Investigation of the summer climate of North America: a regional atmospheric modeling study
Date
2005
Authors
Castro, Christopher Lawrence, author
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Abstract
The Regional Atmospheric Modeling System (RAMS) is used to investigate model sensitivity and the summer climate of North America. The value restored and added by dynamical downscaling is first evaluated. At large scales, RAMS underestimates atmospheric variability and this worsens as the grid spacing increases or domain size increases. The model simulated evolution of kinetic energy relative to the driving reanalysis kinetic energy exhibits a decrease with time which is more pronounced with larger grid spacing. The surface boundary forcing is the dominant factor in generating atmospheric variability and exerts greater control on the model as the influence of lateral boundary conditions diminish. The sensitivity to surface forcing is also influenced by the model parameterizations. Dynamical downscaling with RAMS does not retain value of the large scale of that which exists in the driving global reanalysis. The value added is to resolve smaller-scale features which have a greater dependence on the surface boundary. The NCEP Reanalysis is then dynamically downscaled with RAMS to generate a regional model climatology of the contiguous U.S. and Mexico (1950-2002). The simulations capture climatic features and seasonal transitions associated with the North American monsoon system. The diurnal cycle is the dominant time-varying mode of convective activity and is modulated by the large-scale circulation. Lower frequency modes account for the variability of convection at a remote distance from elevated terrain. The climatology is evaluated with respected to the dominant modes of global sea surface temperature. An additional series of simulations dynamically downscales data from a general circulation model executed with idealized sea surface temperature corresponding to the modes with greatest variability in the Pacific, to establish a casual link to remote sea surface temperature forcing. Time-evolving teleconnections related to tropical Pacific sea surface temperature modulate the evolution of North American summer climate, in particular the low-level moisture transport into the continental interior and convective activity. The most significant response occurs in early summer and affects the distribution of rainfall at that time. A global increase in tropical sea surface temperature over the period has also significantly affected North American summer climate.
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Subject
North America -- Climate -- Mathematical models
Atmospheric models