P3 Radar Scientist Log

Thumbnails of figures for Case 1

A very intense short wave trough formed over the Pacific and moved toward and over the Pacific Northwest during 26-27 November 2001.  The trough moved into and across Oregon between 1200 UTC 28 November and 0000 UTC 29 November (Fig. 1a, Fig. 1b). The 850 mb winds were generally southwesterly as the trough approached (Fig.  2a, Fig.  2b). A surface cyclone and frontal system occurred at the surface in association with this trough (Fig. 3a, Fig. 3b). The warm frontal clouds moved over Oregon between about 0300 and 1200 UTC; and the cold frontal clouds came through Oregon about 1200-2000. Fig. 4 shows the infrared satellite view at 1400 UTC.   The Eta model and MM5 forecast precipitation moving through the area in conjunction with the cloud system (Fig. 5).

The S-Pol radar showed precipitation moving through the Oregon Cascades from 0400 UTC 28 November 2001 through 1600 UTC 29 November 2001. The warm frontal precipitation had maximum intensity at about 1000-1100 UTC (Fig. 6) and was quite stratiform, as can be seen in the cross section along the 112 degree azimuth (Fig. 7). Above 2.0 km the particles consisted of dry snow with some wet snow and light rain below (as seen in the RHI along the 85 degree azimuth, Fig. 8). As the cold frontal region approached the radar, the echoes  had embedded convective cells (as can be seen along the 112 degree azimuth, Fig. 9). The wet snow in the melting layer became more obvious, and some graupel was embedded in the pattern (e.g. along the 85 degree azimuth at 1800 UTC, Fig. 10). As the back portion of the frontal echo approached the radar from the northwest, it exhibited elongated NW-SE wavelike rainbands (Fig. 11). The P3 and Convair aircraft were taking data in the region of the S-Pol radar from about 1900 UTC 28 November until about 0100 UTC 29 November (Fig. 12). The P3 (red track) was flying a dual-Doppler pattern, while the Convair (yellow track) was flying a microphysics sampling vertical profile pattern.

After the cold front passed through Oregon the precipitation took on a more convective character. A secondary trough went through the area between 0900 and 1400 UTC 29 November. The precipitation intensified during this time. Figure (Fig. 13) (PPI of reflectivity at 1.5) shows orographic convection to the east of the radar and the mesoscale echo associated with the secondary trough approaching from the west. The echo tops in the secondary trough were about 7-8 km and there was some evidence of graupel (Fig. 14, Fig. 15).

After 1600 UTC 29 November there were a few decaying convective cells to the east of the radar Fig. 16.  From 1700-1922 UTC 29 November, the Convair flew a microphysics vertical profile across Santiam Pass in these remaining cells (Fig. 17).

The ETL S-band vertically pointing radar showed detailed vertical and horizontal structure during the passage of the storm over Oregon.  The passage of the warm front was evident as a layer of light precipitation lowering to the surface from about 0730 to 1030 UTC 28 November 2001 (Fig. 18a, Fig. 18b).  During the passage of the cold frontal precipitation the echo showed small scale cells in the snow layer extending down through the melting layer as fall streaks in reflectivity and with indication of updrafts in the cells in the radial velocity data (for example, between 1700 and 1900 UTC 28 November, Fig. 18c). During the post frontal period the cells became shallower and less frequent (for example, 0800-1100 UTC 29 November, Fig. 18d).  The echo associated with the secondary trough was more continuous again between 1200 and 1500 UTC (Fig. 18e, Fig. 18f), but much shallower than the echo associated with the primary cold front. After 1500 UTC the echoes became more scattered in the post frontal regime (Fig. 18f).
 
 

Thumbnails of figures for Case 2

The major storm of 28-29 November was followed the next day by a much less intense short wave trough, which passed over the radar area between 0900 UTC 29 November and 2100 UTC 30 November 2001. At 500 mb, this weak short wave moved rapidly from the west northwest from a longitude of 150W at 1200 UTC 29 November (Fig. 1a) to 135W by 0000 UTC 30 November (Fig. 1b). It was over the Oregon-Washington area at 1200 UTC 30 November (Fig. 1c). The trough was evident at 850 at 135W at 0000 UTC 30 November (Fig. 2a), and by 1200 UTC it was affecting Oregon with southwesterly winds and weak warm advection (Fig. 2b). The region of clouds associated with the trough was just off the Oregon coast at 2100 UTC 29 November (Fig. 3a). The S-Pol radar at 2200 UTC showed scattered small convective cells (Fig. 4a). The region of clouds was over western Oregon at 0300 UTC 30 November (Fig. 3b). The coldest cloud tops formed a patchy pattern. At 0900 UTC 30 November, the last of these patches of cold cloud top was approaching the radar area from the west (Fig. 3c).  It was quite convective in character and just preceded the leading edge of the warm frontal clouds of the next short wave. From 0600-0900 UTC the S-band profiler showed the echo lowering to the surface as the trough approached (Fig. 5a), and then from 0900-1200 it showed rather shallow and discontinous convective cells (Fig. 5b). By 1500 GMT the echo on the S-band profiler had become deeper and more continuous (Fig. 5c). This echo area was located over the S-Pol area at 1500 UTC (Fig. 4b), and it continued to be located over the S-band until about 1630 UTC (Fig. 5d).

