Future changes of precipitation over the western United States using variable-resolution CESM

Transcription

1 Future changes of precipitation over the western United States using variable-resolution CESM Xingying Huang, Paul A. Ullrich Department of Land, Air and Water Resources University of California, Davis

2 Background Many former studies have aimed to determine how precipitation will behave in the future based on a variety of global climate forcing scenarios. The impacts of climate change on precipitation vary regionally; regional investigations are needed in order to implement climate adaptation and mitigation strategies. Higher resolution appears to produce higher-quality precipitation features, particularly those associated with orographic forcing (due to better resolution of orography)

3 Background The variable-resolution option in the Community Earth System Model (VR-CESM) uses a relatively coarse global model with enhanced resolution over a specific region; VR-CESM has been employed in many other studies, and demonstrated competitive ability in capturing finescale atmospheric processes compared with uniform-resolution CESM or RCM output VR-CESM does not appear to produce obvious artifacts in the variableresolution transition region

4 Models and Methodology Goal: Assess changes in precipitation over the 21st Century in the U.S. West using long-term ensemble runs conducted by VR-CESM with a finest resolution of 0.25 o under RCP 8.5 forcing. New: a) A novel suite of precipitation indices; b) Climatological simulations performed in VR-CESM; c) Differentiate impacts on precipitation due to ENSO and climate forcing. Simulations: Hist (year ; 2 runs), Mid ( ; 4 runs), End ( ; 4 runs); Global SSTs and sea ice are prescribed in accordance with the AMIP protocol

5 Models and Methodology Indices: Many relevant indices have been examined; best results were observed with indices that quantify the characteristics of the precipitation probability density function (PDF) ENSO: Identified by the Niño 3.4 region (namely, the Oceanic Nino Index (ONI))

6 Figure 1 : (a) Grid mesh used for the VR-CESM 0.25 mesh. (b) Transition from the global 1 resolution to (c) Topography height over study area.

7 Table 1:. Precipitation indices used in this study Names Pr SDII R1mm F1mm R5mm R10mm R20mm R40mm Rxmm P5mm Definition Mean daily precipitation averaged over each time period. unit: mm/day Simple precipitation intensity index: Precipitation amount/rainy days (Pr>1 mm). unit: mm/day Number of days with Pr>1 mm per year averaged over each time period. Unit: day/year Fraction of precipitation contributed by R1mm over total precipitation, similarly for F5mm, F10mm, F20mm, F40mm, Fxmm Similar as R1mm, but for number of days with Pr>1 mm and Pr=<5 mm Similar as R1mm, but for number of days with Pr>5 mm and Pr=<10 mm Similar as R1mm, but for number of days with Pr>10 mm and Pr=<20 mm Similar as R1mm, but for number of days with Pr>20 mm and Pr=<40 mm Similar as R1mm, but for number of days with Pr>40 mm The amount of water resulted from R5mm per year, similarly for P10mm, P20mm, P40mm, Pxmm; unit: mm/year

8 Evaluation VR-CESM CPC Pr SDII R1mm F1mm R5mm F5mm R10mm F10mm VR-CESM CPC Figure 2: VR-CESM and reference datasets over

9 Evaluation VR-CESM CPC VR-CESM CPC R20mm F20mm R40mm F40mm Rxmm Fxmm compared to observations, VR-CESM can well reproduce the spatial patterns of precipitation with quite similar magnitude. Figure 2 continued

10 T2avg (Annual) 2m RH (%) Precipitation (mm/day) Mean Climatology Mid - Hist Hist Mid-Hist Figure 3: 2m average temperature (T2avg), 2m relative humidity (RH) and Pr

11 T2avg (Annual) 2m RH (%) Precipitation (mm/day) Mean Climatology End - Hist Hist End-Hist Figure 3 continued

12 SDII R1mm R5mm R10mm Precipitation Mid-Hist End-Mid mm/day days/year Pr P5mm P10mm Figure 4: Differences of precipitation features from past to future Mid-Hist End-Mid mm/year

13 R20mm R40mm Rxmm Figure 4 Continued Precipitation predicts to be more extreme, with extreme precipitation follows more consistent with Clausius-Clayperon relationship, consistent with former studies Changes of extra water amount is consistent with the changes of frequency. Mid-Hist End-Mid Mid-Hist End-Mid P20mm P40mm Pxmm

14 El Niño La Niña (cold season) SDII R20mm R40mm Rxmm Hist Mid End Figure 5: Difference of precipitation between warm and cool phases of ENSO

15 El Niño La Niña (cold season) Hist IVT kg/m/s Figure 6: Changes of IVT (integrated water vapor transport) During El Nin õ phase, higher precipitation intensity is expected over southwest U.S., and smaller precipitation intensity over the northwest U.S., forming a dipole effect of ENSO, as shown by former studies. Further proved in the IVT stating the major contribution of precipitation by atmospheric rivers over West.

16 Discussion The contribution of human-induced increases in greenhouse gases to the quality of precipitation behaviors is confounded by patterns of variability in the atmospheric circulation. Consistent with previous studies, changes in more extreme precipitation follow the Clausius-Clapeyron relationship more closely than total precipitation amount. The changes of the strength of ENSO remains uncertain. However, the character of ENSO appears to be the largest factor in understanding changing precipitation extremes in the U.S. West.