Climatology of Tropical Cyclogenesis Pathways in the NCAR Nested Regional Climate Model

Previous research has demonstrated that tropical cyclones (TCs) over the North Atlantic basin develop within four general TC genesis pathways: nonbaroclinic, low-level baroclinic, trough-influenced, and tropical transition (TT). The traditional nonbaroclinic (40%) and low-level baroclinic (13%) developments – typically characterized by African easterly wave development in the main development region – have been found to encompass 53% of all TC developments in the North Atlantic. The non-traditional genesis pathways – TT and trough-influenced developments – were found to encompass 28% and 19% of all TC developments in the North Atlantic. The aim of this presentation is to examine the nested regional climate modeling system's (NRCM) ability to simulate the various TC genesis pathways.

The NRCM simulation, generated at NCAR, is nested within version 3 of the Community Climate System Model run using the A2 scenario. The NRCM used in this study was run at 36-km horizontal resolution for three 11-year time slices: 1995–2005, 2020–2030, and 2045–2055. Tropical cyclones in the NRCM were identified by using an objective tracking scheme that follows sea-level pressure minima. A candidate disturbance was classified as a TC if it had a lifetime > 48 h, maximum surface wind speed > 17.0 m s-1, a well-defined warm-core structure with a storm scale thermal perturbation > 2.0 K, and 850 mb relative vorticity > 1.0×10-5 s-1. For each identified TC, the genesis pathway was determined by examining 250 mb geopotential height and 850 mb potential temperature charts at the initial time of detection. The TCs were then subjectively classified as nonbaroclinic (250 mb trough and 850 mb baroclinic zone absent), low-level baroclinic (250 mb trough absent, 850 mb baroclinic zone present), trough-influenced (250 mb trough present, 850 mb baroclinic zone absent), and TT (250 mb trough and 850 mb baroclinic zone present).

Preliminary results show that the genesis pathway climatology in the NRCM for the 1995–2005 period is comparable to previous long-term climatologies for nonbaroclinic (40.2%), with a relatively higher frequency of low-level baroclinic (20.7%) and lower frequency of trough-influenced (15.9%) and TT (23.2%). When comparing the 2020–2030 and 2045–2055 periods with the 1995–2005 period, all of the TC genesis pathways show an increase in the raw number of occurrences, with nonbaroclinic having the largest raw increase in frequency and TT having the smallest increase in frequency. The relative frequency for nonbaroclinic (41.0% in 2020–2030/41.1% for 2045–2055), low-level baroclinic (23.6%/23.3%), and trough-influenced (18.1%/18.5%) all increase for the later periods compared to 1995–2005, while TT (17.4%/17.1%) shows an appreciable decrease in relative frequency. Previous work has noted an equatorward shift in TC genesis events in the NRCM simulations, and is consistent with the relative increase in nonbaroclinic and decrease in TT shown here, which occur preferentially at lower and higher latitudes, respectively. Additionally, previous work has shown that the NRCM simulations show an increase in frequency of major hurricanes. This result is consistent with the notable increase in nonbaroclinic and low-level baroclinic developments, which have been shown to be associated with more intense TCs than the other non-traditional genesis pathways.