Thumbnails of figures for Case 3

The third short wave in this rapid succession passed through during the period 0900 UTC 30 November and 2100 UTC 1 December 2001.  A 500 mb trough formed over the ocean to the west of Oregon (Fig. 1a), moved westward as it intensified (Fig. 1b), and was located so that the 500 mb jet was over Oregon at 1200 UTC on 1 December (Fig. 1c). The accompanying 850 mb trough formed (Fig. 2a) and moved westward while intensifying (Fig. 2b) until it was situated with strong south southeasterly warm advection over Oregon at 1200 UTC on 1 December (Fig. 2c).  The surface low was initially about 987 mb (Fig. 3a). It deepened to 980 mb by 0000 UTC on 1 December (Fig. 3b), and further deepened to 973 mb by 1200 UTC, when it was centered just west of Vancouver Island (Fig. 3c). The infrared satellite imagery showed an enhanced mesoscale cloud area (Fig. 4a) associated with the trough. This feature intensified and was moving over the radar area at 1500 UTC 30 November (Fig. 4b),  just before the leading edge of the warm frontal clouds arrived over the area. The ETL S-band profiler observations from 1400-1700 UTC showed the precipitation to be in a nearly continuous layer of snow containing convective fallstreaks extending downward from about 4 km AGL into a bright band located just above the ground (Fig. 7a, Fig. 7b). At about 0000-0300 UTC the warm frontal portion of the frontal cloud system was moving across the S-Pol radar area (Fig. 4c, Fig. 4d). The S-band profiler showed a continuous layer of cloud with a bright band rising from about 1.0 to 1.2 km AGL in this warm frontal zone (Fig. 7c). In the apparent warm sector, the S-band profiler showed deeper and more discrete convection with a melting level at about 1.3 km (e.g., Fig. 7d). From 0900-1500 UTC 01 December, satellite imagery suggested that the cold frontal portion of the cloud system was moving across the radar area (Fig. 4e, Fig. 4f). The S-band profiler showed a continuous layer of precipitation with the bright band dropping rapidly from 1.3 down to about 0.8 km AGL (Fig. 7e). By 1800 UTC the post frontal region of convective cloudiness was passing over the region (Fig. 4g), and the S-band profiler was showing discrete convective cells (Fig. 7f). By 2100 the comma of the bent back occlusion was moving over the S-Pol area (Fig. 4h). The leading portion of the frontal cloud system had SE-NW oriented bands of enhanced radar echo at 0300 UTC 1 December (Fig. 5a). By 0600 UTC the regions of enhanced echo were more globular (Fig. 5b). By 0900 UTC a band of strong echo, oriented roughly N-S, was just west of the S-Pol radar (Fig. 5c). At 1200 UTC a continuous broad region of echo, apparently associated with the front, was centered over S-Pol, with the bright band evident near the radar location (Fig. 5d). At 1500 UTC the back edge of the continuous frontal echo was over the mountains with cellular post frontal echo moving in behind it (Fig. 5e).  The ETL S-band profiler showed the echo associated with the frontal band aloft and lowering to the surface from 0900-1200 UTC 1 December (Fig. 6a). From 1200-1500 UTC, the echo seen by the S-band profiler was in a thick continuous layer with the bright band dropping as the colder air moved in (Fig. 6b). From 1500-1800 the echo seen by the profiler became a pattern of discrete convective cells (Fig. 6c).

P3 Radar Scientist Log

Thumbnails of figures for Case 4

Post frontal conditions prevailed following the third short wave trough (described above). The satellite infrared sequence shows the convective cloudiness passing over the radar area (Fig. 1a, Fig. 1b, Fig. 1c).  The P3 aircraft flew a mission this day.  When the aircraft was arriving in the S-Pol radar area during 1500-1600 UTC 1 December 2001, the back portion of the frontal precipitation was over the mountains to the east (Fig. 2a, Fig. 2b). The ETL S-band profiler was showing convective cells (as described above (Fig. 3). For about two hours (~1600-1800 UTC) the aircraft executed a dual-Doppler radar pattern in the moderately intense post frontal precipitation over the mountains east of the S-Pol radar (Fig. 2c, Fig. 2d). At 1907 UTC the S-Pol particle identification algorithm indicated graupel at or just above the melting level over an extensive region (Fig. 4). From ~1900-2100 UTC the aircraft executed a microphysical sampling pattern (Fig. 2e, Fig. 2f, Fig. 2g, Fig. 2h ). The aircraft left the region at about 2100 UTC.

Thumbnails of figures for Case 5

The fourth short wave in this multi-day sequence was a comma cloud, which formed in the cold air behind the main frontal zone of the long wave trough.  the comma cloud was associated with a well defined 500 mb short wave trough moving toward and over Oregon from 1200 UTC 2 December 2001 to 1200 UTC 3 December (Fig. 1a, Fig. 1b, Fig. 1c). The passage at 850 mb was accompanied by the winds weakening and changing from southerly to westerly to northerly (Fig. 2a, Fig. 2b, Fig. 2c). The surface low pressure centered moved northeastward across western Oregon while rapidly weakening (Fig. 3a, Fig. 3b, Fig. 3c). The infrared imagery shows this cloud system approaching Oregon at 0600 UTC 2 December 2001 (Fig. 4a), and it was moving into the radar area at 1200 UTC (Fig. 4b). By 1800 UTC it was over the Oregon Cascades (Fig. 4c), and by 0000 UTC  3 December it had moved north into Washington (Fig. 1d). The precipitation area of the comma cloud was moving over the S-Pol radar at 1500-1700 UTC 2 December. The radar echo showed this region of precipitation to be a contiguous region of weaker echo with embedded cells of moderate intensity, suggesting a convective substructure (Fig. 5a, Fig. 5b, Fig. 5c).

