WMOD00561

• Title: Snow Accumulation Algorithm for the WSR-80D Radar: Final Report
• Date: 7/1/1998
• Weather Modification - Doc Type: Report

1. INTRODUCTION

1.1 The NEXRAD and PPS Systems

Approximately 160 NEXRAD (NEXt generation weather RADar) units were installed across the U.S.
during the period 1991-97 (Fulton et aI., 1998). Individual radars in this network are also referred to as
the WSR-88D (Weather Surveillance Radar-l 988 Doppler). The NEXRAD network represents a major
upgrade and improvement over the aging systc~ms it replaced (Crum et aI., 1993). The NEXRAD
network ofWSR-88Ds is operated by three Ftlderal agencies, the National Weather Service, the Federal
Aviation Administration, and the U.S. Air Force. Consequently, NEXRAD is a tri-agency cooperative
effort among the Departments of Commerce, Transportation, and Defense.

The sole precipitation algorithm currently available on the WSR-88Ds is the PPS (Precipitation
Processing Subsystem) designed for rainfall. National Weather Service forecasters and others have an
existing need of a SAA (Snow Accumulation Algorithm) capable of providing real-time snow water
equivalent and snow depth estimates of reasonable accuracy. Awareness ofPPS limitations with snow
led the NEXRAD OSF (Operational Support Facility) in Norman, Oklahoma, to release a request for
proposals during 1994 for development of a SAA.

1.2 The SAA Agreements

Reclamation submitted a proposal to the OSF in mid-October 1994, which was evaluated along with
proposals from other Federal and Federal-supported agencies. An MOU (memorandum of
understanding) was signed the end of May 1995 among the NEXRAD Program, the OSF, and
Reclamation which called for Reclamation to develop the SAA over a 3-year period. The original MOU
was modified in August 1995 to include collection of snow observations parallel to the south shore of
Lake Erie, east-northeast of Cleveland. Subsequent analysis of these snowfall and radar measurements
was expected to evaluate the ability of the developing SAA to detect and quantify lake effect snowfall. A
second modification in October 1996 tasked Reclamation with helping adapt the SAA for real-time use
and working with the NWS (National Weather Service)FOs (Forecast Offices) at Cleveland and
Minneapolis during the 1996-97 winter's real-time SAA testing. Similar real-time testing was performed
at Albany, NY, during the 1997-98 winter.

In their discussion of real-time SAA testing, Naistat et al. (1998) quote a Minneapolis forecaster who
stated that having the SAA output for snow, "sure beats what we have had in the past, which is not
much." Personal communication with several National Weather Service Science Operations Officers and
Lead Forecasters has indicated that the PPS provides only limited usefulness with snow. This should not
be surprising because it was designed for rain and performs best with deep convective rain (Klazura
et aI., 1998).

1.3 The SAA Techniques

The SAA converts radar measurements of Ze (equivalent or effective reflectivity factor) into estimates of
the spatial distribution of S (snow water equivalent). These are integrated into 1 h,3 h, and storm total
accumulations. The SAA also calculates SD (snow depth) by multiplying accumulated S by the inverse
of snow density. A brief overview of the SAA was presented by Super and Holroyd (1997b).