∫
=
,
(13.8)
Multiscale Hydrologic Remote Sensing: Perspectives and Applications
melting layer about 3 km thick. These are plan position indicator (PPI) images of
Z H , Z DR , ρ hv , K DP , and radar echo classification results. The data for these images are
measured by the S-band National Severe Storm Laboratory (NSSL) Polarimetric
WSR-88D (KOUN) radar, which has a beam width of 1°. As shown in the figure, Z H
values within the storm core are large. Because there exists a rain/hail mixture, some
Z H values are close to 60 dBZ. Accordingly, Z DR values within the storm are mostly
greater than 3.5 dB. The “big drop” region generally has a relatively large Z DR and
small Z H compared to normal values for stratiform rain. The image of ρ hv indicates
a large region of rain, where the value is generally greater than 0.95. For rain signals
within the range of 100 km, most ρ hv values are larger than 0.98. For rain signals at
a far range (e.g., >100 km north), the ρ hv value is around 0.95, which is a little lower
than the normal value of rain signals. It is likely that the partial signals come from
the melting layer and that the mixture of melting snow/hail has contaminated the
rain signal. The region around the radar site has a ρ hv around 0.8. The rain signal in
this region has been contaminated by ground clutter. A line of low ρ hv (about 0.4),
which is in front of the squall line, is clearly shown on the image. This is the signature of the gust front, which consists mainly of nonhydrometeor scatters. As the K DP
image shows, K DP is a little noisy in the light-rain region, where there may be a few
small negative values. This is because that the K DP value depends on the Φ DP measurement. However, the Φ DP measurement is noisy when the SNR is low. In this case,
most of the stratiform rain has a K DP below 0.5° km –1 . In the region of convective
rain, K DP is generally larger and sometimes might be greater than 1° km –1 . Generally
speaking, the K DP measurement is more erroneous than the other three variables.
The measurement ρ hv can be used as an indicator of signal type or signal quality.
The measurements Z H , Z DR , and K DP can be applied quantitatively for rain estimation.
13.3 POLARIMETRIC RADAR–RAIN ESTIMATION
In general, the rain variables of interest are rainfall rate (R), total raindrop concentration (N T ), rainwater content (W), and various characteristic sizes (e.g., median
volume diameter D 0 ). All of these variables are closely related to DSD, which can be
expressed based on the following equations:
N
N D dD
D
D
T
3
, [m ]
=
−
∫
( )
min
max
(13.5)
W
D N D dD
D
D
= ×
⋅
−
∫
π
6
10
3
3 ( )
min
max
, [g m ]
3
(13.6)
R
D v D N D dD
D
D
= ×
⋅
−
∫
6 10
3
3
π
( ) ( )
min
max
, [mm h ]
1
(13.7)
D N D dD
D N D dD
D
D
D
D
3
3
0
0
( )
( )
min
max
∫
∫
=
,
(13.8)