Vorticity and Thermodynamics in a Gulf of Mexico Atmospheric River

Abstract:

Atmospheric rivers (ARs) are large-scale moisture transport systems in the atmosphere, characterized by poleward-moving moisture plumes in narrow corridors, extending through mid-latitudes often into the tropics. The physical processes related to tropical moisture exports (TME) are much less understood than the moisture fluxes in higher latitudes. Part of the challenge arises from the definition of AR transport, a predominantly horizontal characterization in which a majority of the moisture flux is expected to be uncoupled from vertical motions associated with convection. We examine the interaction of tropical moisture with an atmospheric river. Our analysis is focused on dropsonde data, collected during the fifth day of the Convective Processes Experiment (CPEX), launched over an area of interest over the central Gulf of Mexico, where a pre-existing mid-level vortex is embedded within an extensive region of stratiform cloudiness. Based on the large-scale picture obtained through satellite imagery, complemented with a broader perspective obtained with NCEP FNL analysis, it appears that this region is under the influence of an AR. Results in this study show an eastward-tilting pattern of mid-level vorticity, coupled with high column relative humidity and low mid-tropospheric moist convective instability in the region. An inverse relation between column relative humidity and mid-tropospheric moist convective instability, as indicated by moisture quasi-equilibrium (MQE), is found in a previously dominant convective regime. Strong vertical shear signals that the vorticity pattern within this stratiform system is being advected poleward into midlatitudes. We present an alternative description to characterize the moisture mechanism of atmospheric rivers near the tropics, in which vorticity is ultimately responsible for the convection, which is associated with the observed moisture convergence. This description includes the upward forcing of moisture leading to precipitation as part of the AR system, in contrast with the conventional paradigm.