One of the most challenging, outstanding research questions concerning tropical cyclones (i.e., hurricanes), which are among the costliest natural disasters in terms of life and property, is how they intensify. The problem of forecasting tropical cyclone intensity change is compounded by the fact that the intensity of the storm is modulated by the environment in which the storm is embedded and processes internal (e.g., clouds and precipitation) to the storm itself.

The focus of this research has been to understand the interactions between tropical cyclones and the environments in which they are embedded, with an emphasis on how vertical wind shear (how the winds change in speed and direction with height) and environmental moisture (i.e., humidity) impact the structure and intensity through ventilation, the injection of cool, dry environmental air into the storm core. This work utilized high-resolution numerical modeling simulations and various airborne observations to study how ventilation occurs in tropical cyclones, and how it modulates their structure, development, intensity, and intensity change.

First, we conducted a suite of idealized model simulations of tropical cyclones embedded in environments with different combinations of initial moisture and shear. A strong, positive relationship was found between the low-level, upward vertical motion in the storm core and intensity (Figure 1). The increase in upward motion with intensity was not due to an increased strength of upward motions, but instead,  was due to an increased area of strong upward motions. This relationship suggests that physical processes that influence the area of upward motions, such as ventilation, directly influence the intensity of tropical cyclones.

Second, we examined the structure of ventilation in the simulations and identified two distinct ventilation pathways: downdraft and radial ventilation (Figure 2). Downdraft ventilation occurs due to the evaporation of rain into dry air undercutting the storm. As a result, this evaporation generates negatively buoyant air that is flushed downward toward the surface. