Orographic Enhancement above the Melting Layer as Observed during the Olympic Mountains Experiment (OLYMPEX)

The west coast of North America is frequented by extratropical cyclones from the Pacific Ocean during the cold season. When these storms pass over coastal mountain ranges, they produce copious precipitation on the windward slopes and significantly reduced precipitation on the leeward slopes. Many past studies describing the degree of precipitation enhancement by complex terrain focused primarily on the surface distribution of precipitation. However, the modification of the airflow by the mountains occurs throughout the vertical column of the atmosphere and the enhancement is not likely to be restricted to the surface. In this study, high-quality range height indicator (RHI) scans from an S-band dual-polarization Doppler radar (NPOL) located on the southwest coast of the Olympic Peninsula during the Olympic Mountains Experiment (OLYMPEX) are used to describe orographic enhancement above the melting level. NPOL operated over an 8-week period from November 2015 through middle of January 2016 and sampled all sectors (prefrontal, warm and postfrontal) of numerous land-falling cyclones. The NPOL scanning strategy included RHI scans over an oceanic sector and a narrower land sector focusing on the Quinault River Valley and surrounding complex terrain. Contoured Frequency by Altitude Diagrams (CFADs) were created using all the ocean sector scans and again using all the land sector scans during the entire OLYMPEX campaign.

Comparison of the ocean and land CFADS reveals that the land CFAD exhibits higher reflectivity than the ocean CFAD at all levels (from 2—8 km) with the effect particularly pronounced in the 4—5 km layer. The land CFAD also exhibits higher reflectivity at levels associated with the bright band than the ocean CFAD. The data were further characterized by environmental conditions, such as melting level height, integrated vapor transport, moist static stability and low-level wind direction and intensity using the North American Regional Reanalysis (NARR) data at a grid point closest to the NPOL location. Although the upper level reflectivity enhancement over land occurs to some degree for all environmental categories, the enhancement is particularly pronounced during environmental conditions characteristic of strong water vapor flux in the warm sector of cyclones (such as during atmospheric river events).

These results have implications for remote sensing precipitation from space. The recently launched core satellite of the Global Precipitation Measurement (GPM) mission has a space-borne radar, the Dual-frequency Precipitation Radar (DPR). CFADS from the Ku-band of the DPR were created from the cold season of 2015-2016 for an area encompassing the Olympic Peninsula and again over a ocean region just offshore to the west of the Peninsula, similar to the RHI scan sectors made by NPOL. The same structure of enhancement aloft over the land region was found in the DPR CFADS as in the NPOL CFADS indicating this signature can be observed from space-borne radars.