OLYMPEX
Science Summary for 12 November 2015
 
OLYMPEX Aircraft and Radars Observe Warm Moist Prefrontal Flow
Impinging on the Olympic Mountains

Prepared by
Robert A. Houze, Jr., Stephen Rutledge, Lynn McMurdie, Walt Petersen, Kristen Rasmussen,
Angela Rowe, and Joe Zagrodnik

NOTES:
The date starts at 0000 UTC.
This report may be updated as new information becomes available.
Data discussed here were compiled in near real time and have not yet been quality controlled.
Updated reports can be found at http://olympex.atmos.washington.edu/index.html?x=Science_Summaries

This day was the fullest day of operation for OLYMPEX. The NPOL radar, DC8, and Citation aircraft all were in use, and the weather consisted of a major Pacific Northwest rainfall event with orographic enhancement over the Olympic Mountains. The flight tracks of the DC8 and Citation are shown in the left panel of Figure 1. Also note from Figure 1 that the flights occurred at the time of a GPM overpass. The DC8 flew to the west and launched two dropsondes to document upstream conditions. Both aircraft flew multiple tracks in the Quinault Valley region. The Citation accomplished a spiral pattern documenting the microphysics in the region covered by the DOW radar. The synoptic conditions consisted of westerly winds at the exit of a strong zonal jet over the Pacific at 500 hPa (Figure 2 left) and southwesterly flow of warm moist air into the Olympic Mountains at 850 hPa to the surface (Figure 2 right, Figure 3). The front was far to the northwest, and the OLYMPEX region thus lay in the warm air ahead of the front during the entire 24 h period. Soundings taken at the NPOL site showed that the incoming flow was stable as well as moist and warm, up to about the 400 hPa level (Figure 4). Figure 5 shows a time series of the winds and radial velocity at Quillayute as seen by a 449 MHz wind profiler. The veering winds in the lower levels produce a classic S-pattern in the radial velocity PPI of the Langley radar (Figure 6). The geosynchronous infrared imagery shows how the frontal cloud band lay to the west-southwest of the Olympic Peninsula during the whole 24 h period (Figure 7). Note the pronounced rain shadow (absence of cloud) on the northeast (leeside) of the Olympic Mountains. The Langley radar PPIs and vertical cross sections (Figure 8) showed that the radar echo in the rain zone was strongest below about 3 km and had an internal cellularity. The lack of bright band in these images is a result of interpolating widely spaced elevation angles. The bright band was evident in PPIs of specific elevation angle (e.g. Figure 9). Note that a secondary reflectivity maximum occurred a higher level, probably due to aggregation in the dendritic growth layer. This feature was also seen on other radars and in dual-polarimetric variables. For example, the NPOL radar data in Figure 10 show the secondary maximum about 2 km above the bright band in the patterns of ZDR and rhohv as well as in the reflectivity. Another significant feature is in the radial velocity RHI in the lower right of Figure 10, which shows the core of maximum cross-barrier wind rising even before reaching the terrain, consistent with the mountain barrier having an upstream influence in the enhancement of the condensation and precipitation. Figure 11 shows a first attempt to calculate the predominant particle type seen by the NPOL radar data. The Ka-band reflectivity seen by D3R showed the upper level echo as the frontal system moved in (Figure 12, upper left). It shows layering of the precipitation that is classic stratiform in character, with smaller non-oriented ice particles at high levels, larger ice in the layer above the bright band, large melting snowflakes in the bright band, with some indications of possibly graupel in places near the top of the bright band, and rain in the lower layer. When the most intense rainfall was present, the D3R echo extended up to about 8 km (Figure 12, lower left). The DOW radar (Figure 13) looking up the Quinault Valley also showed the secondary maximum above the bright band in ZDR and rhohv. The two times shown in Figure 13 were during the flights (Figure 1, left) and GPM overpass (Figure 13, right). Note how the bright band and secondary maxima bend downward over the mountain slope—a feature that has been noted before in this type of precipitation.  An important discovery in the DOW data is the occurrence of down-valley flow at low-levels shown by the radial velocity data in the bottom panels of Figure 13. This feature has been seen in the Italian Alps in the Mesoscale Alpine Programme and in the British Columbia Rockies but has not previously been documented to occur in the Olympic Mountains. There is a layer of very strong shear between the top of this down-valley flow and the strong cross-barrier westerlies at higher levels. The rainfall amounts in the Quinault region (Figure 14) were generally between 20 and 50 mm. Not that on the leeside the amounts were zero, consistent with the strong rain shadow seen in the satellite data (Figure 7). The drop sizes at the surface stations tended to be much larger than in recent days (Figure 15). Small drops began to be more common at the inland sites later in the 24-h period. The Doppler spectrum of fall velocities (Figure 16) was typical of stratiform precipitation, with a wide spectrum of fall velocities, mostly 4-8 m/s, at low levels and a narrow spectrum of velocities of ~1 m/s above the melting level. The Citation scientists noted  pristine stellars at -13C and low liquid water contents in the upper portions of the radar echoes seen at DOW.

Figure 1. Left panel: Flight tracks of DC8 (cyan) and Citation (pink) aircraft on 12 November 2015, superimposed on the Langley radar echo at 1804 UTC. Right panel: GPM overpass during the flights.

Figure 2. Geopotential height, temperature, and wind at the 500 (left) and 850 hPa level sat 1200 UTC 12 November 2015

Figure 3 Surface 2 m temperature, 10 m winds, and sea-level pressure at 1200 UTC 12 November 2015.

Figure 4 Soundings at NPOL site at 1800 and 2100 UTC 12 November 2015.

Figure 5. Radial (vertical) velocity and wind from the ESRL/PNNL wind profiler at Quillayute, 12 November 2015.

Figure 6. Warm advection shown by the classic S-pattern in the PPI of radial velocity observed by the Langley radar at 1023 PST.

Figure 7.  Infrared satellite imagery at 1500 (left) and 1900 UTC 12 November 2015.

Figure 8.  Langley radar reflectivity superimposed on infrared satellite imagery on 12 November 2015. Cross sections are along the black lines in the left panels.

Figure 9.  Langley radar reflectivity  on 12 November 2015. Inner ring is the bright band. A secondary maximum is at a higher level, probably in a layer of dendritic growth and ice particle aggregation

Figure 10.  NPOL radar data for 0132 UTC 13 November 2015. Top row: reflectivity (left) and ZDR. Bottom row: correlation and radial velocity

Figure 11.  Particle identification algorithm results for NPOL radar data for2223 UTC 12 November 2015.

Figure 12.  D3R radar reflectivity in an RHI scan pointing toward the Quinault Valley between 1200 and 2100 UTC 12 November 2015.

Figure 13.  DOW radar reflectivity in RHI scans pointing northeastward up the Quinault River Valley at 1900 (left) and 2100 UTC 12 November 2015.

Figure 14.  24-h precipitation accumulation map for 12 November 2015.

Figure 15.  Raindrop size distributions for 12 November 2015.

Figure 16.  Doppler velocity spectrum at the Fishery site MRR at 1914 UTC 12 November 2015.