Transcript here - Atmospheric Sciences

C41A-0193
Terrestrial and Ecological Impacts of Rain-on-Snow Events in the Circumpolar Arctic
C41A-0193
Kevin Rennert, University of Washington Department of Atmospheric Sciences . [email protected].
Gerard Roe, University of Washington Earth and Space Sciences
Overview
Cecilia Bitz, University of Washington Applied Physics Laboratory
Mechanisms for Rain on Snow to Impact the Soil Surface
I.
ROS in Spitsbergen, from Putkonen and Roe (2002). The top panel shows surface
temperature for winter 1995-96 which featured two large ROS events (orange arrows).
Soil surface temperatures (bottom panel) rise to freezing following the events. When
modeled without the ROS events (green line), the soil surface remains well below
freezing for the entire winter.
The relevant physics for the Rain on Snow events to impact the soil surface are as
follows:
1. During a large ROS event, rainwater percolates through the snow pack to the soil surface.
2. As the liquid water pools at the surface and begins to freeze, the ice-water mixture constrains the
soil surface to 0 degrees Celsius.
3. Latent heat released from the freezing water warms the underlying permafrost and overlying snow.
•
For large events, these conditions may persist for much of the winter, normally a time when the soil surface
is insulated from atmospheric events by the overlying snow pack.
•
Model calculations show that to increase the DJF mean soil surface temperature an equivalent amount as
one 50 mm ROS event would require a winter mean surface air temperature increase of 7 deg. C.
Don Russell , Environment Canada
•
Rain on Snow (ROS) events, while uncommon in the high latitudes, can impact
both the land surface and ecology of the Arctic. Liquid water pooling through the
snow pack to the soil surface warms the underlying permafrost significantly as it
freezes in the weeks after the event. The formation of ice layers within the snow pack
hinders the ability of ungulates (reindeer, caribou, musk oxen) to move and
decreases forage quality. In regions of significant topography, ROS events are
known to trigger avalanches.
Mechanisms for Rain on Snow to Impact Ungulates
II.
•
Our specific interest in rain-on-snow events is to understand more fully the
mechanisms by which they impact the soil surface, ungulates, and avalanche
tendencies. We want to estimate the magnitude of past impacts, to understand what
role these events play and will play in high-latitude climate change, and to project
how ROS will change in the future.
Depending on the size of the event, ROS can impact ungulates (e.g. caribou,
reindeer) in different ways:
•
Our general interest in these events is in the broader type of problem they
represent in understanding and predicting high-latitude climate change. They
represent events which we know have impacted the face of the Arctic and will play a
role in changing it in the future. Given the complexity and of the overall system,
limitations of the data, and the large number of potentially competing effects,
however, quantifying how much of a role ROS play in the high-latitudes is quite
difficult, even though some of the mechanisms are well understood. As a result,
we’ve found our work to understand ROS to be a case study for extracting useful
information about climate change from a wide variety of climate products.
• Extremely large events form ice layers at the soil surface which animals are
unable to penetrate to eat surface lichens, their main source of food.
• Latent heat released from freezing water at the soil surface can also cause lichen
spoilage.
• ROS events too small to reach the soil surface form ice layers in the snow pack,
increasing the difficulty of everyday movement and forage for the animals.
• Increasing the difficulty of movement and forage from any size of event can be
thought of in terms of an energetic penalty to the animal.
Guide to Poster
• Panel I: The physics behind Rain on Snow impacts on the permafrost.
Past Rain On Snow Events
Jaakko Putkonen, University of Washington Earth and Space Sciences
Case Study October 2003: 20,000 die from Banks Island Rain on
Snow
• Panel II: Mechanisms for ROS to impact ungulates.
III.
• Panel III: An exploration of the frequency and location of past occurrences of
Rain on Snow in the circumpolar Arctic using the ERA 40 data set.
Ungulate ranges by herd and species
(left panel), and areas of permafrost
(right panel).
• Panel IV: A case study of ROS that killed an estimated 20,000 musk oxen on
Banks Island.
• Panel V: Future ROS frequency using the CCSM model under a typical
climate change scenario.
• Keeping in mind regions of
importance for permafrost and
ungulates (above right), we’ve
evaluated frequency of ROS in the
European 40 Year Reanalysis (ERA
40) data set for different thresholds
for the winter season. The ERA 40
rain product compared favorably with
arctic meteorological station data
(not shown).
• Small ROS ( > 1 mm) are fairly
common in regions of importance to
ungulates.
• Panel VI: ROS events are placed in the context of large scale climatic modes.
(a)
(b)
Summary of Results
(c)
(d)
• Events large enough to form ice layers and severely impact the forage of
ungulates occur much more rarely and are generally limited to the coastal
areas of their range.
• Even slightly larger events ( >
3mm) are much rarer in the inland
Arctic in the reanalysis record, and
especially in regions of continuous
permafrost.
• Large events, however, are not as
rare in areas of the Arctic with more
maritime climates, and many of
these areas are within the ranges of
caribou and reindeer.
V.
• Small Rain on Snow events occur throughout the circumpolar arctic with a
fair degree of frequency each year, primarily during the transitional months
leading into and out of winter.
Total ROS in the ERA 40 data set (1957-1999) for
varying rain thresholds. The minimum amount of total
daily rainfall required to be considered significant ROS
varies is 1, 3, 5, and 10 mm for panels (a)-(d) respectively.
