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. Mechanisms for Rain on Snow to Impact the Soil Surface 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. Mechanisms for Rain on Snow to Impact Ungulates Depending on the size of the event, ROS can impact ungulates (e.g. caribou, reindeer) in different ways: 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. Past Rain On Snow Events Overview 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. 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. 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. Ungulate ranges by herd and species (left panel), and areas of permafrost (right panel). (a)(b) (c)(d) Total ROS in the ERA 40 data set ( ) 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 and 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. 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 Gerard Roe, University of Washington Earth and Space Sciences Cecilia Bitz, University of Washington Applied Physics Laboratory Jaakko Putkonen, University of Washington Earth and Space Sciences Don Russell, Environment Canada 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. 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. Panel I: The physics behind Rain on Snow impacts on the permafrost. Panel II: Mechanisms for ROS to impact ungulates. 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. 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. Panel VI: ROS events are placed in the context of large scale climatic modes. Summary of Results 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. 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. 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. Case Study October 2003: 20,000 die from Banks Island Rain on Snow 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 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. 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 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. 3: 500 mb Geopotential Heights and Surface Temperature Prior to the Event Oct. 5: Synoptic conditions during the Event 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). Banks Island 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. Large Scale Climate Mode Controls on ROS 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. Percentage of Total ROS by Mode Percent of total ROS occurring when the NAM and PNA are more than 2 standard deviations positive or negative. NAMPNA The NAM (left) and PNA (right) expressed as regression maps in the Sea Level Pressure (mb) and 500 mb geopotential height (m) fields respectively. PNA + PNA - NAM - NAM + I. II. III.IV. V. VI. Guide to Poster ROS in Spitsbergen, from Putkonen and Roe (2002). The top panel shows surface temperature for winter 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. Total ROS events in ERA40 (top) for the time period of the CCSM control run (middle), and the future (bottom). Rain minimum is 1 mm.