A multiscale examination of tropical cyclogenesis

An extensive investigation of tropical cyclogenesis (genesis) over the Atlantic and eastern Pacific Oceans has revealed that tropical cyclone formation is a multiscale process. Extensive use of compositing methods and case studies of genesis and tropical cyclolysis (lysis) events has shown that genesis requires a cooperation between planetary-scale, synoptic-scale, and mesoscale flows resulting in the production of persistent, deep moist convection. Synoptic-scale storm-centered genesis composites are created with the Analysis of the Tropical Oceanic Lower Level (ATOLL; ∼900 hPa) and 200 hPa winds for June--November produced by the National Hurricane Center for the years 1975--1993. Results show that over the Atlantic Ocean genesis is commonly accompanied by synoptic-scale forcing for tropospheric-deep ascent in association with cyclonic vorticity in the lower troposphere and weak but non-zero vertical shear of the horizontal wind. Composites in the western part of the basin show a 200 hPa trough (ridge) located to the west (east) of the ATOLL disturbance. In the eastern half of the basin composites of genesis show a sprawling 200 hPa ridge centered northeast of the ATOLL disturbance.;A combination of global gridded output from operational forecast models and reanalysis projects, and flight-level data from research aircraft were used in three case studies of developing tropical cyclones. Pseudo-dual-Doppler analyses showed that during genesis a rnesoscale convective vortex (MCV), present within a region of persistent deep, moist convection, penetrates to the surface as a cyclonic surface circulation. As the surface cyclonic circulation intensifies bands of higher equivalent potential temperature and precipitation curve around the vortex. It appears that once a spiral rain band, and its associated deep ascent, become co-located with a lower tropospheric cyclonic vorticity maximum the cyclonic circulation intensifies upwards into the updraft through vortex tube stretching. As the vortex intensifies it expands in size while the spiral rain band(s) migrate away from and envelop the vortex center, forming an eye-like feature. Descent is then observed at upper levels within the eye-like feature, which then may help to form the warm core of the vortex.