P3 Radar Scientist Log

Thumbnails of figures for Case 6

The fifth and sixth short waves in this multi-day sequence moved in rapidly from the west northwest on 3-4 December 2001. At 500 mb the first short wave trough was evident at 1200 UTC 3 December at about 145 W (Fig. 1a). By 0000 UTC 4 December, this trough was at about 132 W (Fig. 1b), and it was centered over the Washington and Oregon Cascades at 1200 UTC  4 December (Fig. 1c). The infrared satellite imagery shows a quasi frontal cloud system associated with the first short wave trough moving into the S-Pol radar area in Oregon at 2300 UTC 3 December 2001 (Fig. 2a).  At 0500 UTC 4 December the cloud area of the short wave was centered over the radar area, with one major component still northwest of the radar, and another band of enhanced brightness east of the radar (Fig. 2b).  By 1100 UTC, the quasi frontal cloud band had moved east of the radar, and post frontal convective clouds were located in the western part of the radar area (Fig. 2c). By 1700 UTC , the enhanced cloud pattern of the second short wave trough was moving over the radar and had the appearance of a jet streak cloud system (Fig. 2d).  This feature strengthened and moved in and became centered over the southern portion of radar area (Fig. 2e). The S-Pol low-level PPI scans were picking up echoes of precipitation moving into the area at 0500 UTC 4 December (Fig. 3a). At 0800 UTC a solid frontal like band of precipitation about 80 km in width was centered over the radar (Fig. 3b). By 1100 UTC this main band of echo had moved over the mountains to the east, and the echoes in the vicinity of the radar had begun to look more post frontal in character (Fig. 3c). Cellular echoes were still present at 1400 UTC but were smaller and weaker (Fig. 3d).

The P3 and Convair flew in the second short wave, as it continued to intensify over the radar area. At 0000 UTC 5 December the enhanced infrared cloud top image was covering the southwestern quadrant of the S-Pol radar area, and the cloud area was being cut off abruptly on its eastern edge, apparently, by orographically induced downward motion (Fig. 4a).  The P3 entered the radar area at about 2300 UTC 4 December and was starting its dual-Doppler pattern at 0000 UTC 5 December (Fig. 5a).  At 0300 UTC the satellite infrared enhance cloud top formed a frontal like band oriented SW-NE and centered over the S-Pol radar (Fig. 4b).  At this time the P3 was flying a microphysical sampling pattern on an E-W track across the mountains east of the S-Pol radar (Fig. 5b). The ETL S-band profiler showed that the precipitation at this time was highly convective, with updrafts appearing at about 1.5 km AGL (Fig. 6).

Thumbnails of figures for Case 7

 Another short wave and frontal cloud band crossed Oregon between about 0000 and 1200 UTC 6 December 2001. The 500 mb trough was at 135 W at 0000 UTC (Fig. 1a ) and had moved past Oregon by 1200 UTC (Fig. 1b). The 850 mb wind was turning to southwesterly at 850 mb at 0000 UTC (Fig. 2a) and had turned westerly following the front at 1200 UTC (Fig. 2b).  The satellite imagery for the 12 hour period preceding the passage of the frontal system showed an interesting development in the enhanced cloud top pattern in the southern half of the  S-Pol radar area (Fig.  3a, Fig. 3b). Precipitation associated with this feature was present on the S-Pol radar at 1200 UTC 5 December (Fig. 4a). This feature was passing over the ETL S-band profiler between 1200 and 1500 UTC 5 December (Fig. 5a). The S-band profiler showed convective structure in the form of vertically extensive fallstreaks. By 1700 UTC only post-frontal-type orographic showers were evident over the mountains east of the radar (Fig. 4b).  The Convair flew a brief mission in the radar area at around 2200 UTC, as the echo aloft (as defined by the circular echo at the 1.5 deg elevation angle of the S-Pol display) was moving into the S-Pol radar area (Fig. 6). At 0000 UTC 6 December, the satellite imagery showed a solid SSW-NNE oriented band of cold cloud top passing over the radar area in association with the frontal system (Fig. 3c). On radar it appeared as a broad continuous region of light precipitation, with a bright band evident as a circle of high reflectivity near the radar in the 1.5 degree elevation base scan (Fig. 4c). Most of the echo in the picture is evidently snow above the 0 deg C level.  At 0300 UTC, the echo in the frontal cloud band took on a fine scale cellular structure, seen both on the S-Pol radar and by the ETL S-band profiler (Fig. 4d, Fig. 5b) There was a double structure in the high cloud band associated with the frontal system. The second band of enhanced infrared imagery was passing through the radar area at 0600 UTC (Fig. 3d). The back edge of this feature corresponded to the surface frontal passage. The radar echo associated with this cloud band associated with the front was generally very light except over the mountains where it was manifested as showers of moderate intensity, which were aligned in the manner of wavelike rainbands (Fig. 4e). For many hours following the frontal passage orographic clouds remained over the mountains in the westerly flow. The cloud tops were quite low (e. g. at 1200 and 1800 UTC; Fig. 3e , Fig. 3f). Despite the low cloud tops the precipitation in this post frontal orographic regime was of moderate intensity (Fig. 4f, Fig. 4g). This persistent post frontal orographic precipitation was at least as intense and continuous as the earlier pre-frontal (Fig. 4c) and frontal (Fig. 4e) periods. The persistent post frontal precipitation appeared on the ETL S-band profiler as a continuous layer of precipitation with a well defined melting layer and fine-scale embedded cellular structure (Fig. 5c).

