high-resolution vertical pro les of x-band polarimetric ... · high-resolution vertical pro les of...
TRANSCRIPT
High-resolution vertical profiles of X-band polarimetric radarobservables during snowfall in the Swiss Alps
M. Schneebeli1, N. Dawes2, M. Lehning2 and A. Berne1
1Environmental Remote Sensing Laboratory, EPFL, Lausanne, Switzerland2WSL Institute for Snow and Avalanche Research SLF, Davos, Switzerland
ERAD 2012 - June 26 2012
Introduction Measurements Results Conclusions
Motivation
Snow crystal habit
Can snow microphysical processesbe observed with an X-bandpolarimetric radar?
Can such a radar distinguishdifferent snow particles?
General atmospheric behavior
Do X-band polarimetric variablesexhibit a general behavior withheight?
Can such a behavior be related toatmospheric processes?
2 / 12
High-resolution vertical profiles of X-band polarimetric radar observables during snowfall in the Swiss Alps
Introduction Measurements Results Conclusions
Motivation
Snow crystal habit
Can snow microphysical processesbe observed with an X-bandpolarimetric radar?
Can such a radar distinguishdifferent snow particles?
General atmospheric behavior
Do X-band polarimetric variablesexhibit a general behavior withheight?
Can such a behavior be related toatmospheric processes?
2 / 12
High-resolution vertical profiles of X-band polarimetric radar observables during snowfall in the Swiss Alps
Introduction Measurements Results Conclusions
Overview
In order to properly quantify the instrumental errors, it is nec-
essary that the accuracy of the local deviation estimates is
significantly smaller than the considered errors. The first step
in checking this concerns the quality of the sun ephemeris.
) provides an accu-
racy for the sun position in azimuth and elevation with a max-
imum error of about 0.003 deg from 2003 to 2023, which is
far high enough. In order to reach this accuracy, the geo-
graphic position (referred to WGS84 ellipsoid) of the cen-
ter of rotation of the antenna has to be known with an er-
ror smaller than 0.001 deg (i.e. about 110m, reachable with
a standard GPS beacon). It must be noted that the vertical
deflection can be non-negligible, especially in mountainous
regions, and in that case must be taken into account. This
angle represents the difference at the same position between
the true zenith, which is the reference for the astronomical
Radar at 2133 m above sealevel.
Considered period: End ofFebruary to end of April2010.
Around 110 hours ofsnowfall collected abovethe melting layer.
Contrasting snow events:cold dry snow, aggregates,graupel, dendrites.
3 / 12
High-resolution vertical profiles of X-band polarimetric radar observables during snowfall in the Swiss Alps
Introduction Measurements Results Conclusions
Overview
3 / 12
High-resolution vertical profiles of X-band polarimetric radar observables during snowfall in the Swiss Alps
Introduction Measurements Results Conclusions
Radar calibration
Radar constant determined with a corner reflector.
Radar orientation determined by sun tracking.
Zdr is calibrated by rotating the antenna at 90◦ elevation.
4 / 12
High-resolution vertical profiles of X-band polarimetric radar observables during snowfall in the Swiss Alps
Introduction Measurements Results Conclusions
Experimental data (1)
Radar
RHI scan every 5 min.
150 samples per ray at 1◦ resolution.
6 vertical profiles extracted between 5and 10 km distance from the radar.
Water vapor
Water vapor path (WVP) inferredfrom GPS signal.
WVP separated into three temperaturesegments by using the humidity andtemperature measurements at differentheight levels.
-5 0 5 10 15 20
01
2
3
4
5-7.0 0.0 7.0 14.0 21.0 28.0 35.0
Reflectivity
Fri Mar 26 15:27:22 2010
Radial distance [km]
Heig
htagl[k
m]
-5 0 5 10 15 20
01
2
3
4
5-0.5 0.0 0.5 1.0 1.5 2.0 2.5
Differential reflectivity
Radial distance [km]
Heig
htagl[k
m]
-5 0 5 10 15 20
01
2
3
4
50.76 0.80 0.84 0.88 0.92 0.96 1.00
Copolar correlation
Radial distance [km]
Heig
htagl[k
m]
-5 0 5 10 15 20
01
2
3
4
5-0.60 0.10 0.80 1.50 2.20 2.90 3.60
Spec. differential phase shift
Radial distance [km]
Heig
htagl[k
m]
5 / 12
High-resolution vertical profiles of X-band polarimetric radar observables during snowfall in the Swiss Alps
Introduction Measurements Results Conclusions
Experimental data (1)
Radar
RHI scan every 5 min.
