An examination of atmospheric river moisture transport and hydrology using isotope-enabled CAM5.

摘要

Atmospheric rivers are a feature of the atmospheric circulation that play a major role in terms of precipitation, flooding, drought, and even the global climate system itself. Thus understanding what the main controls on these weather systems are is critical if one wants to be able to determine the impact they could eventually impose on society. Also too, almost all climate projections are performed by global climate or earth system models. Thus there is a need to ensure that these sorts of models can accurately simulate atmospheric rivers, and the global hydrologic cycle in general, if one is to have confidence in the projections generated by these programs.;These concerns are examined in this thesis. In particular, the CAM5 model is used to generate a climatology of extreme moisture transport from transient eddies and atmospheric rivers, which is compared to a reanalysis. It is found that although the average climatological results are similar, the average moisture flux per event was too weak, indicating that the model may not adequately simulate the more extreme flux and/or precipitation events, which can have the largest impact on society.;To further investigate what might be causing this bias, water tracer and isotope physics were added to CAM5, where the biases present in the isotope-enabled simulations show that CAM5 generates too much precipitation. A sensitivity analysis is performed to try and determine the specific cause of the bias, and it is found that CAM5 generates deep convection too frequently, particularly in the winter midlatitudes over the ocean. This could also help explain the weakened moisture fluxes in atmospheric rivers, as too much moisture is lost in the model due to overly active convection.;Finally, water tracers are used to examine the moisture sources for the West Coast of the United States, including in atmospheric rivers. It is found that atmospheric rivers pull more moisture from the tropics than average. It is also found that in the future, the fraction of locally-sourced moisture decreases, compensated for by an increase in long-distance moisture transport. This new information provides a new way to examine the atmospheric hydrologic cycle, and eventually, create better climate projections.