P3 Radar Scientist Log

Thumbnails of figures for Case 8

A 500 mb short wave trough was at 137 deg W and approaching Oregon at 1200 UTC 8 December (Fig. 1). At 850 mb, the winds over Oregon were southwesterly ahead of the trough (Fig. 2 ), and the band of moisture being advected from the southwest at this level extended to 20 deg N (Fig. 3  ). The surface pressure pattern exhibited a rather weak trough off the coast at this time (Fig. 4). The frontal cloud band was approaching the northwest Oregon coast at 1200 UTC 8 December 2001 (Fig. 5a). By 1800 UTC the main frontal cloud zone was centered over the S-Pol radar (Fig. 5b). By 2100 UTC, S-Pol was seeing echo associated with the frontal cloud band ; it was a contiguous region of weak echo with some embedded cellular elements; and the P3 aircraft was beginning to fly a dual-Doppler pattern in this echo (Fig. 6a). The surface front was reportedly near the leading edge of this cloud band. The rear portion of the frontal cloud region had a more intense appearance with a sharp back edge and patches of colder cloud top that looked wavelike in some enhancements. Possibly these are another example of wavelike rainbands. This cloud band was moving across the northwestern portion of the S-Pol area by 0000 UTC 9 December (Fig. 5c ). The radar echo was a contiguous region of diffuse snow echo of moderate intensity with a bright band surrounding the radar location in the 1.5 deg elevation PPI, and the two aircraft were flying a coordinated pattern, with the P3 continuing the dual-Doppler pattern and the Convair flying a microphysics vertical profile (Fig. 6b, Fig. 6c ).  The back edge of the radar echo associated with the frontal cloud band had a wavy continuous pattern of narrow intense echo when it passed the radar at 0300 UTC (Fig. 6d ).  It looked like a narrow cold frontal rain band but apparently was not, as the front was reportedly in the leading edge of the cloud band, as noted above. By 0600 UTC the frontal cloud region had passed the radar area (Fig. 5d ), and the radar showed only weak post frontal cells over the mountains to the east of S-Pol (Fig. 6e).

Thumbnails of figures for Case 9

A short wave trough in the northwesterly jet moved from the northwest into Oregon at 500 mb from 0000 to 1200 UTC 10 December 2001 (Fig. 1a,  Fig. 1b). The 850 mb trough moved in a similar manner, and the 850 mb wind shifted to weak westerly by 1200 UTC (Fig. 2a, Fig. 2b). The tongue of high relative humidity did not extend far south in this case (Fig. 3a , Fig. 3b).

The cloud pattern associated with this short wave had a frontal structure in the infrared satellite imagery. It was approaching the S-Pol radar area from the northwest at 0000 UTC 10 December    Fig. 4a  and was directly over the region at 0600 UTC   Fig. 4b   . The Portland radar showed a broad region of continuous radar echo at 0637 UTC (Fig. 5a). By 1200 UTC, the coldest cloud top band was past the S-Pol radar area  Fig. 4c   . The Portland radar nonetheless showed a broad region of radar echo with post frontal convective lines offshore (Fig. 5b). The precipitation over and to the east of the radar at this time was either frontal, orographic, or both. By 1800 UTC the infrared imagery showed midlevel cloud tops over the mountains just east of S-Pol (Fig. 4d). The Portland radar showed the orographic precipitation over the mountains distinctly at this time (Fig. 5c).

Orographic precipitation in the Oregon Cascades continued through 11 December. The Little Meadows OR Snotel observing site, located on the western slope of the Cascade Mountains just east of the S-Pol radar, recorded 1.6 inches of precipitation between 1200 UTC 10 December and 1200 UTC 12 December.

Thumbnails of figures for Case 10

A trough barely perceptible at 137 W at 1200 UTC 12 December 2001 UTC 12 December 2001 became better defined and was located at the Oregon coast at 0000 UTC 13 December (Fig. 1a, Fig. 1b). At 1200 UTC on the 12th, warm advection from a previous system was evident along the Oregon coast, and the formative trough at 137 W was just barely perceptible (Fig. 2a, Fig. 2b).  By 0000 UTC the wind was stronger and southwesterly at 850 mb. The tongue of high relative humidity at 850 mb did not extend especially far south, only to about 25 deg N (Fig. 3a, Fig. 3b).  At the surface, the pressure gradient and warm advection intensified along the coast from 1200 UTC 12 December to 0000 UTC 13 December (Fig. 4a, Fig. 4b). Cold advection following this system was extremely weak.

The infrared satellite imagery for 1200 UTC  12 December (Fig. 5a) showed clouds along the coast, apparently associated with the warm advection lingering there after the previous passed over the coast. This region of clouds eventually merged with the warm frontal region of the trough moving in from the west. The warm frontal region of the cloud system was over the coastline and moving over the S-Pol radar area by 1800 UTC (Fig. 5b).  By 2100 UTC (Fig. 5c), the warm frontal part of the cloud system had become enhanced over the mountains, while the more intense cold frontal part of the cloud system was still over the ocean west of the S-Pol radar area.

