Moisture flux convergence: its history and application in convective initiation forecasting

Since the early 1970s, surface moisture flux convergence has been successfully applied as a short-term (0-6 h) prognostic quantity in subjective and objective forecasts of convective initiation. At present, surface moisture flux convergence is often used by forecasters for assessing the spatial location of potential thunderstorm development.

A literature review reveals that moisture flux convergence for the purpose of severe storms forecasting evolved initially from vertically integrated approaches to predict rainfall associated with synoptic-scale systems in the 1950s and 1960s, and from the Kuo convective parameterization scheme – also predicated on vertically integrated moisture flux convergence - developed in the 1960s for tropical convection. Likely owing to the higher temporal resolution of surface observations, moisture flux convergence was then applied using surface data exclusively, starting in the 1970s. However, the uneven density of surface observing stations can have a negative impact in the computation of derivatives using traditional methods of objective analysis. This contributed to varying degrees of success when using this parameter to predict the time and location of surface-based convection. In addition, this application assumed that surface conditions were representative of a deeper layer, which often is not justified. This presentation explores aspects of moisture flux convergence as it relates to the modern ingredients based approach to thunderstorm forecasting.

A scale analysis is performed to show that moisture flux convergence is directly proportional to the convergence field, allowing moisture flux convergence to be highly effective in highlighting mesoscale boundaries that can be resolved by surface data and appropriate grid spacing. However, the effectiveness of surface boundaries in generating thunderstorms is influenced by many factors including the depth of the vertical circulation along the boundary and the presence of CAPE and/or CIN near the boundary. Operational forecasters should be cognizant of two primary points. First, the potential exists for boundaries to act in a manner analogous to a warm front, such that initiating storms are displaced from surface moisture flux convergence maxima (the storms are rooted above the local boundary layer or are “elevated”). Second, convergence is an equally effective tool for highlighting surface boundaries that may be important for convective initiation. For researchers, it is noted many thunderstorms are elevated during all or part of their lifetimes, limiting the effectiveness of surface moisture flux convergence in some situations. Lower and upper tropospheric jets, frontogenesis, and other forcing mechanisms may provide the necessary lift to bring parcels to their level of free convection. Case examples are used to illustrate the points discussed above as a context for applying moisture flux convergence.