Water Vapor Phase Changes and the Mesoscale Dynamics of Heavy Precipitation

The potential vorticity (PV) framework provides a convenient tool for analyzing the role of diabatic processes on atmospheric dynamics, including those processes associated with water phase changes. Many previous studies have documented the contribution of condensational heating to circulations on various spatial and temporal scales. When positive feedbacks occur, particularly interesting and important examples arise, such as atmospheric rivers and tropical cyclones. In this talk, I will review two distinct physical processes associated with water phase changes, one that is well known, the other more subtle.

Examination of lower-tropospheric moisture content global model simulations (or analyses) reveals beautiful filaments of enhanced vapor content extending poleward from the tropics. These zones are narrow, and often coincide with a low-level jet (LLJ), hence the term “atmospheric rivers” coined by Zhu and Newell (1992). If one repeats a similar model simulation but withholds the heating contribution from water phase changes, a different picture soon emerges: The atmospheric rivers become more diffuse and the LLJ weakens. A positive feedback in these situations has been documented in previous studies, in which a LLJ is associated with strong horizontal moisture transport into the vicinity of a precipitation band. Moisture convergence, precipitation, and latent heat in turn strengthen an associated lower-tropospheric cyclonic PV maximum, which is linked to the LLJ. This is often evident as a sharp edge to the atmospheric river, with the “bank” of the river being the axis of strong cyclonic lower-tropospheric PV. Without latent heating, atmospheric rivers lose the “bank” on the cold side and become diffuse, featuring reduced moisture transport. Full physics and “dry” simulations with the MPAS model are presented for a case study in order to illustrate this process, and demonstrate that condensation plays a key dynamical role in driving atmospheric rivers.

A more obscure dynamical role of water vapor phase change involves the “precipitation mass sink” effect. The pressure equivalent of 1” of rain is ~2.5 hPa. With extreme rain rates, this mass sink effect can produce a positive feedback via changes in the pressure gradient, as the associated mass removal and resulting pressure fall lead to additional moisture convergence beyond what would otherwise occur. Of course, this effect must be scaled against that resulting from condensational heating. Though the mass sink is small as measured by a scale analysis of the continuity equation, this process does not feature cancellation in the vertical, and has been shown to play a non-negligible role in heavy rainfall events. In terms of PV, vapor mass removal from a given isentropic layer via falling precipitation results in concentration of “PV substance” there, giving rise to a cyclonic PV anomaly. The role of this effect in shaping mesoscale and synoptic-scale circulations is difficult to diagnose, and additional studies are needed to quantify the role of this mechanism in flooding events. Experiments with the WRF model are presented in an effort to quantify this mechanism.