Rain Observations
with a vertically
looking Micro
Rain Radar (MRR)
Peters Gerhard, Fisher
Bernd and Andersson
Tage.
Boreal Environment
Research: 7, 353-362,
(2002)
BALTEX- GEWEX & PEP
• Baltic Sea Experiment : Hydrological Cycle
• Global Energy and Water Cycle Experiment
• Precipitation and Evaporation Project
• Radar Rainfall
– RR and Z depends on DSD
– Height of Measuring Volume
• Extrapolation of radar measuring volume to
surface includes significant uncertainties
MRR
•
•
•
•
FM-CW Doppler radar (24.1 GHz)
Power transmit 50mWatt
30 heights
Minimum RR is 0.01mm
 ( D)
N ( D) 
 ( D)
Z

6

6
N
(
D
)
D
dD

0
• DSD and W
where  ( D ) is the single particle backscattering cross section,
and
 ( D) is the spectral reflectivity as a function of the
drop diameter D.
Z is referred to as the Equivalent Radar Reflectivity factor with
units of mm6 mm-3
R

6

3
N
(
D
)
D
v ( D ) dD

0
LWC   w


N ( D) D
6
3
dD
0

W

2
  f  fdf
0

  f  df
0
Set Up
• Rainfall measurements
– 1min
– 50 m
– German Baltic coast Zingst Peninsula
• Comparison with tipping bucket Rain Gauge
– 30 minute
– 5 months (summer)
MRR vs Weather Radar
• What is the Biggest source of Error with MRR measurements ?
Vertical Updrift Winds
• When Mie theory is Applied?
Mie scattering occurs when the particles in the
atmosphere are the same size as the wavelengths
being scattered
• When the Rayleigh Approximation is Applied?
Drops Diameter smaller in
comparison to the wavelength of
light
• What’s About the Weather radar wavelength (λ = 5 cm to 10 cm)
σ = 2700 to 3000 MHz (S-band)
• MRR (σ = 24.1 GHz or 1,24 cm)
DSD and RR versus Z
Peters et. al (2002)
• What is main Assumptions Peters et al.
did raise for this study:
Liquid Precipitation
• What is the Rainfall Rate classification
scheme they used
0.25 mm hr-1
Height Resolution was set to: 50m
Which Gate was Selected: 10th
Peters et. al (2002)
• Rain Gauge:
Rainfall
F and Resolution ΔF
• Rain Rate Resolution
ΔR = ΔF / Δt
• Define the contribution of
certain R to the total
rainfall.
Frequency of High RR has increased
Duration of most events < 30 min
Averaging process:
underestimation of the actual occurring RR
They concluded that:
1 minute resolution is not enough to reveal the true distribution of rain rate
Which Rain Rates Contributed most?
0 – 0.25 mmhr-1
Peters et al. (2002)
ΔR / R = 0.05
• Differential water column (mm/mm hr-1)
Maximum Contribution
From rain rate around
0.2 mm hr-1
Highest rainfall Contribution
From rain rate around
≤ 0.3 mm hr-1
The resolution of the gauge is not sufficient to represent rain rate classes
Peters et al. (2002)
Spectral Peak of velocity is about
6 m s-1 at lower gates
Above 1100m peak shift to
2 m s-1
Meting level appears as:
1- Enhanced Reflectivity
2- Step in Fall Velocity
3- Apparent increase of RR
Peters et al. (2002)
Comparison with Weather Radar: DWD
9 hours of sampling
51.48 km
900m lowest WR measuring volume
Averaged of two subsequent samples of WR volumes
MRR data averaged between 500 and 1400 m
MRR vertical resolution 100 m
Peters et al. (2002)
Reflectivities below -5dBZ are missing,
in weather radar (WR) data probably
because they fall below the detectable
threshold
Agreement between MRR
Reflectivity and WR suggests that
MRR measurements could be
used for continuously updating
the Z-R relationship
WR measurements could be
linked to the rainfall at the
surface by the use of MRR
profiles
Z a R
b
Investigation of
vertical profile of
rain microstructure
at Ahmedabad in
Indian tropical
region
Saurabh Das, Ashish K.
Shuklaand and Animesh
Maitra.
Advances in Space Research 45
(2010) 1325-1243
Set Up
Das et al. (2010)
• Rain Attenuation
• Rain Classification
• Vertical Profiles of rain
microstructure
–
–
–
–
1- MRR
30 seconds
200m (up to 6km)
2- Disdrometer
30 seconds
DSD
Rain Rate
Liquid water content
Average Fall speed of drops
• Compare MRR DSD to Disdrometer
Assumptions and Corrections for MRR:
1- W for lower air velocities
2- Errors due to non-spherical drops ~ 6% at 10mm/hr are neglected
Das et al. (2010)
• Disdrometer
– Transform vertical momentum of drops to an electrical signal (where:
amplitude is f(DD))
– Terminal Velocity  Drop diameter (Gunn & Kinzer 1949)
– Drop Size in
20 bins
– DSD  Rain Rate
Assumptions
1- Momentum is due entirely to fall velocity
2- Drops are spherical
3 - Acoustic Noise Errors are neglected
4- Dead Time correction not applied
Range 0 – 5.5 mm
Rain Classification Scheme Based on:
Vertical Reflectivity profile
1- Stratiform
2- Mixed
3- Convective
Das et al. (2010)
Radar reflectivity profile for:
(a) 16:21:30–20:43:30 UTC
(b) 02:42:30–11:47:30 UTC
(c) 20:39:30-23:55:00 UTC
16:21:30–20:43:30 UTC
Max R is 10.02 mm/hr
BB at
4.6 - 5.2 km
of 15/08/2006;
of 01/08/2006;
of 03/07/2006.
of 15/08/2006
16:42:30 UTC no bright band is visible
17:10:30 UTC bright bands with two peaks
17:33:30 UTC a clear bb structure is visible
Convective
Mixed
Stratiform
Das et al. (2010)
16:42:30 UTC
17:10:30 UTC
Convective
Mixed
17:33:30 UTC
Stratiform
Das et al. (2010)
15/08/2006
16:40:00 to 16:44:30
Convective
3a Z
3b Rain Rate
3c LWC
3d W
3e Mean Drop Diameter
3f DSD
Das et al. (2010)
15/08/2006 17:08:30 to 17:13:30
Mixed
3a
3b
3c
3d
3e
3f
Z
Rain Rate
LWC
W
Mean Drop Diameter
DSD
Das et al. (2010)
15/08/2006
17:28:30 to 17:33:30
Stratiform
3a Z
3b Rain Rate
3c LWC
3d W
3e Mean Drop Diameter
3f DSD
Das et al. (2010)
MRR vs Disdrometer
1- good agreement between MRR and
Disdrometer rainfall (r = 0.8)
2- Scattering is higher above 2.5 mm/hr
3- Stratiform Rain Bigger Drop Sizes near
ground level (7c)
4- Convective Drops 0.5 mm are dominant
5- Stratiform Drops are also around 1mm
6- Mixed showed a mixture of drops sizes (7b)
MRR show very high concentration of smaller drops
(7a)
MRR measures drops in range 0.245 – 5.03 mm
Disdrometer measures drops in range 0.3-5.5 mm
MRR is more sensitive to smaller Drops