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Abstract This study examines how midlevel dry air and vertical wind shear (VWS) can modulate tropical cyclone (TC) development via downdraft ventilation. A suite of experiments was conducted with different combinations of initial midlevel moisture and VWS. A strong, positive, linear relationship exists between the low-level vertical mass flux in the inner core and TC intensity. The linear increase in vertical mass flux with intensity is not due to an increased strength of upward motions but, instead, is due to an increased areal extent of strong upward motions ( w > 0.5 m s −1 ). This relationship suggests physical processes that could influence the vertical mass flux, such as downdraft ventilation, influence the intensity of a TC. The azimuthal asymmetry and strength of downdraft ventilation is associated with the vertical tilt of the vortex: downdraft ventilation is located cyclonically downstream from the vertical tilt direction and its strength is associated with the magnitude of the vertical tilt. Importantly, equivalent potential temperature of parcels associated with downdraft ventilation trajectories quickly recovers via surface fluxes in the subcloud layer, but the areal extent of strong upward motions is reduced. Altogether, the modulating effects of downdraft ventilation on TC development are the downward transport of low–equivalent potential temperature, negative-buoyancy air left of shear and into the upshear semicircle, as well as low-level radial outflow upshear, which aid in reducing the areal extent of strong upward motions, thereby reducing the vertical mass flux in the inner core, and stunting TC development. 

Abstract This study demonstrates how midlevel dry air and vertical wind shear (VWS) can modulate tropical cyclone (TC) development via radial ventilation. A suite of experiments was conducted with different combinations of initial midlevel moisture and VWS environments. Two radial ventilation structures are documented. The first structure is positioned in a similar region as rainband activity and downdraft ventilation (documented in Part I) between heights of 0 and 3 km. Parcels associated with this first structure transport low–equivalent potential temperature air inward and downward left of shear and upshear to suppress convection. The second structure is associated with the vertical tilt of the vortex and storm-relative flow between heights of 5 and 9 km. Parcels associated with this second structure transport low–relative humidity air inward upshear and right of shear to suppress convection. Altogether, the modulating effects of radial ventilation on TC development are the inward transport of low–equivalent potential temperature air, as well as low-level radial outflow upshear, which aid in reducing the areal extent of strong upward motions, thereby reducing the vertical mass flux in the inner core, and stunting TC development. 

While the frequency and structure of Atlantic basin tropical cyclone diurnal cooling and warming pulses have recently been explored, how often diurnal pulses are associated with deep convection was left unanswered. Here, storm-relative, GridSat-B1, 6-h IR brightness temperature difference fields were supplemented with World Wide Lightning Location Network (WWLLN) data to answer that question. Electrically active, long-lived cooling and warming pulses were defined objectively by determining critical thresholds for the lightning flash density, areal coverage, and longevity within each pulse. Pulses with lightning occurred 61% of the time, with persistently electrically active pulses (≥9 h, ACT) occurring on 38% of pulse days and quasi–electrically active pulses (3–6 h, QUASI) occurring on 23% of pulse days. Electrically inactive pulses (<3 h, INACT) occurred 39% of the time. ACT pulse days had more pulses located right-of-shear, the preferred quadrant for outer-rainband lightning activity, and were associated with more favorable environmental conditions than INACT pulse days. Cooling pulses were more likely to occur in lower-shear environments while warming pulses were more likely to occur in high-shear environments. Finally, while the propagation speeds of ACT and INACT cooling pulses and ACT warming pulses did lend support to the recent gravity wave and tropical squall-line explanations of diurnal pulses, the INACT warming pulses did not and should be studied further. 

Recent research has found that diurnal pulses are ubiquitous features of tropical cyclones. To gain further insight into the characteristics of these pulses, a case study of an electrically active (ACT) cooling pulse and an off-the-clock ACT cooling pulse that occurred in Hurricane Harvey (2017) was conducted. Using GridSat-B1 IR brightness temperatures, World Wide Lightning Location Network (WWLLN) lightning data, the 85–91-GHz channels on microwave satellite imagers, and Level-II Doppler radar reflectivity data from WSR-88D stations (i.e., NEXRAD), these pulses were found to share many similar characteristics: both propagated outward on the right-of-shear side of Harvey and were associated with elevated cloud ice content and high reflectivity. Additionally, using HRRR model output, both pulses were found to be associated with 1) column-deep total condensate, 2) a surface cold pool, 3) an overturning circulation, and 4) an enhanced low-level jet. These characteristics are similar to those found in tropical squall lines, supporting the tropical squall-line interpretation of diurnal pulses put forth in recent studies. A hypothesis for ACT pulse initiation was then introduced, tested, and confirmed: inner rainbands that propagated outward into a more favorable environment for deep convection reinvigorated into ACT pulses that had tropical squall-line characteristics. 

Storm-centered IR brightness temperature imagery was used to create 6-h IR brightness temperature difference fields for all Atlantic basin tropical cyclones from 1982 to 2017. Pulses of colder cloud tops were defined objectively by determining critical thresholds for the magnitude of the IR differences, areal coverage of cold-cloud tops, and longevity. Long-lived cooling pulses (≥9 h) were present on 45% of days overall, occurring on 80% of major hurricane days, 64% of minor hurricane days, 46% of tropical storm days, and 24% of tropical depression days. These cooling pulses propagated outward between 8 and 14 m s⁻¹. Short-lived cooling pulses (3–6 h) were found 26.4% of the time. Some days without cooling pulses had events of the opposite sign, which were labeled warming pulses. Long-lived warming pulses occurred 8.5% of the time and propagated outward at the same speed as their cooling pulse counterparts. Only 12.2% of days had no pulses that met the criteria, indicating that pulsing is nearly ubiquitous in tropical cyclones. The environment prior to outward propagation of cooling pulses differed from warming pulse and no pulse days by having more favorable conditions between 0000 and 0300 LT for enhanced inner-core convection: higher SST and ocean heat content, more moisture throughout the troposphere, and stronger low-level vorticity and upper-level divergence.