The NASA Global Precipitation Measurement (GPM) program together with the NASDA space agency of Japan will launch a satellite with a dual frequency (Ka and Ku band) radar and passive microwave sensors (10-183 GHz frequencies) for measuring precipitation over the Earth. This satellite will serve as a calibration reference for a constellation of satellites operated by several countries. It is crucial to validate the GPM satellite measurements at various location around the world. The NASA GPM Ground Validation Program is coordinating ground validation field campaigns at key locations. One of the most comprehensive ground validation field campaigns for GPM will be held from November 2015 through February 2016 on the Olympic Peninsula in the Pacific Northwest of the United States. The primary goal of this campaign, called OLYMPEX, is to validate rain and snow measurements in midlatitude frontal systems moving from ocean to coast to mountains and to determine how remotely sensed measurements of precipitation by GPM can be applied to a range of hydrologic, weather forecasting and climate data.

The Olympic Peninsula is an ideal location to conduct a GV campaign for GPM. It is situated within an active mid-latitude winter storm track in the northwest corner of Washington State. It reliably receives among the highest annual precipitation amounts in North America ranging from over 2500 mm on the coast to about 4000 mm in the mountainous interior. This unique venue is of an ideal size for a field campaign involving aircraft, radars and other ground-based sensors. OLYMPEX will be able to monitor the storm characteristics and processes over the ocean, their modification over complex terrain and the resulting hydrologic impacts. OLYMPEX is ideally suited to quantify the accuracy and sources of variability and uncertainty inherent to GPM measurements in such a varied region.

OLYMPEX will be conducted as an integrated validation program that will simultaineously address several GPM goals:

• Physical validation of the precipitation (rain and snow) algorithms for both the GPM MIcrowave Imager (GMI) and Dual-Frequency Precipitation Radar (DPR).
• How precipitation mechanisms in midlatitude frontal systems and their modification by terrain affect GPM rainfall estimation uncertainties.
• Quantifying the accuracy and uncertainty of the GPM precipitation data and its hydrologic applicability.
• Merging numerical modeling and satellite observations to optimize precipitation estimation in hybrid monitoring systems of the future.

To address the broad focus of OLYMPEX, the ground validation program will include accurate measurements of all aspects of the hydrological cycle on a range of spatial and temporal scales. Specifically, OLYMPEX will monitor and measure the following quantities:

• Seasonal accumulation of the snowpack (in terms of snow water equivalent, or SWE) over the higher terrain, and its variability in the rain/snow transition zone.
• The storm-by-storm liquid and frozen precipitation at multiple sites over the coast, the lowlands, foothills and mountains.
• The upstream meteorological conditions over the Pacific Ocean.
• Brightband variability in height and over ocean, coastal and mountain surfaces.
• Microphysical properties within all sectors of midlatitude storms, before and after their passage over complex terrain.
• Emissivity of a variety of surfaces including the ocean, coastal lowlands, forest and snow-covered mountains.
• River response and runoff.

These requirements will be met with an integrated approach, including:

• Surface precipitation gauge networks and snowpack monitoring instrumentation.
• Accurate surface measurements of falling liquid and solid precipitation using a combination of radar and gauge instruments.
• Disdrometer networks to measure microphysical properties of falling hydrometeors.
• A suite of scanning, dual-polarization and vertically pointing surface-based radars.
• Satellite-simulator and microphysical measurements from aircraft.
• Stream flow monitoring.
• Use of numerical modeling tuned with ground-based data to estimate microphysical properties of precipitation and accumulated snow on the ground for the more remote areas where it is difficult to obtain direct surface measurements.

Through this combination of surface-based instrumentation, snowpack monitoring strategies, multi-frequency radars, aircraft satellite simulators, aircraft and surface-based microphysical measurements, hydrologic measurements and numerical model estimates of many of these quantities, OLYMPEX will provide an unprecedented integrated picture of the surface and in-cloud microphysical properties and their variability that can be transferred to wide-range meteorological and topographic conditions.

For questions, contact Lynn McMurdie