Precipitation and Propagation Characteristics of Narrow Cold-Frontal Rainbands during the OLYMPEX Field Campaign

The Olympic Mountain Range in the Pacific Northwest often experiences frontal passages associated with mid-latitude cyclones to the north. Narrow cold-frontal rainbands (NCFRs) associated with these frontal passages approach the coast from the west, forming over the ocean and propagating over land across the mountain range. The OLYMPEX field campaign performed focused precipitation measurements in this mountainous region for the ground validation of GPM. Since NCFRs are characterized by localized intense precipitation, it is important to examine the effects of orographic enhancement on their formation, structure, and evolution. In this region, these NCFRs are unique since they traverse several types of terrain from formation to decay as they develop over the ocean, propagate towards the coast, and are quickly met by the mountain range.

In this study, analysis of three cases of NCFR passages during OLYMPEX relate NCFR precipitation distribution, as seen by the WSR-88D radar, with the mesoscale and synoptic-scale frontal structure. Beginning with their first detection by radar, the individual NCFRs are separated into three distinct zones: Zone A is the portion of the band that spends the most time on land; Zone B serves as a transition zone straddling land and ocean; and Zone C is the portion of the band that spends the most time over the ocean. This zonal breakdown allows for a closer analysis relating the changing radar characteristics of the bands to the terrain they propagate over, such as the precipitation intensity as seen in the maximum reflectivity values, the propagation speeds, and the physical dimensions of the NCFR in terms of its narrowness. These characteristics of the NCFR are related to the sharpness in the gradient of the wind shift and temperature change in the frontal zone.

One case with a mean propagation speed nearly twice as slow as the other two cases had a poorly defined wind-shift at the surface as well as a 10 dBZ weaker mean maximum reflectivity. Conversely, the other two cases both exhibited well-defined frontal boundaries, similar mean propagation speeds, and similar mean maximum reflectivities. Despite the differences in intensity, speed, and frontal boundary definition, the weaker case and one of the stronger cases are both considered “atmospheric river” events. This influx of moisture aided the stronger case to appear on radar as a textbook example of a NCFR, while the weaker case appeared as a small NCFR surrounded by scattered precipitation in the region. Interestingly, the other well-defined case was not considered an atmospheric river event, although it possessed the highest mean propagation speeds and mean maximum reflectivities. Further analysis will be conducted on the effects of the changing terrain on the different zones of the NCFR.