How might climate change affect heavy lake-effect snowstorms

1. How might climate change affect heavy lake-effect snowstorms Kenneth Kunkel, Nancy Westcott, and David Kristovich Illinois State Water Survey Champaign, IL

2. Great Lakes-Climate • The Great Lakes have a number of effects on local climates, particularly on locations immediately downwind • Such locations experience enhanced snowfall in winter, warmer winters, and cooler summers compared to locations distant from the shoreline

6. Seasonal Cycle of Air Temperature and Lake Surface Temperature From Kristovich 1998

7. Lake-effect Snow • In the late fall and early winter, cold frontal passages bring in air that is much colder than the lakes • This cold air is warmed and moistened by the warm lake surface waters • Snow forms in this warmer (but still cold) and moister air and is deposited on the lee shores

8. Main pressure and frontal features associated with lake-effect snow From Laird and Kristovich (2002)

9. Satellite loop showing development of clouds over lake after frontal passage

10. 60 20 30 10 40 50 Radar image of snow bands B2 B1 0700 UTC 11 Nov 1996 2000 UTC 11 Nov 1996

11. Assessment Study • Examined the weather conditions that were conducive to heavy lake-effect snowstorms (8 inches or more occurring over a 24 hour period) • Initial focus was on Lake Erie, but the results were extended to Lakes Superior and Michigan

12. Weather Conditions Associated with Heavy Lake-effect Snow • Surface air temperature in the range of 14-32°F (-10 to 0°C) • Difference between lake surface temperature and air temperature greater than 13°F • Wind speed greater than 14 mph • Wind direction allowing a long passage over lake

13. Climate Model Analysis • Used two future climate simulations-one from the Hadley Centre (United Kingdom) climate model and the other from the Canadian climate model • Examined differences in the frequency of heavy lake-effect snow conditions between the present and the end of the 21st Century

14. Climate Models • There are differences in the way these two models are constructed • Climate models break the atmosphere into boxes and determine values of weather conditions for each box • The boxes in the Hadley Centre model for the atmosphere are smaller than those in the Canadian model

15. Climate Models • However, the boxes in the Canadian model for the ocean are smaller than those in the Hadley Centre model • The treatment of the land surface is more sophisticated in the Hadley Centre model than in the Canadian model • Model differences lead to different projections for the future

16. Results • The Hadley Centre model projects a greater than 50% reduction in the frequency of heavy lake-effect snow by 2100 while the Canadian model projects a more than 90% reduction • The reduction is almost entirely due to warmer winter temperatures (about 5°F for the Hadley Centre model and 10°F for the Canadian model) • An increase in lake-effect rain is likely to accompany the decrease in lake-effect snow

17. Results (continued) • For southern Lake Michigan, the decrease is about the same as Lake Erie. However, there is a much smaller decrease for the Lake Superior snowbelt (only about 10% for the Hadley Centre model)

18. Confidence in results • Although there are model differences, these two models (and all other climate models) indicate significant warming over the Great Lakes • Thus, the eventual decline in the frequency of heavy lake-effect snowstorms seems highly likely

19. Potential Effects of Ice Cover? • Lake-effect snow is greatly diminished once ice cover develops • Duration of ice cover will decrease in a future warmer climate • Longer season for lake-effect snow could compensate • Our results for the end of the 21st Century take this into account; however, this could be an important compensation in the early portion of the 21st Century (which we did not examine)

20. This study indicated that air temperature is the key The following analysis is based on the primary importance of air temperature

21. Historical Climate Analysis • An examination of historical climate data provides some insight into possible future outcomes • This analysis assumes that, as temperatures rise, the day-to-day variations remain the same • This assumption may not be valid, but the analysis of the two GCMs did not indicate significant changes in variations

23. Heavy Snow

27. Houghton Iron Mt Traverse City Benton Harbor

28. New Climate Model Results • Recently available projections from a newer version of the Hadley Centre model and a U.S. model are consistent in indicating significant warming in the Great Lakes region

29. Provided by Katharine Hayhoe and Donald Wuebbles

30. Provided by Donald Wuebbles

31. What About Total Snowfall? • In our assessment, we did not study the effects of climate change on total snowfall • An examination of past historical data may provide some guidance • This examination shows that warmer winters are generally less snowy

33. Conclusions • Climate models suggest that during the 21st Century, heavy lake-effect snowstorms will dramatically decrease in frequency in the southern portions of the Great Lakes basin • Changes in the northern portions of the basin may be much less • These changes are caused almost entirely by warmer temperatures; other conditions favorable for heavy snow do not change much in the climate models

34. Conclusions • What we were not able to examine is the speed with which these changes will take place. Will decreased frequencies begin to occur early in the 21st Century, or later? • The models do show warming in the early part of the 21st Century, so mid-winter melting events may become more frequent, even if total snowfall does not change

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