Water Year 2013 CODOS Summary for
Berthoud Summit
Summary | Snowpack & Dust | Melt Rates | Stream Flows
Summary
Water Year 2013 entered late winter with dismal snowpack development at ten of eleven CODOS dust-on-snow monitoring sites, Willow Creek Pass being the single exception. Through March 2013, snowpack SWE totals at all other sites resembled or even fell short of the very dry winter of WY 2012, raising concerns about back-to-back drought seasons. That broadly consistent spatial pattern in scant precipitation began to diverge in April, 2013. CODOS sites in the northern Front Range and Colorado River headwaters benefitted most from a series of April and May winter storms that augmented snowpacks and eventually resulted in average or even above average peak SWE levels on average or later-than-average dates. In the southwestern mountains, fewer and/or smaller April/May storms failed to offset the dry beginning to WY2013 winter. CODOS sites in the San Juan Mountains experienced, for the second year, substantially sub-par peak SWE values, on near-average dates. In between, the Central Mountains, Grand Mesa, Park, and northern Gore ranges came closer to but still fell short of average snowpacks. Berthoud Summit was among those sites where WY 2013 snowpack development and peak SWE lagged WY 2012 into March but then surged in late winter and spring to eventually reach slightly above-median levels.
At the CSAS Senator Beck Basin Study Area at Red Mountain Pass, the primary CODOS monitoring site, WY2013 produced a total of ten separate dust-on-snow events, a lower total count than in the past three seasons. However, the actual mass of dust deposited at Senator Beck Basin in WY 2013 was greater than in any prior season, including the “Martian Winter” of WY 2009. Dust season began at Senator Beck Basin in November with a single event, February and March delivered four additional events, but the vast majority of dust deposition occurred during events D6 on April 8th and the long-duration D8 event of April 15-17. As of late March, dust layers observed at Senator Beck Basin were either weak or not present at CODOS sites farther north and east, creating some hope that WY 2013 might be dust-free in parts of the state. Then, dust events D6 and D8 spelled the end to that possibility with significant depositions statewide near the top of the snowpack.
As at all CODOS sites, dust layers D6 and D8 played the principal role in very significantly reducing snowcover albedo in the Berthoud Summit and Berthoud Pass locale as they became exposed, merged, and remained exposed. Snowmelt rates at the Berthoud Summit Snotel site were often high but periods of reduced solar insolation because of cloud cover, and/or higher albedo after fresh snowfalls, disrupted the potential for maximum radiative forcing of snowmelt in those layers. Those periods of unsettled weather also interrupted snowmelt runoff but, once underway, the Fraser River sustained and exceeded median levels before a final surge to and well above median peak flow, close to the median date.
SnowPack & Dust
SWE accumulation and ablation for Water Years 2012 and 2013, with dust-on-snow events shown as brown bars, by date (as observed at the Senator Beck Basin Study Area). The long-duration D8 event of April 15-17, 2013, is shown as a heavier bar. Note that not all dust layers observed at Senator Beck Basin were also found at other CODOS sites. Snowpack formation at the Berthoud Summit CODOS site and nearby Berthoud Summit Snotel site began early but soon stalled, falling into the lower quartile of observed SWE values at that site and far behind WY 2012 levels. A period of stormy weather and SWE gains followed in December (CSAS Storms #4-8) but early- and mid-January saw almost no further gains. Snow accumulations finally resumed in late January (CSAS Storms # 10, 11, 12) and proceeded during February and early March at a near-median rate, paralleling and finally exceeding WY 2012. Despite having logged four dust-on-snow events at Senator Beck Basin by March 18, the March 19, 2013 snow profile at the Berthoud Summit CODOS site found no discernible dust layers at that location, raising hopes that WY 2013 might be dust-free in the Front Range. That snow profile found some cold content remaining in the snowpack, at a mean snow temperature of -2.6° C.
Snowpack development then began to accelerate. On April 8/9 a storm (CSAS Storm #19) delivered both fresh snow as well as dust event D6, clearly present in the April 12 snow profile (perhaps merged with layer D5). That profile presented an isothermal snowcover, at a mean temperature of -0.05° C. Also observed was evidence of a recent snowmelt episode in and below the former snowpack surface containing dust layer D6/D5 (since buried in new snow). That melt episode contributed to the snowpack warming since March 19. Another winter storm on April 14/15 (CSAS Storm #20) delivered substantial additional snow as well as the long-duration D8 dust event.
Yet more snow followed in early May. A site visit on May 10 found 16” of clean new snow over dust layer D8 and D6 another 9” below D8. The snowpack was fully isothermal and wetted throughout. Layer D8 had clearly been exposed prior to the recent storms and forced rapid surface melt, but that exposure did not last long enough for D8 to ablate the snow between it and layer D6, and merge with D6. Virtually no exposed dust was visible anywhere in the Berthoud Pass locale at that time.
Peak SWE at the Berthoud Summit Snotel followed on May 11 at 22.9”, 9.9” higher than the WY 2012 peak and 1.1” above the median peak SWE value of 21.8”. Peak SWE WY 2013 was twelve days later than the median date of Peak SWE (April 29) and over two months later than peak SWE in WY 2012 (note the very large 81 day range in dates of peak SWE at Berthoud Summit). Snowmelt accelerated quickly following peak SWE at the Snotel site and snowpack ablation was much more rapid than in WY 2012. The Berthoud Summit Snotel recorded “snow all gone” (SAG) on June 11, 2013, almost five weeks later than in WY 2012. See the melt rate discussion below.