For all figures, the winter season is Oct-Mar, and the snow
depth minimum is 5mm snow water equivalent.
Future ROS under Climate Change Scenarios
• Climate change is expected to increase
atmospheric water vapor concurrently with
increases in lower tropospheric temperature.
This could be expected to result in an increase in
frequency, magnitude, and total area impacted by
ROS.
• To test this hypothesis we’ve used daily model
output for the time periods from 1980-1999 and
2040-2059 from the Community Climate System
Model (CCSM). The future projection was driven
by the A1B SRES emissions scenario, which
includes a doubling of CO2 by midcentury.
Total ROS events in ERA40 (top) for the time period of
the 1980-99 CCSM control run (middle), and the future
(bottom). Rain minimum is 1 mm.
• The model does show an increase in ROS
frequency for areas of the circumpolar arctic that
were already affected, as well as an increase in
total area impacted. Sensitive regions for caribou
and permafrost in northern Canada and Alaska
become more affected, and ROS events become
more evident in parts previously unaffected parts
of Siberia. Previous estimates have placed the
increased area at close to 40% by 2080.
• Regions of permafrost are far enough inland that ROS is unlikely to have
played a large role in its evolution in the past. Climate change scenarios,
however, indicate that future vulnerability to ROS is plausible.
• Large events such as the one on Banks Island are uncommon, but can
have catastrophic impacts on a herd that last for decades past the event
itself.
• The daily synoptic conditions that lead to rain on snow project strongly onto
large scale climatic modes such as the Northern Annular Mode and the
Pacific North American pattern. Positive NAM anomalies are conducive to
ROS in Scandanavia and Northern Russia. Positive PNA anomalies favor
ROS in northwestern Canada, while a negative PNA favors ROS in Alaska
and eastern Russia.
Future Work
• Create diagnostics based on large scale patterns to compensate for difficulty in
determining ROS from station data and model output.
• Model impacts of ROS within the snowpack itself to determine the cumulative
impacts of ROS throughout a winter in representative sites throughout the
circumpolar arctic.
• Explore alternative methods of delivering liquid water into the snowpack (e.g.
melt-freeze events) and their relative impact on permafrost and ungulates.
• Determine predictive skill of ROS by large scale climatic modes like the NAM
and PNA.
• In the first few weeks of October 2003, one
large Rain on Snow event was responsible
for the eventual starvation of approximately
20,000 musk oxen on Banks Island, the
westernmost Island in the Canadian
Archipelago
Oct. 3: 500 mb Geopotential Heights and Surface
Temperature Prior to the Event
IV.
Banks
Island
• The synoptic conditions leading to this
event were almost a week of strong southerly
flow from the tropics as a result of a strong
positive excursion of the PNA pattern (see
panel VI).
• This relatively warm, moist flow was lifted at
Banks Island by a short wave system passing
through, resulting in snow followed by rain.
Both positive vorticity advection and
advection of warm air contributed to the lift.
• Rapidly falling temperatures following the
event turned the wet snowpack into a solid
mass of ice, which the musk oxen were
unable to penetrate to forage.
The Setup: Almost a full week of strong southerly
flow from strongly positive PNA conditions created
anomalously warm conditions on Banks Island.
500 mb height contour interval is 75 m. Colors are
surface temperatures in deg. C.
Oct. 5: Synoptic conditions during the Event
• Widespread starvation from the event killed
off approximately half of the entire Banks
Island population of musk oxen.
• It is worth noting that, despite the severity of
this event, it was largely missed by both the
one reporting meteorological station and the
precipitation forecast for the island. Our
ground truth is provided by biologists in the
area at the time.
The Event: Warm advection at low levels is
evident in the sea level pressure field (left, contour
interval = 4 mb) giving rise to lift and precipitation.
An upstream short wave is also evident in the 500
mb geopotential height field (right – contour 75 m).
VI.
Large Scale Climate Mode Controls on ROS
NAM
PNA
• The Northern Annular Mode (NAM) and Pacific North
American Pattern (PNA) are the two primary modes of
wintertime atmospheric variability in the Northern
Hemisphere. The NAM represents the strength of the
polar vortex, while the wave train of the PNA primarily
influences large scale flow over North America.
The NAM (left) and PNA (right) expressed as
regression maps in the Sea Level Pressure (mb) and
500 mb geopotential height (m) fields respectively.
• To quantify the connection between these large scale modes and
incidences of ROS, we’ve composited ROS events about extremes
(> 2 standard deviations positive or negative) of each of these
indices. The results fit with our view of the requirements for these
events.
• The extreme positive phase of the NAM is indicative of a tight polar
vortex with strong zonal flow over Northern Europe and Asia, and is
conducive to ROS in those regions. A negative NAM is more
conducive to ROS for more southern parts of Europe and Asia,
where the concern is more likely to be avalanches than permafrost
degradation.
• As in our case study (above), southerly flow into Northwestern
Canada from a strongly positive PNA pattern is extremely conducive
to ROS there. A weak Aleutian low from a strongly negative PNA
favors ROS in interior Alaska and eastern Russia.
Percentage of Total ROS by Mode
NAM -
PNA +
PNA -
Percent of total ROS occurring when the
NAM and PNA are more than 2 standard
deviations positive or negative.