150 samples per ray at 1◦ resolution.
6 vertical profiles extracted between 5and 10 km distance from the radar.
Water vapor
Water vapor path (WVP) inferredfrom GPS signal.
WVP separated into three temperaturesegments by using the humidity andtemperature measurements at differentheight levels.
0
2
4
6
8
10
12
14
60 70 80 90 100 110 120
Day of year 2010
Integrated
water
vapor
[mm]
5 / 12
High-resolution vertical profiles of X-band polarimetric radar observables during snowfall in the Swiss Alps
Introduction Measurements Results Conclusions
Experimental data (2)
Snow intensity
Snow accumulation per time inferredfrom several snow height sensors.
Temperature profile
Determination of the 0◦ level by fittingtemperature measurements at differentheight levels.
60 70 80 90 100 1100
1
2
3
4
5
Snow
rate
[cm
h−1]
Day of year 2010
6 / 12
High-resolution vertical profiles of X-band polarimetric radar observables during snowfall in the Swiss Alps
Introduction Measurements Results Conclusions
Experimental data (2)
Snow intensity
Snow accumulation per time inferredfrom several snow height sensors.
Temperature profile
Determination of the 0◦ level by fittingtemperature measurements at differentheight levels.
1
1.5
2
2.5
3
3.5
-12 -10 -8 -6 -4 -2 0
Heig
ht above s
ea level [m
]
Temperatur [C]
MeasurementsFit
6 / 12
High-resolution vertical profiles of X-band polarimetric radar observables during snowfall in the Swiss Alps
Introduction Measurements Results Conclusions
Distribution of polarimetric observables
−0.5 0 0.5 1 1.5 2 2.5
5
4
3
2
1
0 0
−5
−10
−15
−20
−25
−30
−5 5 15 25 35
5
4
3
2
1
0 0
−5
−10
−15
−20
−25
−30
0
0.05
0.1
0.15
0.2
0.25
−0.5 0 0.5 1 1.5
5
4
3
2
1
0 0
−5
−10
−15
−20
−25
−30
0.75 0.8 0.85 0.9 0.95 1
5
4
3
2
1
0 0
−5
−10
−15
−20
−25
−30
Heigh
tab
ove0◦C
level[km]
Heigh
tab
ove0◦C
level[km]
Heigh
tab
ove0◦C
level[km]
Heigh
tab
ove0◦C
level[km]
Zh
[dBZ]
Zdr
[dB]Kdp
[◦ km−1]
ρhv
[-]
temperature
[◦C]
temperature
[◦C]
temperature
[◦C]
temperature
[◦C](a) (b)
(c) (d)
7 / 12
High-resolution vertical profiles of X-band polarimetric radar observables during snowfall in the Swiss Alps
Introduction Measurements Results Conclusions
Distribution of polarimetric observables
0.75 0.8 0.85 0.9 0.95 11.5
2
2.5
3
3.5
4
−10
−15
−20
−25
0.05 0.1 0.15 0.2 0.25Heigh
tab
ove0◦C
level[km]
Tem
perature
[◦C]
Zdr [dB]
Kdp[◦km−1]
Dendrification signal in Zdr and Kdp → Kennedy et al., JAMC, 2011.
7 / 12
High-resolution vertical profiles of X-band polarimetric radar observables during snowfall in the Swiss Alps
Introduction Measurements Results Conclusions
Mean Hydrometeor identification
0 0.2 0.4 0.6 0.8 10
1
2
3
4
5
0
−5
−10
−15
−20
−25
−30
Heigh
tab
ove0◦C
level[km]
fraction
Hydrometeor identification
AG CR HDG
LDG temperature
[◦C]
Hydrometeor identification with thealgorithm of Dolan and Rutledge,JTECH, 2009.
