Lessons learned in field studies about weather radar observations in the western US and other mountainous regions Socorro Medina and Robert Houze Department of Atmospheric Sciences University of Washington OROGRAPHY ANNUAL PRECIPITATION

Sponsored in part by: NSF Award# ATM NSF Award# ATM NASA Award# NNX07AD59G

Relevant characteristics for radar purposes:  Complex terrain  High annual precipitation accumulation

The 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

In addition, the western WA region is characterized by a low 0ºC level  low bright band (BB) Plots by Justin Minder Nov-Jan 0ºC climatology for low-level moist onshore flow (Quillayute) Occurrence Height (km)

Field experiments in the region IMPROVE-1 IMPROVE-2 NOAA Hydro- Meteorological Testbed (HMT) CASCADE CYCLES SCPP (Sierra)  Lessons learned from these past experiments

IMPROVE-1 domain (Jan – Feb 2001) NCAR S-band dual polarimetric radar (S-Pol) at the coast Picture credit: Peggy Taylor

Lesson 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

Lesson 2: Sea clutter was only a problem within the first ~20 km from the radar

Lesson 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

IMPROVE-2 domain (Nov – Dec 2001) S-Pol looking directly at the Cascades  Ground clutter was a big issue

Use of polarimetric variables for ground clutter elimination (87º RHI) CLEAR AIR REFLECTIVITY (CLUTTER MAP) PARTICLE IDENTIFICATION ALGORITHM BASED ON POLARIMETRIC VARS RAW REFLECTIVITY CORRECTED REFLECTIVITY Height  Range   Ground clutter dBZ

Lesson 4: Polarimetric variables were critical in dealing with severe ground clutter issues in mountainous regions CORRECTED REFLECTIVITY RAW REFLECTIVITY

NOAA Hydro- Meteorological Testbed (HMT)  Time (4 Jan 2008) 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 Lesson 5: High resolution information on vertical structure of the reflectivity helpful in estimating rain-snow line

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 Lesson 6: RHIs provide high resolution information on the vertical profile of reflectivity (IMPROVE-1 example)

Lesson 7: RHIs are also useful in providing the vertical structure of the cross-barrier wind (RHI at 90º across the OR Cascades, IMPROVE-2)

Besides 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

Area of interest and possible locations 100 km range rings from Camano and Portland radars

Beam blockage for a radar at Westport (PRO: good visibility over full domain at 1º) (CON: A bit far from precipitation maxima in SW Olympics) Blockage simulator of Lang et al. (2009) PPI = 0.0º PPI = 0.5ºPPI = 1.0º

Beam blockage for a radar at Pacific Beach (PRO: excellent visibility over ocean, close to precip max) (CON: blockage over S. Puget Sound) Blockage simulator of Lang et al. (2009) PPI = 0.0º PPI = 0.5ºPPI = 1.0º

Beam 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) Blockage simulator of Lang et al. (2009) PPI = 0.0º PPI = 0.5ºPPI = 1.0º

View along 0º azimuth from Ocean city

View along 20º azimuth from Ocean city

View along 40º azimuth from Ocean city

View along 60º azimuth from Ocean city

View along 80º azimuth from Ocean city

View along 100º azimuth from Ocean city

View along 120º azimuth from Ocean city

View along 140º azimuth from Ocean city

View along 160º azimuth from Ocean city

To 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)

CONCLUSIONS 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