Event Title

Observed and Projected Changes in Precipitation Regime over Utah

Presenter Information

Shih-Yu Simon Wang
Robert Gillies

Location

Eccles Conference Center

Event Website

http://water.usu.edu/

Start Date

4-21-2010 10:05 AM

End Date

4-21-2010 10:10 AM

Description

The intermountain west region of the United States, in which the state of Utah resides, relies both economically and socially on the development of a heavy winter mountain snowpack. The retention of water in the snowpack and its timely release is an important part of the hydrological cycle for both the state and the region. Observational evidence of the hydroclimate in Utah shows a reduction in the snowpack along with changing conditions in melting and subsequent runoff - both are occurring earlier in the season. What has not been examined in the region is the proportion of snow-to-rainfall and whether the aforementioned declining trends are evident in the winter precipitation regime, i.e., more rain and less snow. The opening analysis presented here examines this and uses three long term (1915-2003) precipitation and temperature gridded datasets for the state of Utah to determine if the total and snow/rain partition has changed. Results show that the ratio of snow-to-rain has declined over the period. Sensitivity tests were also conducted using a number of temperature threshold methods to differentiate rain versus snow and when applied, all were statistically significant in indicating the decline and as anticipated, the trend is stronger at lower elevations. The declining ratio of snow-to-rain is very likely attributed to global climate change because the pronounced decadal and inter-decadal variations that characterize the central intermountain climate are much less significant in the variation of snow-to-rain ratio. The observations of change in the precipitation regime are consistent with likewise observed changes in temperature that are warming the West. In addition, the analysis indicates a north-south stratification in total precipitation that is placed at the southern boundary of Utah. The logical progression from the aforesaid observational assessment with associated indications is an examination of future projections of snow-to-rain ratios for the state of Utah. Simulations generated by the Canadian Regional Climate Model (CRCM) and the Regional Climate Model version 3 (RCM3), both participants of the North American Regional Climate Change Assessment Program (NARCCAP), were examined from this perspective. The future climate simulations are prescribed by general circulation models and cover the period 2038-2069. Results indicate a similar declining trend in the simulated ratio of snow-to-rain for the state of Utah. The decline in ratio of snow-to-rain is consistent with increases of daily temperature maximums and minimums in both models. However, compared with the simulations forced by the NCEP-DOE Reanalysis (i.e., 1979-2004), the simulated future climate, as driven by general circulation models, exhibits a strong cold bias in the daily temperature minimum with a comparable trend but weaker cold bias in daily temperature maximum. These cold biases enhance the ratio of snow-to-rain in the models, leading the future projection of snow-to-rain ratio to be higher than that in the past decade which is contradictory to the observed trend mentioned previously. The cold bias also forms a cooling, instead of a warming, of the future intermountain climate.

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Apr 21st, 10:05 AM Apr 21st, 10:10 AM

Observed and Projected Changes in Precipitation Regime over Utah

Eccles Conference Center

The intermountain west region of the United States, in which the state of Utah resides, relies both economically and socially on the development of a heavy winter mountain snowpack. The retention of water in the snowpack and its timely release is an important part of the hydrological cycle for both the state and the region. Observational evidence of the hydroclimate in Utah shows a reduction in the snowpack along with changing conditions in melting and subsequent runoff - both are occurring earlier in the season. What has not been examined in the region is the proportion of snow-to-rainfall and whether the aforementioned declining trends are evident in the winter precipitation regime, i.e., more rain and less snow. The opening analysis presented here examines this and uses three long term (1915-2003) precipitation and temperature gridded datasets for the state of Utah to determine if the total and snow/rain partition has changed. Results show that the ratio of snow-to-rain has declined over the period. Sensitivity tests were also conducted using a number of temperature threshold methods to differentiate rain versus snow and when applied, all were statistically significant in indicating the decline and as anticipated, the trend is stronger at lower elevations. The declining ratio of snow-to-rain is very likely attributed to global climate change because the pronounced decadal and inter-decadal variations that characterize the central intermountain climate are much less significant in the variation of snow-to-rain ratio. The observations of change in the precipitation regime are consistent with likewise observed changes in temperature that are warming the West. In addition, the analysis indicates a north-south stratification in total precipitation that is placed at the southern boundary of Utah. The logical progression from the aforesaid observational assessment with associated indications is an examination of future projections of snow-to-rain ratios for the state of Utah. Simulations generated by the Canadian Regional Climate Model (CRCM) and the Regional Climate Model version 3 (RCM3), both participants of the North American Regional Climate Change Assessment Program (NARCCAP), were examined from this perspective. The future climate simulations are prescribed by general circulation models and cover the period 2038-2069. Results indicate a similar declining trend in the simulated ratio of snow-to-rain for the state of Utah. The decline in ratio of snow-to-rain is consistent with increases of daily temperature maximums and minimums in both models. However, compared with the simulations forced by the NCEP-DOE Reanalysis (i.e., 1979-2004), the simulated future climate, as driven by general circulation models, exhibits a strong cold bias in the daily temperature minimum with a comparable trend but weaker cold bias in daily temperature maximum. These cold biases enhance the ratio of snow-to-rain in the models, leading the future projection of snow-to-rain ratio to be higher than that in the past decade which is contradictory to the observed trend mentioned previously. The cold bias also forms a cooling, instead of a warming, of the future intermountain climate.

https://digitalcommons.usu.edu/runoff/2010/Posters/9