The Convair flew in this system from 2332 UTC 12 December-0548 UTC 13 December. The cold frontal cloud and precipitation was moving over the western part of the S-Pol radar area at 0000 UTC (Fig. 5d, Fig. 6a). By 0100, the cold frontal clouds had intensified over the mountains (Fig. 5e, Fig. 6b). At 0200 UTC the intensified frontal clouds over the mountains were still apparent, while weak post frontal convective cells were appearing west of the radar (Fig. 5f, Fig. 6c). At 0300-0400 UTC the post frontal convection was over the mountains and more was west of the radar, while a patch of intensified clouds was evident over the oceans on the leading edge of the warm advection region of the next frontal system approaching from the west (Fig. 5g, Fig. 5h, Fig. 6d, Fig. 6e). This patch of cloudiness was moving over the S-Pol radar area during the period 0500-0600 UTC(Fig. 5i, Fig. 5j, Fig. 6f, Fig. 6g).

P3 Radar Scientist Log

Thumbnails of figures for Case 11

A rapidly deepening 500-mb short-wave trough approached the Pacific Northwest between 1200 13 December and 0000 14 December 2001 (Fig. 1a, Fig. 1b). During this period the surface cyclone rapidly deepened from 997 to 983 mb as it approached Vancouver Island (Fig. 2a, Fig. 2b). Meanwhile, a well defined baroclinic cloud shield was moving over the West Coast by 1900 UTC (Fig. 3), which was a little faster than forecast by the operational models. The 0000 UTC 13 December AVN run seemed to have the best timing for this event (not shown).

Before the start of the IOP at 1900 UTC, the precipitation was somewhat convective over the windward slope, with interesting N-S parallel bands over the barrier (Fig. 4). The convective plumes to 4 km ASL seemed to be situated near the various ridges along the western Cascades (Fig. 5). By 21:39 UTC widespread heavy stratiform precipitation was occurring over the IOP site (Fig. 6), with a well defined bright band to 45 dBZ near 2 km ASL and cloud tops above 8 km (Fig. 7). There was tremendous cross-barrier flow at 850 mb (Fig.8). As a result, there was significant orographic precipitation enhancement over the windward slopes around this time (Fig. 9), with an an interesting area of reflectivity enhancement sloping downward from 4 to 2 km ASL from the first set of windward ridges. In addition, there was significant precipitation shadowing in the lee, an ice overhang above 7 km ASL, and low-level precipitation spillover in the immediate lee of the crest.

The P-3 and Convair reached the study area at about 2300 UTC. At this time there was still widespread stratiform precipitation. The P-3 successfully completed the full doppler N-S patterns between 2300 and 0100 UTC (Fig. 10a, Fig. 10b). During this period, the precipitation turned slightly more convective aloft (Fig. 11). This could also be seen on IR satellite, with the "ragged" area of cold cloud tops over the study area (Fig. 12).  There was also significantly more precipitation in the lee at 2300 UTC compared to two hours ago (Fig. 9, Fig. 11), therefore allowing the P-3 to complete all N-S Doppler legs. Meanwhile, the Convair executed SW-NE flight tracks descending down from around 5.5 km ASL. Winds at the S-Pol radar site were gusting strongly. The peak gusts gradually increased from 30 to 37 m/s (the absolute peak value) during the period of about 2300 UTC 13 December through 0100 UTC 14 December.  (Exposure of the anemometer was not ideal, so these values are only an indication of the actual speed. In any case it was exciting to be there!)

At 0000 UTC, the S-Pol particle identification algorithm indicated graupel above the bright band over an extensive region of echo (Fig. 17). The ETL S-band profiler showed a deep continuous layer of echo with a bright band at about 1.7 km AGL between 0000 and 0300 UTC (Fig. 18a). The radial velocity data in this figure showed a considerable region echo between 2 and 3.5 km AGL in which the radial velocity was zero or upward, indicating updraft of a meter per second or more. The pattern showed alternating convective scale cells of upward velocity, just above the melting layer. This pattern is consistent with the appearance of graupel at this level in the S-Pol particle identification field.  The apparent upward vertical air motions were greatest when the cold air came in and the bright band dropped about 0200 UTC. After 0300, the S-band profiler showed the bright band drop eventially to the ground in the post frontal period (Fig. 18b, Fig. 18c). Bob Houze and Brad smull visited the S-band profiler site on the morning of the 14th and found a few inches of snow on the ground (Fig. 19a). Snow was falling while they were at the site, and Bob's dog Schaefer collected some aggregates (Fig. 19b).

The P-3 began its second set of N-S Doppler legs along the lower windward slope by 0130 UTC. During this second set of legs the back edge of the deep upper level cloud/precipitation shield was moving across the study area (Fig. 13), while a second band was moving in from the west (Fig. 14, ). The P3 continued to fly over the windward slope as these frontal precipitation bands moved across the region (Fig. 10c, Fig. 10d, Fig. 10e). The surface front made landfall at the coast shortly before 0300 UTC (Fig. 15).With the passage of the upper level frontal band the winds diminished at flight level (2-3 km ASL) from near 40 m/s to 30 m/s but with no apparent wind shift and no surface frontal passage was reported yet. The precipitation character also turned more convective, with tops 4-6 km ASL. The P-3 encountered some lee wave activity at 0226 UTC and saw dramatic evidence of precipitation spillover to the north of Santiam Pass at 0232 UTC. The P-3 did not complete easternmost N-S leg because there was limited precipitation downwind of the Cascades.

