A climatology of bow echo mesovortices

An important finding of the Bow Echo and MCV experiment was that the strongest and most damaging winds in bow echoes are frequently associated with embedded mesovortices. Despite the importance of mesovortices to severe weather, the current state of mesovortex forecasting and nowcasting is limited due to the short lifetimes of mesovortices and the lack of understanding of mesovortex genesis. Several conceptual models for mesovortex development in mesoscale convective systems (MCSs) have been proposed, most of which are based on the tilting of baroclinically generated horizontal vorticity in downdrafts. Tilting up (down) of vortex lines parallel to the MCS gust front by updrafts (downdrafts) results in couplets of cyclonic and anticyclonic mesovortices. Although model simulations suggest that anticyclonic mesovortices should occur frequently, though not as often as cyclonic mesovortices, only a few anticyclonic mesovortices have been documented. Trapp and Weisman (2003) found that convergence of planetary vorticity was responsible for weakening anticyclonic mesovortices, a reason that is often used to explain the lack of observed anticyclonic mesovortices. However, the modeling simulations suggested that anticyclonic mesovortices should be weaker and shorter-lived than their cyclonic counterparts and does not necessarily explain why they are almost altogether absent in observations.

A radar-based climatology of bow echo mesovortices will be created to determine whether anticyclonic mesovortices are really as infrequent as is believed and other related questions about mesovortex strength, intensity, and genesis. A sufficiently large number of mesovortex events will be examined to draw statistically significant conclusions about the mechanisms responsible for mesovortex formation. Events will be selected based on the criteria of either having a large number of severe thunderstorm wind reports or several high-end severe wind reports. Bow echoes will be manually tracked in radar mosaics and will be associated with circulations identified by the mesocyclone detection algorithm to generate mesovortex tracks. In addition to counting the number of cyclonic and anticyclonic mesovortices, the duration and intensity of cyclonic and anticyclonic mesovortices will be compared. Typical storm environments exhibit seasonal variability, which could affect the mechanisms responsible for mesovortex formation. A sufficiently large climatology would be capable of assessing whether there is statistically significant seasonal variability in mesovortices and the processes responsible for their genesis. Also, planetary vorticity varies with latitude and potentially could result in weaker, shorter-lived, and fewer anticyclonic mesovortices at higher latitudes where the vertical component of planetary vorticity is stronger. Preliminary results of the climatology will be presented.