AT690 Polarimetric
Radar Meteorology
Review by Larry Carey
9/16/04
Petersen, W. A., L. D. Carey, S. A. Rutledge, J. C. Knievel, N. J. Doesken, R.
H. Johnson, T. B. McKee, T. Vonder Haar, and J. F. Weaver, 1999: Mesoscale
and Radar Observations of the Fort Collins Flash Flood of 28 July 1997. Bull.
Amer. Meteor. Soc., 80, 191-216.
Overview
• Summary of the Fort Collins Colorado Flash
Flood of 28 July 1997
–
–
–
–
Events
Rain maps from gauge and bucket survey
Synoptic environment
Meso-analysis
• Radar observations
–
–
–
–
Horizontal storm structure and evolution
Vertical storm structure and evolution
Single and dual-Doppler observations of flow field
Polarimetric radar
• Perspective on cloud microphysical processes
• Radar rainfall estimation
Events
Events
Urban Street Flooding…
Events
Heavy, quasi-steady rain combined
with failure of hydrological systems
resulted in a deadly flash flood.
Rain maps (inches) – gauge/bucket survey
a. 27/1600 – 28/1300 MDT:
Antecedent moisture
b. 28/1730 – 28/2300 MDT: The Flood
Synoptic Environment – 500 mb analysis
• 500-mb ridge
•Moist, tropical
Southerly to
Southwesterly
Flow aloft
•Short wave
Synoptic
Environment Summary
• post-frontal upslope
(easterly) flow along Front
Range of Colorado.
•Moist low-levels
•Cold (T < -20C) cloud top
evolution appeared to
track evolution of short
wave.
Synoptic Environment – Denver Sounding
(29/00Z) and Thermodynamic characteristics
• Tropical! – unstable,
low-mod CAPE, warm
cloud base (deep warm
cloud layer), extremely
moist troposphere.
• Consistent with late
July onset of the North
American Monsoon in
Colorado.
Mesoscale
Analysis –
Surface
Conditions
•Strong easterly upslope
flow
•Denver cyclone
initiated bow echo squall
line
Mesoscale Analysis –
Cloud Drift Winds and
Visible Satellite Imagery
from GOES-9
1416 MDT
Fuel for the fire…
1715 MDT
GOES-9 derived cloud drift winds show
dramatic late afternoon increase in
strength of moist easterly upslope flow
Mesoscale
Analysis – NLDN
CG Lightning
(1800-2300 MDT)
Bow echo to the
southeast produced
copious cloud-to-ground
(CG) lightning while
quasi-steady convection
over Fort Collins.
What does this say about
nature of the convection
over Fort Collins??
Fort Collins
Bow
Echo
KCYS WSR88D Radar
Observations
– Horizontal
Structure
and
Evolution
Training echoes
becomes quasi-steady
convection over Fort
Collins and Spring
Creek by 2045 MDT
(Internet radar loop:
http://olympic.atmos.colostate.edu/~walt/flood_anim.html)
Radar observations – Evolution of vertical structure from time
height cross-section of mean KCYS WSR-88D reflectivity
• Convection came in
pulses.
• Final pulse produced the
most rain mass flux
• highly unusual
compared to
“typically” assumed
conditions for
flooding.
•Significant factor in
flash flooding.
CG lightning
Radar observations
– CSU-CHILL Lowlevel horizontal flow
field from single
Doppler velocity (Vr)
•Outflow from convection
interacted with strong easterly flow
to create continual regeneration of
cells on south-southeast side.
•Cells then slowly trained toward
the northeast
•Continual regeneration gives the
sense of a single cell parked over
Fort Collins.
Radar Observations – Synthesized
horizontal winds using dual-Doppler
methods on CSU-CHILL and KCYS
WSR-88D radars
Radar
Meteorology:
Polarimetric
Perspective
•Large rain drops (high
Zdr) at leading edge of
convection – warm-rain
collision-coalescence.
•Nearly collocated
with high Zh
•Kdp proportional to rain
rate. High rain rate
located to rear of
convection with smaller
drops.
•Implication for Z-R
rainfall estimation.
Horizontal Structure
Zh (shaded),
Zdr (black),
Kdp (white)
Zh (contour),
R (shaded)
Radar Meteorology: Polarimetric Perspective
Vertical
Structure
Zh (shaded)
Vr (contour)
Zh (shaded)
Kdp (blue)
Zdr (black)
Zh (shaded)
Fi – ice fraction (black)
LDR (blue)
Polarimetric
radar rainfall
estimation
KCYS
NEXRAD
Z-R
CHILL
Blended
R(Kdp,Zdr) and
Z-R
WSR-88D vs. polarimetric radar
rainfall estimation
KCYS: NEXRAD Z-R
unpublished
CSU-CHILL: R(Kdp) and Z-R blend
Polarimetric Tuning of Z-R relations
Radar Cumulative Rainfall (inches)
12
pol-tune Z-R
WSR-88D Z-R
10
Linear (pol-tune Z-R)
Linear (WSR-88D Z-R)
8
6
4
2
0
0
2
4
6
8
10
12
Gauge Cumulative Rainfall (inches)
Loop of cumulative rainfall on Internet:
http://olympic.atmos.colostate.edu/~carey/flood_Z-R_anim.html
Poltune Z-R: -6.5 % bias
88D Z-R: -44% bias
unpublished
Summary
• Polarimetric radar is a powerful tool for
– Microphysical/precipitation physics inferences
– Radar rainfall estimation
• When combined with Doppler data, radar provides
complete kinematic and precipitation observations of
convection in general and, in this case, a flash flood.
– Polarimetric data provided unique microphysical information that
complimented and was consistent with the mesoscale, synoptic,
and Doppler data of the Fort Collins Flash flood.
– Polarimetric radars will provide improved real-time rainfall
estimation (Quantitative Precipitation Estimation – QPE) when
the WSR-88D is upgraded.
• Obvious application for flash flooding.