As the Convair began its second mission around 0400 UTC, the P-3 executed a NE-SW cross mountain transect over Santiam Pass at 10 kft (Fig. 10f). The surface front reached the study area at McKenzie Bridge by 0430UTC (Fig. 16). At 0500 UTC, scientists aboard the P3 reported seeing a well defined orographic cloud extending 20-30 km upwind of Santiam Pass, to about 4.5 km AGL. The S-Pol radar during this time period showed SW-NE oriented wavelike bands of echo within this orographic cloud (Fig. 10g). The P-3 ended its mission at 0514 UTC, after completing its final south to north Doppler leg just west of Santiam Pass. The Convair continued to fly in postfrontal convection on the windward slopes till after 0730 UTC (Fig. 10h,  Fig. 10i).
 
 

P3 Radar Scientist Log

Thumbnails of figures for Case 12

The P3 flew from 2200 UTC 15 December-0400 UTC 16 December 2001. At 2200 UTC 15 December 2001, the infrared satellite imagery showed a frontal cloud pattern extending far to the southwest, into the tropics (Fig. 1a). A wave on the front centered at about 128 W was just moving into the Pacific Northwest. A second wave was forming along the frontal cloud band at 150 W.  The warm frontal sector of the first wave was moving across the S-Pol radar area (Fig. 2a). At 0100 UTC 16 December, the two waves on the frontal system were well defined and apparent at 125 and 145 W (Fig. 1b). The warm frontal clouds had moved east of the mountains. The region of most enhanced cloud cover, i. e. the leading portion of the cold frontal cloud band, was moving across the radar area (Fig. 2b). At 0400 UTC, the cold frontal cloud band of the first wave was over the radar area, with an apparent suppression of cloudiness east of the Cascade Range (Fig. 1c, Fig. 2c).

The two waves in the frontal cloud pattern were identifiable in the MM5 initial fields. At 0000 UTC 16 December, the 500 mb flow over the eastern Pacific was southwesterly, extending from a long wave trough line at 150 W all the way to the Oregon-Washington coast (Fig. 3a). The first short wave (hard to see in the 500 mb flow) was moving over the coast. The second short wave trough lay in the axis of the long wave trough at 150 W. By 1200 UTC, the first short wave had moved east of Washington and Oregon as had the long wave ridge, and the second trough had moved eastward over the ocean to about 145 W (Fig. 3b). The 850 mb map for 0000 UTC 16 December showed a frontal zone in the temperature pattern, extending from Vancouver Island southwestward to 160 W (Fig. 4a). The first wave seen in the satellite imagery was manifest as a warm front moving into Washington and Oregon. The P3 flight described below took place in the warm frontal and warm sector region seen in this 850 mb pattern. By 1200 UTC 16 December the warm frontal region had moved east of Oregon and Washington, and the frontal zone over the ocean had moved rapidly eastward (Fig. 4b). The 850 mb winds on these maps were strong southwesterly throughout the 0000-1200 UTC time period. The relative humidity pattern at 850 mb showed that this frontal system was drawing on humidity from well south of 20 N latitude (Fig. 5a, Fig. 5b). The surface map at 0000 UTC showed the warm front moving into the Washington-Oregon region (Fig. 6a). The surface map at 1200 UTC showed that the whole frontal system had moved rapidly eastward during the 0000-1200 UTC time period (Fig. 6b).

During the P3 flight, the radar echo had a rather convective appearance, and the echo pattern was concentrated over the windward slopes just east of the S-Pol radar (Fig. 7a, Fig. 7b, Fig. 7c, Fig. 7d, Fig. 7e, Fig. 7f). The P3 flew a short dual-Doppler pattern (N-S legs) followed by a cloud physics pattern. From the radar/flight track images, it can be seen that during the time of the flight the precipitation on the windward slopes varied in intensity.  Convective bands oriented N-S appeared to move W-E into the mountains and intensify.

P3 Radar Scientist Log

Thumbnails of figures for Case 13

On 15-16 December, two frontal waves were located along the frontal system moving into the Pacific Northwest (see previous discussion). At 1000 UTC 16 December, the infrared satellite imagery showed the first frontal wave to the east of the Cascades; a region of enhanced cloudiness in advance of the leading edge of the warm frontal clouds of the second wave was located over Washington at this time (Fig. 1a). A southward extension of the feature over Washington was over the S-Pol radar area (Fig. 2a). Warm frontal clouds of the second wave were moving over the S-Pol area at 1600 UTC (Fig. 1b, Fig. 2b). By 2200 UTC, the cold frontal cloud zone of the second wave was moving across the S-Pol radar area from the west (Fig. 1c), and clouds were being suppressed east of the Cascades (Fig. 2c).

