Bell, M. M., M. T. Montgomery, : Mesoscale Processes during the Genesis of Hurricane Karl (2010). Journal of Atmospheric Sciences, 76, 2235–2255 , https://doi.org/10.1175/JAS-D-18-0161.1
Observations from the Pre-Depression Investigation of Cloud Systems in the Tropics (PREDICT), Genesis and Rapid Intensification Processes (GRIP), and Intensity Forecast Experiment (IFEX) field campaigns are analyzed to investigate the mesoscale processes leading to the tropical cyclogenesis of Hurricane Karl (2010). Research aircraft missions provided Doppler radar, in situ flight level, and dropsonde data documenting the structural changes of the predepression disturbance. Following the pre-Karl wave pouch, variational analyses at the meso-β and meso-α scales suggest that the convective cycle in Karl alternately built the low- and midlevel circulations leading to genesis episodically rather than through a sustained lowering of the convective mass flux from increased stabilization. Convective bursts that erupt in the vorticity-rich environment of the recirculating pouch region enhance the low-level meso-β- and meso-α-scale circulation through vortex stretching. As the convection wanes, the resulting stratiform precipitation strengthens the midlevel circulation through convergence associated with ice microphysical processes, protecting the disturbance from the intrusion of dry environmental air. Once the column saturation fraction returns to a critical value, a subsequent convective burst below the midlevel circulation further enhances the low-level circulation, and the convective cycle repeats. The analyses suggest that the onset of deep convection and associated low-level spinup were closely related to the coupling of the vorticity and moisture fields at low and midlevels. Our interpretation of the observational analysis presented in this study reaffirms a primary role of deep convection in the genesis process and provides a hypothesis for the supporting role of stratiform precipitation and the midlevel vortex.
FIG. 11. Analyzed time series of meso-a-averaged quantities from initial aircraft mission to Karl’s genesis. (a) Circulation tendency [1025 s21 3 (600 km)2 day21] at 850 (red circles) and 500 hPa (blue squares) using the left abscissa and saturation fraction up to 700 hPa (green triangles) and cold-cloud fraction below 2608C from Davis and Ahijevych (2012) using the right abscissa. (b) Relative humidity at 850 hPa (green triangles, %), virtual potential temperature anomaly at 850 hPa (purple squares, K), and virtual potential temperature anomaly at 500hPa (black diamonds, K). The anomaly was calculated by removing the mean uy from the time series at each respective level. Red stars below (b) denote mission times, with 0000 and 1200 UTC 13 Sep highlighted.
This study was supported by the National Science Foundation (NSF) Award AGS- 0851077. MMB also acknowledges support by the NSF CAREER Award AGS-1701225 and the Office of Naval Research (ONR) Director of Research Early Career Grant N000141613033. MTM acknowledges the support of NSF Grant AGS-1313948, IAA-1656075, NOAA HFIP Grant N0017315WR00048, NASA Grant NNG11PK021, ONR Grant 0001417WX00336, and the U.S. Naval Postgraduate School. The authors thank the PREDICT, GRIP, and IFEX teams for collecting the data used in this study. We would also like to thank David Raymond and two anonymous reviewers for their insightful re- views that improved the manuscript. All code and data used in this study are available upon request.