Abstract
For decades now, atmospheric scientists have had a strong understanding of the generalized flow of the secondary circulation of tropical cyclones (TCs); the cycle defined by boundary level inflow that rises convectively in the eyewall then disperses radially outward at the tropopause. But observations of radial wind throughout the depth of TCs indicate that there can be outflow in the mid-levels nearly equal in strength to the outflow at the tropopause as seen from azimuthally-averaging over hundreds of cases. Vertical cross sections, contoured frequency by altitude diagrams (CFADs), difference plots, and composite plots of radial wind have provided clear indications of the structural signature of this feature, with clear distinctions of the MLO initializing in the downshear quadrants before spiraling to feed into the main outflow layer. Analysis of the TC-RADAR database—which is comprised of 1,510 tail Doppler radar (TDR) flight passes for 436 TCs spanning 1997–2024—is utilized to define mid-level outflow (MLO) cases as flights where the value of the 95th percentile of radial wind (Vrad) in the height slice from 6.5 to 9.5 km is greater than on equal to 11 m/s. We also defined non-MLO cases (NON) as flights where the 95th percentile value was less than 8 m/s. The flights in between are left as ambiguous (AMB). From this partitioning of cases we see a statistically significant increase of best-track maximum wind speed (Vmax), environmental vertical wind shear magnitude, and vortex tilt magnitude with stronger MLO and a statistically significant decrease in 24-hr intensity change. Analysis into reflectivity and tangential wind found that there are slight differences with respect to these variables, but none that can be solely attributed to the MLO itself. Additionally, the TC-DROPS database is explored to ensure robustness and confirms the existence of the MLO.