OLYMPEX
Science Summary for 13 November 2015
 
Huge Rain Accumulations in the Mountains in the Warm Prefrontal Regime
Followed by a Narrow Cold Frontal Rainband

Prepared by
Robert A. Houze, Jr., Angela Rowe, Kristen Rasmussen, Simone Tanelli, 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

An enormous amount of rain fell on the windward side of the Olympic Mountains as the prefrontal warm air very slowly moved across the region. A narrow cold frontal rainband marked the end of the main rainy period. In Figure 1, the prefrontal precipitation is mainly to the east of NPOL and the frontal band is to the east. More details of these features will be described below. The DC8 and Citation flying between 1400 and 2000 UTC sampled both the prefrontal and frontal zones (tracks in Figure 1). Figure 2 shows the great amounts of rain that fell in the 24-h period of 0000-2359 UTC 13 November. Note that these were maximum in the Quinault valley, where amounts were between 100 and 300 mm. These were in addition to the 50-100 mm in the same region the previous 24 h. The runoff nearly flooded the DOW radar (Figure 3), where Lake Quinault was rising by about half a foot every hour for several hours. At 500 hPa, the OLYMPEX region lay on the south side of a jet east of a trough, which was digging moving slowly eastward (Figure 4). At 850 hPa, the wam flow continued stream toward the Olympic Mountains (Figure 5). The soundings  taken at NPOL showed showed moisture up to 200 hPa, with an occasional dry layer between cloud decks and slight moist instability in the lower troposphere (Figure 6). The satellite imagery in Figure 7 shows that the OLYMPEX area lay under the frontal cloud band for the whole period. Note that by 2100 UTC wave development on the front was manifesting. The Langley radar data in the left panel of Figure 8 show the large prefrontal echo over the Olympic Mountains and the narrow cold frontal rainband far to the northeast. The right panel shows the narrow cold frontal rainband arriving over the OLYMPEX region at about 1600 UTC. It had not yet reached NPOL (cross in the center of Figure 8). Figure 9 shows NPOL PPI displays of reflectivity and radial velocity at 1619 UTC. The narrow cold frontal rainband is the narrow line in the reflectivity, and the wind shift at the line is evident in the radial velocity display. Figure 10 shows RHIs across the cold frontal rainband. The reflectivity maximum closest to the radar is the front. The velocity cross section shows the sharp frontal wind shift showed the westerly component concentrated in the lowest half kilometer with a bulbous nose, like a density current. The vertical profile of ZDR and particle identification signals look similar in the cell at the frontal convergence line to those elsewhere ahead of and behind the line. Figure 11 shows how the low-level velocity maximum rises over the terrain. This rise is a principal factor in the orographic enhancement of the precipitation over the mountains (accounting for the huge rain amounts seen in Figure 2). Figure 12 shows RHI displays from the DOW radar during two times (0253 and 0543 UTC) during the tie of enhanced prefrontal precipitation. The echo was basically stratiform but contained embedded convection, as seen prominently in the later example in Figure 12. In the earlier example, the stratiform radar echo showed clearly the persistent secondary maximum 2-3 km above the melting level (i.e. in the dendritic growth zone, where ice particle aggregation occurs); it appears in dBZ, ZDR, and rhohv. Figure 13 shows examples of the Ku and Ka band APR radar data obtained on the DC8 flight shows the extensive prefrontal stratiform precipitation. The earlier section (left panel) shows the high bright band in the prefrontal precipitation over the mountains. The later section (right panel) shows the rise of the bright band from behind to ahead of the front. Detailed observations of the precipitation at the Prairie Creek site in Figure 14 shows the accumulation of 300 mm by the dual tipping bucket gauge (left panel). Rain rates hovered around 15-20 mm/h continuously for over 16 h as the moist flow kept rising over the mountain ridge (middle panel). The drop diameters tended to be small but with some larger drops of 4-5 mm (right panel). The MRR data in Figure 15 show that the fall speeds were mostly ~6 m/s, occasionally reaching 7-8 m/s. Hurricane ridge had smaller drops and lower fall speeds (~2-3 m/s) and almost no snow (Figure 16).

Figure 1. Flight tracks of DC8 (cyan) and Citation (pink) aircraft on 13 November 2015 superimposed on the Langley radar echo at 1623 UTC.

Figure 2. 24-h precipitation accumulation on 13 November 2015.

Figure 3. DOW radar on, left to right 3 , 13 and 14 November 2015.

Figure 4. Geopotential height, temperature, and wind at the 500 hPa level at 0600 UTC and 1800 UTC 13 November 2015

Figure 5. 850 hPa geopotential height, temperature, and winds at 0600 and 1800 UTC 13 November 2015.

Figure 6. Soundings at NPOL, 0600 and 1800 UTC 13 November 2015

Figure 7. Infrared satellite imagery for 13 November 2015. Left to right: top 0300 and 0900 UTC, bottom 1500 and 2100 UTC.

Figure 8. Langley radar imagery for 1200 and 1600 UTC 13 November 2015 UTC.

Figure 9. NPOL reflectivity (left) and radial velocity (right) at 1619 UTC 13 November 2015.

 

Figure 10. RHIs of dual polarimetric variables seen by the NPOL radar at 1631 UTC 13 November 2015.

 

Figure 11. NPOL radial velocity cross sections pointing northeast at 0055, 1335, and 1414 UTC 13 November 2015. Streaks of side lobe echoes associated with the rough terrain obscure the velocity patterns.

Figure 12. DOW radar RHIs pointing up the Quinault Valley at 0253 and 0543 UTC 13 November 2015.

Figure 13. Reflectivity from APR2 radar frequencies Ka and Ku on DC8 on 13 December 2015 (top two panels). Left: Crossing NPOL east to west. Right: Crossing NPOL west to east.

Figure 14. Detailed precipitation data collected at the Prairie Creek site 13 November 2015.

Figure 15. MRR data from Bishop/CRN site for13 November 2015. Data interrupted by power outage

Figure 16. Detailed data from Hurricane Ridge site, 13 November 2015.