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Department of Atmospheric Science

Thurs., Nov. 30, 3:10 pm, EN6085

Evaluation of a variable resolution version of CESM in simulating the South-Asian Monsoon - Part I

Stefan Rahimi

University of Wyoming


The South-Asian Monsoon (SAM), occurring annually from June through September, is responsible for providing the region with three-quarters of its annual rainfall. A weakening trend in the SAM’s intensity has been observed in the last century due potentially to increased burdens of light-absorbing aerosols, as these forcing agents alter the vertical thermodynamic structure of the atmosphere. For these anthropogenic (or natural) aerosol effects to be properly quantified, it is essential that simulations accurately capture the establishment and evolution of the SAM, which is highly modulated by the region’s complex terrain (i.e. the Tibetan Plateau). This study implements a variable resolution (VR) version of CESM (regionally refined at 1/8o resolution) to assess the improvement in simulating the SAM at higher horizontal resolutions compared to its uniform (UN) 1o counterpart. The VR experiment is more comparable to satellite and surface-based observations in simulating summertime (JJA) temperature across the Tibetan Plateau (TP), and TP foothills, respectively. The spatial pattern of monthly precipitation is also improved in the VR experiment, simulating a thinner zone of orographic precipitation in the TP foothills compared to its UN counterpart, and VR more realistically captures the monthly bimodal variability of precipitation across the western TP. The number of days in which effective (>2.5mm) and heavy (>25mm) precipitation events occurred are better captured in VR when compared to surface-based observations. Finally, snow cover fraction in the VR experiment is better simulated for all months compared to UN, with the largest improvements in JJA. These improvements in simulated precipitation and snow cover are due to VR’s ability to resolve more complex topographic features than UN, which drive highly localized but important patterns in the vertical motion.

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