At 0000 UTC 16 December, the 500 mb trough was at 132 deg W and moving rapidly toward Oregon and Washington (Fig. 3a). By 1200 UTC it had pass through the region (Fig. 3b). The 850 mb winds were strong southwesterly at 0000 UTC over Oregon in the warm air ahead of the trough (Fig. 4a). These winds turned to westerly with the trough passage (Fig. 4b). The tongue of high relative humidity extended southwestward into the tropics (Fig. 5a, Fig. 5b).  At 0000 UTC, the cold front was approaching the coast (Fig. 6a), and by 1200 UTC it had passed through Oregon with the ridge building up behind it (Fig. 6b).

The P3 flew a full duration mission from 2300 UTC 16 December-0900 UTC 17 December in this second wave (see further discussion and graphics below). At 0100 UTC 17 December, the cold frontal cloud band was very well defined and intense in the infrared satellite imagery, and it was located over western Washington and Oregon (Fig. 1d, Fig. 2d). At this time, Bob Houze and Brad Smull were each driving along I-5 in southwest Washington in continuous heavy rain . Around Centralia and Chehalis, they experienced some of the heaviest rain they've seen in a Washington winter storm. Between 3 and 4 inches of rain fell at those locations. The P3 was flying at this time (Fig. 7a). The radar echo at 1.5 deg elevation showed continuous snow over the S-Pol area with a well defined bright band. The echo maintained this structure for several hours. The P3 was flying a dual-Doppler pattern through the snow layer. By 0400 UTC , the higher cloud tops had moved east of the mountains of central Oregon (Fig. 1e), but continuous cloud top of lesser height remained over the S-Pol radar area (Fig. 2e). The P3 aircraft was flying a SW-NE microphysical track at this time, and the echo on S-Pol was beginning to appear more cellular in the western part of the region of radar coverage (Fig. 7b).  At 0600 UTC, the S-Pol particle identification algorithms indicated a cellular echo with graupel just above the melting level (Fig. 8). At 0700 UTC, clouds related to the second frontal wave lingered over the Cascades, with a clear zone east of the mountains separating the clouds over the mountains from the higher clouds of the frontal wave far to the east (Fig. 1f, Fig. 2f). The P3 was flying through the clouds over the mountains at this time, with isolated small cells of a post frontal nature appearing over the western foothills of the mountains (Fig. 7c). By 1000 UTC 17 December, the cloud regime over western Oregon was entirely post frontal (Fig. 1g, Fig. 2g).

P3 Radar Scientist Log

Thumbnails of figures for Case 14

The 500 mb map for 1200 UTC 18 December 2001 showed a trough at about 130 deg W approaching the Pacific Northwest coast (Fig. 1a). By 0000 UTC 19 December it had moved into Oregon (Fig. 1b). At 850 mb at 1200 UTC 18 December, the trough was well defined with moderately strong winds, but ahead of the trough, over the Oregon Cascades, the winds were only about 15 m/s from the SSW (Fig. 2a). By 0000 UTC 19 December, the 850 winds were 10-15 m/s from the SW over the Oregon Cascades (Fig. 2b). The tongue of high humidity at 850 mb did not extend south of 20 deg N (Fig. 3a, Fig. 3b). The surface map at 1200 UTC 18 December showed a moderately strong front over the ocean west of Oregon and a strong SW-NE pressure gradient across western Oregon (Fig. 4a). By 0000 UTC, the front had moved over the mountains and the pressure gradient over Oregon was weak (Fig. 4b).

The infrared satellite imagery for 1200 UTC 18 December 2001 showed an intense frontal cloud band moving over the western portion of the S-Pol radar area (Fig. 5a). The back edge of the cold cloud top was offshore. The southwestern portion of the cloud band showed a small scale transverse banded structure, as has been seen in previous cases during the project. By 1500 UTC, the frontal cloud band was directly over the radar area, and a comma cloud was forming south of Vancouver Island (Fig. 5b).

The P3 aircraft flew from 1700 UTC 18 December-0200 UTC 19 December. The first part of the flight documented flow in the Columbia Gorge. The rear portion of the frontal cloud band was over the Gorge, with precipitation occuring and thus allowing for airborne dual-Doppler radar measurements. The aircraft therefore headed southeast out of Seattle to a point along the Columbia River and then flew WSW along the Gorge till about 1800 UTC (Fig. 5c). Then the P3 headed south into the S-Pol radar area where they arrived about 1900 UTC (Fig. 6a). From about 1800-2000 UTC the P3 flew a SW-NE track through the back edge of the frontal cloud and precipitation pattern over the mountains just east of the S-Pol radar (Fig. 5d, Fig. 5e). The 1.5 deg elevation echo in this region in which the P3 was flying, had the smooth uniform appearance of stratiform snow, as has been typical of the frontal precipitation regions in previous systems seen in the project (Fig. 6b, Fig. 6c) The SW-NE oriented band of echo moving toward the radar from the west of the radar during this time period had a more convective appearance and was located in the tail of the comma cloud seen in the satellite imagery. From 2000-2300 UTC, the P3 flew a dual-Doppler pattern in the tail of the comma cloud (Fig. 5f, Fig. 6d). The comma cloud was extremely prolific producer of liquid water (multiple excursions above 0.5 g/m**3) and moderate-to-severe icing.  As the P3 flew in the tail of the comma cloud, another cloud band, separated from the tail of the comma by a relatively clear gap in the satellite image, was moving in from the west (Fig. 5e , Fig. 6e, Fig. 5f). By 0000 UTC 19 December, the comma cloud was east of the Cascades, and the P3 was flying in the next cloud band as it moved over the mountains (Fig. 5g,  Fig. 6f). From the viewpoint of the P3 this latter part of the flight pattern was described by Brad Smull as, "cold-core showers congealing into a more continuous tho shallow precip shield as they piled into the Cascades." The shoulder of the next warm frontal cloud pattern was in the southwest part of the S-Pol radar area. By 0300 UTC the regime over the S-Pol radar was generally post frontal, with the shoulder of the next warm frontal pattern still in the southwest quadrant of the radar area (Fig. 5h).

