Synoptic-scale Rossby Wave Precursors to heavy Alpine Precipitation

Synoptic-scale Rossby waves (RW) at the tropopause are a key meteorological feature closely associated with surface baroclinic development and energy propagation in the atmosphere. From a potential vorticity (PV) perspective the waves exist and propagate along the bands of maximum PV gradients at the dynamical tropopause breaks.

For our analysis we use a climatology of RWs derived from the ERA-40 reanalysis data set on a 6-hourly basis. Their propagation is depicted in a novel form of the Hovmoeller diagram that accounts for the instantaneous shape of the PV waveguide. This eliminates the effect of meridional averaging and non-zonal wave propagation, precludes attenuation of the amplitude by spurious cancellation of wave peaks and circumvents difficulties in estimating propagation velocities.

The RW climatology can shed light on the dynamical understanding of the origin and development of severe weather. This is exemplified for severe precipitation events along the Alpine south-side, that were identified in an observational Alpine precipitation climatology (Frei and Schaer,1998). These events are triggered by breaking synoptic-scale Rossby waves. The general characteristics and the seasonal variation of the RW precursor signal to these breaking waves are presented as well as potential interactions of the precursor RWs with mid-tropospheric diabatic processes.