Increasing abundance of aggregatestowards higher temperatures.
Increasing abundance of graupeltowards higher temperatures.
Aggregates, Crystals, High density graupel, Low density graupel
8 / 12
High-resolution vertical profiles of X-band polarimetric radar observables during snowfall in the Swiss Alps
Introduction Measurements Results Conclusions
Polarimetric profiles as a function of humidity
0 1 2
0
-5
-10
-15
-20
-25
-30
0.8 0.9 1-0.5 0 0.50 0.5 1
0
1
2
3
4
5
-5 5 15
Heigh
tab
ove0◦C
level[km]
(a) (b) (c) (d) (e)
Zh
[dBZ]
Zdr
[dB]
Kdp
[◦ km−1]
ρhv
[-]
# of profiles
[×103]
temperature
[◦C]
low water vapor
high water vapor
9 / 12
High-resolution vertical profiles of X-band polarimetric radar observables during snowfall in the Swiss Alps
Introduction Measurements Results Conclusions
Polarimetric profiles as a function of humidity
0 0.5 10
1
2
3
4
5
0
−5
−10
−15
−20
−25
−30
0 0.5 10
1
2
3
4
5
← c
0
−5
−10
−15
−20
−25
−30
Heigh
tab
ove0◦C
level[km]
Heigh
tab
ove0◦C
level[km]
fraction
(a) (b)
fraction
Low water vapor High water vapor
temperature
[◦C]
temperature
[◦C]
Signature ofdendrification in highwater vapor conditions.
Increased abundance ofgraupel in high watervapor conditions.
Increased abundance ofcrystals in low watervapor conditions.
Aggregates, Crystals, High density graupel, Low density graupel
9 / 12
High-resolution vertical profiles of X-band polarimetric radar observables during snowfall in the Swiss Alps
Introduction Measurements Results Conclusions
Hydrometeor identification vs. snowfall rate
0 0.5 10
1
2
3
4
5
0
−5
−10
−15
−20
−25
−30
0 0.5 10
1
2
3
4
5
0
−5
−10
−15
−20
−25
−30
Heigh
tab
ove0◦
level[km]
Heigh
tab
ove0◦
level[km]
fraction
(a) (b)
fraction
Low snow accumulation High snow accumulation
temperature
[◦C]
temperature
[◦C]
Strongly increasedsignature of graupelformation for highsnowfall rates. →Harimaya and Nakai,JMSJ, 1999; Houze andMedina, JAS, 2005.
Increased abundance ofaggregates for highsnowfall rates.
Aggregates, Crystals, High density graupel, Low density graupel
10 / 12
High-resolution vertical profiles of X-band polarimetric radar observables during snowfall in the Swiss Alps
Introduction Measurements Results Conclusions
Conclusions and Outlook
Conclusions
The average behavior of around 8000 vertical polarimetric profilesmeasured with an X-band radar above the melting layer has beenstudied.
X-band polarimetric profiles as a function of the height above 0◦Care related to microphysical processes such as dendrification,aggregation and riming.
High snowfall rates are coupled to increased riming occurrence.
Outlook
Are we able to theoretically reproduce and confirm theseobservations by coupling an electrodynamical model to a snowmicrophysics model?
11 / 12
High-resolution vertical profiles of X-band polarimetric radar observables during snowfall in the Swiss Alps
Introduction Measurements Results Conclusions
Conclusions and Outlook
Conclusions
The average behavior of around 8000 vertical polarimetric profilesmeasured with an X-band radar above the melting layer has beenstudied.
X-band polarimetric profiles as a function of the height above 0◦Care related to microphysical processes such as dendrification,aggregation and riming.
High snowfall rates are coupled to increased riming occurrence.
Outlook
Are we able to theoretically reproduce and confirm theseobservations by coupling an electrodynamical model to a snowmicrophysics model?
11 / 12
High-resolution vertical profiles of X-band polarimetric radar observables during snowfall in the Swiss Alps
Introduction Measurements Results Conclusions
12 / 12
High-resolution vertical profiles of X-band polarimetric radar observables during snowfall in the Swiss Alps