Title: Lessons learned in field studies about weather radar observations in the western US and other mountainous regions
1Lessons learned in field studies about weather
radar observations in the western US and other
mountainous regions
ANNUAL PRECIPITATION
OROGRAPHY
Socorro Medina and Robert Houze Department of
Atmospheric Sciences University of Washington
2Sponsored in part by NSF Award ATM-0505739 NSF
Award ATM-0820586 NASA Award NNX07AD59G
3- Relevant characteristics for radar purposes
- Complex terrain
- High annual precipitation accumulation
4The MM5 climatology was successfully validated
over the Olympic Peninsula maxima (Minder et al.
2008)
Orography (km) and 5-yr MM5 Nov-Jan precipitation
climatology (mm)
Minder et al. 2008
5In addition, the western WA region is
characterized by a low 0ºC level ? low bright
band (BB)
Nov-Jan 0ºC climatology for low-level moist
onshore flow (Quillayute)
Occurrence
Height (km)
Plots by Justin Minder
6Field experiments in the region
? Lessons learned from these past experiments
7IMPROVE-1 domain (Jan Feb 2001)
NCAR S-band dual polarimetric radar (S-Pol) at
the coast
Picture credit Peggy Taylor
8Lesson 1 Low level PPIs (0.0 0.5º) observed
the leading edge of storms several hours before
they reached the coast
Initial time 2103 UTC 1 Feb Final time 0206
UTC 2 Feb ? 5 hours lead time for this case
Radar beam at 4.5 km MSL
9Lesson 2 Sea clutter was only a problem within
the first 20 km from the radar
10Lesson 3 Ground clutter was a big problem,
especially over the Olympic Mountains
However this study was focused on the systems
offshore, so it was not an issue for their
purposes
11IMPROVE-2 domain (Nov Dec 2001)
S-Pol looking directly at the Cascades ? Ground
clutter was a big issue
12Use of polarimetric variables for ground clutter
elimination (87º RHI)
RAW REFLECTIVITY
Height ?
? Ground clutter
Range ?
dBZ
PARTICLE IDENTIFICATION ALGORITHM BASED ON
POLARIMETRIC VARS
CLEAR AIR REFLECTIVITY (CLUTTER MAP)
?Ground clutter
CORRECTED REFLECTIVITY
13Lesson 4 Polarimetric variables were critical
in dealing with severe ground clutter issues in
mountainous regions
RAW REFLECTIVITY
CORRECTED REFLECTIVITY
14Lesson 5 High resolution information on
vertical structure of the reflectivity helpful in
estimating rain-snow line
- Rain-snow line Altitude on the surface where
snow changes to rain - Critical for hydrological purposes and flooding
forecasting -
- Lundquist et al. (2008) found that using bight
band heights measured by vertically pointing
S-band radars improved the estimates of rain-snow
line
NOAA Hydro- Meteorological Testbed (HMT)
? Time (4 Jan 2008)
15Lesson 6 RHIs provide high resolution
information on the vertical profile of
reflectivity (IMPROVE-1 example)
Zooming in over the rectangle, it can be shown
that the vertical resolution at close ranges from
the radar 0.03 km (similar to vertically
pointing radar) ? Detailed information on
vertical structure, including bright band height
16Lesson 7 RHIs are also useful in providing the
vertical structure of the cross-barrier wind (RHI
at 90º across the OR Cascades, IMPROVE-2)
17Besides ground clutter contamination, an
additional concern when deploying a radar in
mountainous terrain is beam blockage, which
reduces visibility
- Pink shading indicates radar coverage at 3 km MSL
- Hatching indicates substantial blockage
18Area of interest and possible locations
100 km range rings from Camano and Portland radars
19Beam blockage for a radar at Westport (PRO good
visibility over full domain at 1º)(CON A bit
far from precipitation maxima in SW Olympics)
PPI 0.0º
PPI 0.5º
PPI 1.0º
Blockage simulator of Lang et al. (2009)
20Beam blockage for a radar at Pacific Beach(PRO
excellent visibility over ocean, close to precip
max)(CON blockage over S. Puget Sound)
PPI 0.0º
PPI 0.5º
PPI 1.0º
Blockage simulator of Lang et al. (2009)
21Beam blockage for a radar at Ocean City(PROs
Near precipitation maximum in SW Olympics, good
coverage) (Area of blockage in NW Olympics due
to closeness ? needed to map the SW Olympics well)
PPI 0.0º
PPI 0.5º
PPI 1.0º
Blockage simulator of Lang et al. (2009)
22View along 0º azimuth from Ocean city
23View along 20º azimuth from Ocean city
24View along 40º azimuth from Ocean city
25View along 60º azimuth from Ocean city
26View along 80º azimuth from Ocean city
27View along 100º azimuth from Ocean city
28View along 120º azimuth from Ocean city
29View along 140º azimuth from Ocean city
30View along 160º azimuth from Ocean city
31To obtain unbiased reflectivity and high quality
precipitation estimates, it is necessary to
correct for beam blockage
- Techniques used
- Use a digital elevation model to identify blocked
rays and add a reflectivity correction based on
the vertical profiles of the reflectivity at
unblocked azimuths or obtained from climatology
(e.g., Germann et al. 2006) - Use polarimetric data to correct the reflectivity
(e.g, Gfiangrande and Ryzhkov 2005, Lang et al.
2009)
32CONCLUSIONS
- Low-level (0 and 0.5º) PPI scans provide
information far upstream, over the ocean - Dual polarimetric variables are important since
they - Help quality control the data, in particular
eliminate ground clutter contamination from
mountains - Aid in producing high quality QPE
- Can potentially be used to compensate for lost of
radar energy due to beam blockage by the terrain - RHI scans provide vertical profiles of
reflectivity and radial velocity, which are
useful in - Estimating rain-snow line
- Compensating the reflectivity at blocked azimuths
- Characterizing the low level flow with high
vertical resolution