The Influence of an Upper Tropospheric Potential Vorticity Anomaly on Rapid Tropical Cyclogenesis

Tropical cyclones (TCs) that undergo rapid tropical cyclogenesis (RTCG) close to land are especially dangerous due to little advanced warning time. In order to better understand the environments in which RTCG has been observed, the ERA-Interim is utilized to examine upper tropospheric differences among newly formed TCs in the North Atlantic basin from 1980 to 2013. This study classifies TCs into three groups based on the maximum sustained surface wind change (ΔVmax) 24 hours after genesis from the best track database. The groups are: 1) RTCG, if ΔVmax ≥ 25 kt; 2) slow tropical cyclogenesis (STCG), if ΔVmax < 25 kt, but > 5 kt; and 3) neutral tropical cyclogenesis (NTCG), if ΔVmax ≤ 5 kt, but ≥ -5 kt. The synoptic-scale environments of the analyzed TCs and their pre-existing disturbances are examined over a 72-hour period, commencing 48 hours prior to genesis.

Many pre-genesis disturbances interact with an upper tropospheric potential vorticity (PV) anomaly. PV anomalies are calculated by subtracting the 12-hour time mean of PV, centered at the time of genesis, by the background 30-day base state. If a PV anomaly within 1000-km from the TC center exceeds 1.5 PVU on the 350 K isentropic surface, the TC is classified as being part of a “high-PV” subgroup. Otherwise, the TC is placed into a “low-PV” subgroup. Results from this study show a higher percentage of RTCG cases are embedded in a high-PV environment (~45%) compared to ~38% of STCG cases and ~31% of NTCG cases.

We hypothesize that an upper tropospheric PV anomaly is typically favorable for intensification of a newly formed tropical cyclone, provided the spatial scale of the PV anomaly is similar to the spatial scale of the TC. Composites of environmental conditions and the three-dimensional structure of the upper tropospheric PV anomalies were created and normalized by the direction of the environmental vertical shear. Results show RTCG cases are embedded in a wide range of environmental wind shear magnitudes, with time series bulk median values similar to STCG and NTCG cases. In the composite framework, however, RTCG upper tropospheric PV anomalies are greater in amplitude, smaller in zonal scale, deeper in the vertical, and are more amplified directly upshear of the TC center than STCG and NTCG events. Additionally, RTCG events have greater and more symmetric values of quasi-geostrophic forcing for ascent around the TC center, which matches the cloud pattern observed from infrared brightness temperatures.