Identifying and Characterizing Tropical Moisture Exports in Atmospheric Rivers

Atmospheric rivers (ARs) are large-scale moisture transport systems in the atmosphere, commonly associated with an extratropical cyclone. Two key variables are used to characterize ARs and estimate their impact, that is Integrated Vapor Transport (IVT) and Integrated Water Vapor (IWV). This vertically-integrated approach to atmospheric rivers has limitations related to moisture sourced in the tropics, loosely defined as tropical moisture exports (TMEs). Part of the challenge is understanding how tropical convection fits in this vertically-integrated moisture model, and ultimately how it affects the precipitation caused by this large-scale disturbance. Even though the physical mechanisms behind TMEs are not well understood, many researchers have hypothesized that tropical moisture fluxes can exacerbate the hydrologic impact caused by atmospheric rivers, particularly at landfall. In recent years, the concept of moisture quasi-equilibrium (MQE) has elucidated the average behavior of moisture in the tropics, by setting an anti-correlation between vertically integrated moisture (saturation fraction) and moist convective instability, which depends on vertical temperature anomalies. This moisture feedback mechanism, which has proven to perform significantly well in cloud-resolving models and observations of tropical cyclogenesis, is not unique to the tropics. I propose an alternate approach to identify and characterize tropical moisture exports in atmospheric rivers from the perspective of MQE, its governing role over moisture convergence and precipitation, and its persistence in AR tropical-extratropical interactions. I will first describe the ideas that led to the development of the MQE concept in the tropics, as well as the ideas that led to its application in atmospheric rivers. I will then elaborate on the role played by TMEs in NE Pacific atmospheric rivers that impact the US West Coast. Results in this study show that the TMEs that evolve in moist convectively stable environments have higher column-integrated moisture (saturation fraction) and a higher precipitation rate, in agreement with MQE. A correlation between saturation fraction and precipitation peaks is also observed in the water vapor budget in atmospheric rivers with TMEs. I finally will discuss how the MQE signal can be used as a tool to predict precipitation extremes brought by landfalling TMEs within ARs.