A final site visit on May 21 again found clean but very wet new snow over a merged D8/D6 layer, and a still largely clean snowpack surface in the Berthoud Pass locale. That layer of clean new snow was removed and a concentrated sample of the merged D8/6/5 dust was collected for chemical and other analyses by USGS.
The March 19, 2013 snow profile at the open and sunny Berthoud Summit CODOS site. No dust was discernible in this snowpack,
raising
hopes
that this could be a dust-free season at this Front Range site. Snow temperatures were already fairly warm, at a mean of -2.6° C.
Dust layer D6 was weakly present in the April 12 snow profile at the Berthoud Summit CODOS site, buried 9” below the surface and dashing hopes for a clean WY 2013 snowpack. D6 may have been merged with event D5. Clear evidence of a surface melt episode was observed in the snow immediately below D6 and the snowpack had become effectively isothermal.
Snowpack accumulation was still ongoing in early May. The CODOS snow profile of May 10 found a very strong D8 dust layer buried under
16” of recent, clean new snow. However, clear evidence was found of surface melt associated with late April
exposure of layer
D8. That
melt episode was brief and layer D8 was still separated from merged layers D6/D5 below.
During the final visit to Berthoud Summit on May 21, 2013 a concentrated sample of merged layers D8-D6/5 dust was collected for chemical and other analyses by USGS. The site and Berthoud Pass locale still retained a high snow albedo on that date but would soon reveal those merged dust layers, only to be re-buried in late May before a final, sustained emergence on June 3, with a very rapid acceleration in SWE losses at the Berthoud Summit Snotel (seen in the background).
Melt Rates
As was the case at Senator Beck Basin, layers D8 and D6 were the dominant dust-in-snow features driving snowmelt in the Berthoud Summit locale, although their effects on snowmelt occurred much later in the spring in the Front Range. Those dust layers were deposited high in the snow column and quickly experienced brief periods of exposure at the surface under clear, sunny skies, producing melt episodes and accelerating snowpack warming. However, subsequent active and unsettled weather delivered substantial amounts of fresh snow, deeply burying those dust layers and restoring a higher albedo to the snowpack, delaying the maximum potential radiative forcing in the dust. Overall, dust emergence was significantly delayed and the snowmelt cycle was pushed slightly later than median timing.
Analysis of Berthoud Summit Snotel data for Water Years 2006-2013 snowmelt seasons showing date and quantity of peak SWE, days from peak SWE to “snow all gone” (SAG), total additional precipitation after the date of peak SWE, an “adjusted” mean daily rate of snowmelt adding the additional precipitation to the peak SWE total, the maximum five-day moving average of daily melt, and the mean air temperature over the entire snowmelt period, from peak SWE to SAG. The Berthoud Summit Snotel site experiences very good solar access, with no shading from the east, south, or west. This site is unique in this regard, among the CODOS Snotel sites, and snowmelt rates at this site most fully reflect the impacts of direct solar (radiative) forcing in dust-in-snow.
Following peak SWE at the Berthoud Summit Snotel of 22.9” on May 11, 2013, an additional 2.4” of SWE was added to the snowpack during the ablation cycle for an adjusted total of 25.3” of SWE available to melt. Snowmelt in spring 2013 began fully two months later than in 2012 and, consequently, under much warmer mean air temperatures than in spring 2012. WY 2013 snowpack ablation to SAG took just 31 days to melt 25.3” of SWE, 9.1” more than was available in WY 2012, when it required 60 days to melt just 16.2” of SWE. Spring 2013 melt rates were as high as 1.9” SWE loss per day and the mean daily SWE loss over the entire snowmelt period was 0.82” per day. Even with the disruptions to melt described above, the maximum 5-day moving average daily melt rate was 1.52” SWE loss per day. SAG occurred on June 11, very near the median date of SAG at Berthoud Summit and over a month later than in WY 2012.
WY 2013 SWE accumulation and ablation at the Berthoud Summit Snotel station, with upper and lower quartiles of period of record shown in light grey, middle quartiles in dark grey, and the median trace for the period of record shown as a dashed line.
Stream Flows
Comparison of WY 2011, 2012 and 2013 (in progress) hydrographs for the Fraser River Near Montezuma gauge highlights variations in snowmelt runoff timing and volume over those three seasons. WY 2011 snowmelt runoff in the Fraser River watershed began erratically and oscillated and lagged median ascending limb timing as dust-in-snow was alternately exposed and reburied under relentless spring storms. Eventually, flows surged strongly to a far above-average peak near the average date of peak discharge, as dust finally emerged and remained exposed. Flows then remained high for an extended period, well above median values during the descending limb, and remained above median levels throughout the summer.
WY 2012 snowmelt runoff began early, in March, and flows quickly surged to above median levels. However, the very poor snowcover was quickly consumed and the somewhat early peak in flows fell well short of median values and then quickly declined. Descending limb flows were well below median levels and WY 2012 total snowmelt runoff fell far short of average yield.
This season, thanks to stormy March and April weather, onset of WY2013 snowmelt runoff in the Fraser River watershed was delayed almost a month past median timing. Once underway, flows at the gauge quickly reached median levels, ratcheting upward in a series of surges followed by partial retreats. As of this writing (June 17, 2013), the Fraser River has surged to a far above-average peak, resembling WY 2011, with near-average timing. With summer solstice approaching, sustained radiative forcing in the merged D8/D6 dust is likely to maintain very high melt rates in the remaining snowcover in the Berthoud Pass and Fraser River watershed locales until all snowcover is ablated. As a result, WY 2013 may produce a steeper-than-normal descending limb on the Fraser River At Upper Station gauge hydrograph.