Thumbnails of figures for Case 15

This case differed from all the previous IMPROVE II cases in that a low to the southwest of Oregon produce easterlly flow over the study area, so that the eastern slopes of the Cascades became the windward slope of the terrain. Thus, the orographic enhancement occurred on the eastern rather than the western slopes when the easterly flow became predominant. At 500 mb, a well defined short wave trough was moving eastward, intensifying, deepeniing, and beginning to cut off between 19 December 1200 UTC and 20 December 0000 UTC ( Fig. 1a, Fig. 1b). As the intensifying low moved eastward, the 850 mb flow changed from weak southerly to strong east-southeasterly (Fig. 2a, Fig. 2b). The tongue of high relative humidity at 850 mb reached into the storm from 15 deg S (Fig. 3a, Fig. 3b). The sea-level pressure and temperature pattern showed a warm front moving over the study area during this time period (Fig. 4a, Fig. 4b).

The Convair flew its first mission from 2229 UTC 19 December-0419 UTC 20 December.  The infrared satellite data for 19 December 2200 UTC and 20 December 0400 UTC showed the warm frontal clouds moving over the region during this time period (Fig. 5a, Fig. 5b, Fig. 5c, Fig. 5d). The Convair flew north-south legs both east and west of the crest of the Cascades to see the warm frontal structure on both sides of the mountains. Before the aircraft reached the study area, the S-Pol showed a layer of precipitation aloft and gradually lowering (indicated by the shrinking circle of no echo around the radar at 1.5 deg elevation, Fig. 6a, Fig. 6b, Fig. 6c). At 2200 UTC, approximately the beginning of the Convair's first flight, the echoes reached the surface (Fig. 6d). At 2300 UTC, the echo appeared as a relatively featureless continuous layer of snow above a well defined intense bright band (at the first range ring in Fig. 6e). By 0000 UTC 20 December, the bright band was weaker and at a higher altitude, indicating general warming with the passage of the warm frontal region (Fig. 6f). By 0100 UTC, the echo was weakening and disappearing to the southwest of the radar, while the echo to the northeast was more continuous (Fig. 6g).  From 0200-0300, the echo to the northeast weakened and more cellular echo appeared to the northeast (Fig. 6h, Fig. 6i), as the cold frontal part of the system approached (cf Fig. 5b).

The Convair's second flight was from 0506-0609 UTC 20 December. The infrared satellite data for 0400 UTC-0600 UTC showed that the S-Pol radar was in a gap between warm frontal clouds to the east of the mountains and cold frontal clouds moving in from the west across the Oregon coast (Fig. 5d, Fig. 5e). At 0400 UTC, the S-Pol radar showed the warm frontal precipitation aloft to the northeast of the radar, and convective (warm sector?) echo just west of the radar (Fig. 6j). At 0600 UTC, the warm frontal echo to the northeast had disappeared, and a fairly continuous echo was moving in from the southwest (Fig. 6k) in asociation with the cold frontal cloud band (Fig. 5e). From 0800-1200 UTC, the satellite imagery showed the cold frontal cloud band moving slowly across the radar area (Fig. 5g, Fig. 5h, Fig. 5i). The satellite images sugges that the frontal cloud band had embedded substructure in the form of transverse wavelike bands (esp. Fig. 5h), and the radar showed weak echo with embedded cellular structure (Fig. 6l, Fig. 6m).

Thumbnails of figures for Case 16

A nearly cut off short wave at 500 mb moved into Oregon during 1200 UTC 22 December and 0000 UTC 23 December 2001 (Fig. 1a, Fig. 1b). The wind at 500 mb turned from southerly to southeasterly during this period. The wind at 850 mb was southerly and weakening during this period (Fig. 2a, Fig. 2b). The tongue of high relative humidity reached southwestward to only about 20 deg N (Fig. 3a, Fig. 3b). The west-east gradient of sea level pressure was very strong at 1200 UTC but decreased during the period (Fig. 4a, Fig. 4b). This west-east gradient favored easterly flow through the Columbia Gorge.

The satellite imagery showed an occluded frontal cloud band moving across western Oregon from 1200-1500 UTC 22 December (Fig. 5a, Fig 5b, Fig. 5c). At 2100 UTC, a comma cloud was moving over western Oregon (Fig. 5d).

The Convair flew two flights between 1400 UTC and 2300 UTC. The Medford radar showed warm frontal rainbands oriented NW-SE moving through western Oregon from 0900-1200 UTC (Fig. 5e, Fig. 5f). At 1500 UTC, the Medford radar showed the last warm frontal rainband to the northeast of the radar, while the rainbands over and to the southwest of the radar were changing over to north-south oriented cold frontal rainbands (Fig. 5g). By 1800 UTC the cold frontal rainbands were still evident but much weaker (Fig. 5h). At 2100 UTC post frontal convective echoes, located just ahead of the comma cloud were detected to the southwest of the radar (Fig. 5i).