14B.3 The impact of spring subsurface soil temperature and snow anomaly in the Western U.S. on Southern U.S. summer precipitation and the Texas drought 2011

Thursday, 10 January 2013: 2:00 PM
Ballroom C (Austin Convention Center)
Yongkang Xue, University of California, Los Angeles, CA; and R. Vasic, S. li, C. oaida, D. A. Robinson, F. De Sales, Z. Janjic, P. C. Chu, and Y. Liu

The association of anomalies of snowpack, surface temperature, and precipitation over western North America had been extensively investigated based on observational data. It has been found that the conditions with heavy snow cover and cold subsurface soil temperature (SUBT) in the western U.S. in the spring have high probably to associate with drier condition in southern U.S., including Texas. Based on these observed based associations, this study explores the impact of spring SUBT and snow anomaly in the Western U.S. on southern U.S. summer precipitation, especially the Texas Drought 2011, and possible mechanisms using two regional climate models (RCM) and a general circulation model (GCM). The GCM produces the lateral boundary condition (LBC) for the RCMs. The study has found that the snow effect is greatly enhanced though the SUBT anomaly memory. In the first experiment, two initial SUBT conditions (one cold and another warm) on May 1st were assigned for the GCM runs and the corresponding RCM runs, to explore the SUBT effect. The results suggest that antecedent May 1st warm (cold) initial SUBT in the Western U.S. contributes positive (negative) June precipitation over the southern U.S. and less (more) precipitation to the north, consistent with the observed anomalies between a year with a warm spring and a year with a cold spring in the Western U.S. The anomalous cyclone induced by the surface heating due to SUBT anomaly propagated eastward through Rossby waves in westerly mean flow. In addition, the steering flow also contributed to the dissipation of perturbation in the northeastern U.S. and its enhancement in southeastern U.S. However, these results were obtained only when the RCM model run was driven by the corresponding GCM run. When the same reanalysis data were applied for both (cold and warm initial SUBT) Eta runs' LBCs, the precipitation anomalies could not be properly produced, indicating the intimate dependence of the regional climate sensitivity downscaling on the imposed global climate forcing, especially when the impact was through wave propagation in the large-scale atmospheric flow (Xue et al., 2012). In the second experiment, the effects of both snow and SUBT anomalies on the Texas Drought 2011 are investigated. The winter and spring of 2010-2011 had extreme heavy snow over the western U.S. and very cold SUBT during the spring. Two initial SUBT and snow cover anomaly conditions (one cold SUBT with heavy snow cover based on 2011 observation/reanalysis and another warm SUBT with less snow cover) on May 1st, 2011 were assigned for the GCM runs and the corresponding RCM runs to explore their effect on the 2011 Texas drought. The preliminary results suggest that the snow and SUBT contribute to the 2011 drought, and further confirm the discoveries in the first study. The special characteristics of snow, SUBT, and the Texas rainfall interactions are analyzed. Xue, Y., R. Vasic, Z. Janjic, Y. M. Liu, and P. C. Chu, 2012: The impact of spring subsurface soil temperature anomaly in the Western U.S. on North American summer precipitation – a case study using regional climate model downscaling. Journal Geophysical Research. 117, D11103, doi:10.1029/2012JD017692
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