Dynamical processes and forecast uncertainty associated with an extreme-rain-producing atmospheric river over the southeastern U.S. during late October 2007

Abstract:

This study examines the dynamical processes and forecast uncertainty associated with an intense atmospheric river (AR) that contributed to widespread drought-mitigating extreme rainfall (maxima >200 mm) across the southeastern U.S. during 22–27 October 2007. This AR is found to be dynamically linked to anticyclonic wave breaking (AWB) over central North America and the attendant formation of an elongated potential vorticity (PV) streamer. The AR formed in connection with a pronounced increase of water vapor flux into the southeastern U.S., forced in conjunction with the equatorward and eastward propagation of the PV streamer and an attendant lower-tropospheric cyclone toward the western flank of a subtropical anticyclone. Trajectory calculations demonstrate that the AR was associated with poleward transports of moist air parcels originating over the Caribbean Sea, the Gulf of Mexico, and the western North Atlantic Ocean.

Examination of ECMWF ensemble forecasts from TIGGE reveals large uncertainty associated with the AWB and, correspondingly, large uncertainty and low skill in the water vapor flux and precipitation forecasts over the central and eastern U.S. It is found that the uncertainty associated with the AWB is closely related to ensemble variability in the representation of upstream cyclogenesis resulting from the interaction between a diabatic Rossby vortex and a tropopause-level polar disturbance over the eastern North Pacific. Specifically, ensemble members in which the upstream cyclogenesis is too strong exhibit overly intense ridge amplification over western North America and, accordingly, AWB occurs too far west. These members exhibit large water vapor flux and precipitation forecast errors over the central and eastern U.S., displacing the AR and the heavy precipitation area to the west and underforecasting the water vapor flux and precipitation amounts. Conversely, ensemble members more accurately representing the upstream cyclogenesis exhibit AWB farther east and produce relatively accurate water vapor flux and precipitation forecasts over the central and eastern U.S.