Mirror Images of Atmospheric Flow

Abstract

We address whether there are pairs of instantaneous 500-mb flow patterns that are, relative to the climatology, as much as possible each other's opposite (which we term ?mirror images? or ?antilogs?), and we investigate whether such flows are followed by opposite 12-h time tendencies. Over eastern North America for almost all wintertime flow patterns in a 15-yr dataset it is almost as easy to find an antilog as an analog. Exceptions are very deep lows for which no mirror-imaged highs exist. In addition, antilogs make for 12-h height forecasts at a skill level almost as good as those based on analogs. Therefore the multivariate height distribution (i.e., flow patterns) is almost symmetric, and mirror imaging an observed flow is likely to yield a physically plausible pattern although not necessarily observed so far. Note that time tendencies tend to be opposite for opposite initial conditions for short periods of time even though the perturbations (full anomalies) are not small. An explanation of the latter is sought by running a global barotropic model from both regular and mirror-imaged initial conditions. Out to 12 h the tendency of the midlatitude streamfunction is primarily determined by the linear part of the absolute vorticity advection. However, on small scales (i.e., the vorticity field) forecasts deteriorate after about 6 h when the nonlinear term is either omitted (linear run) or represented wrongly (in the mirror-imaged run).