Effect of Landscape Modification on the Synoptic and Inland Patterns of Atmospheric River (AR) Events in the Western United States: Observational and Modeling Analysis.

Abstract:

This study addresses the following two important science questions: 1) “Have long term trends in landscape change modified the inland patterns of ARs that are responsible for occurrence of storms and flooding in the western US?”, and 2) “What are the relative sensitivities of landscape modifications, particularly in the form of human-induced land-use/land cover (LULC) change, on the synoptic and inland patterns of atmospheric river (AR) events?” The first question was addressed through observational analysis of AR-related storms that occurred from 1970 to 2000. Thirty years (1970-2000) of daily precipitation data from the Global Historical Climatology Network (GHCN) were analyzed to obtain the highest intensity storm event for each decade. For each station and decade, anomalies of precipitable water and integrated vapor transport (IVT) were evaluated as first-hand indicators of the interdecadal synoptic structure and inland propagation of ARs. The results of the study indicated that, large and extensive precipitable water anomaly was observed in the latest decade (the 1990’s) that penetrated deep to the leeward sides of the Cascades and Sierras. The second question was addressed through simulations of the Regional Atmospheric Modeling System (RAMS, version 6.2) on predefined LULC scenarios that covered portions of western U.S. The established LULC scenarios were intentionally formulated to represent the natural undisturbed landscape and the contemporary landscape as a control. AR duration, AR frequency, IVT-differences and synoptic anomalies have been evaluated in coarser (Grid-1) and finer (Grid-2) grids as essential indicators of the LULC-induced modification in AR hydrometeorology. Our RAMS simulations indicated that the contemporary LULC scenario represented by the control exhibited an increase in the mean AR duration (by over 5hrs) and frequency (by over 10%) as compared to the undisturbed scenario. IVT differences between control and undisturbed scenarios reached up to 200kg m^-1 s^-1 at the time of maximum precipitation.