The Role of Marine Thermal Gradient Structure on Gulf Stream-related Extratropical Cyclogenesis

Abstract

Mid-latitude cyclones, or Nor'easters, have a long history of producing severe, and sometimes catastrophic, blizzard conditions along the eastern seaboard. These winter weather events have been perplexing meteorologists and forecasters for as long as records have been kept. The coastal region east of the Carolinas, in association with the warm Gulf Stream current, is considered by most to be an epicenter of extratropical cyclogenesis. This is a three part study combining a 20 year climatology, a numerical model simulation, and a sensitivity study which links both of the above.

The climatological part of this study extends the previously published Atlantic surface cyclone intensification index (ASCII) database to include dates from October 1991 to April 2002. This added another 115 extratropical cyclogenetic cases to double the total number of storms to 231. As in the first study, linear regression fits were done on the comparison of surface-level thermal gradient with the deepening rate of the coastal low. The statistical relation for the 1991-2002 data are within 2% of the 1982-1990 data, thus verifying the results of the original study. Although the climatological comparison was in full agreement, 69% of the variance in storm deepening rate remains unexplained by this method. To address this issue of scatter, storms were separated into 3 bins based on their 500-mb absolute vorticity. Separate fits were done for the individual bins which were found to explain as much 74% of the variance.

For the second part of this study, a numerical mesoscale weather model (MM5) was employed to test the cyclogenetic response to high resolution sea surface temperature (SST) initialization data using the 24-25 January 2000 winter storm case. While leaving all other model parameters and initialization the same, two different SST distributions were used for the two simulations. The control simulation was initialized with the standard NCEP Eta-212 gridded analysis, while the experimental simulation was initialized with a very high resolution (1.1 km) SST file. The experimental's results show a noticeable response with a more defined coastal front, increased deepening rate, and more accurate track.

A final 2-part study was conducted on the sensitivity of lower-tropospheric cyclogenesis to the sea surface thermal gradient induced by the Gulf Stream. The first part was carried out by systematically reducing the magnitude of the SST gradient for three consecutive mesoscale model simulations. Results show significant decrease in deepening rate with each successive run. The combined results are also in agreement with the regression fits from the ASCII climatology. In the second part, the Gulf Stream was shifted to the east while leaving the unique features such as curvature and SST values unchanged. Results reveal an ASCII limitation within the Gulf Stream Front position parameter, and show that by altering the track of the surface low, the feedback link to the upper-level trough is weakened thus reducing the surface-level cyclogenesis.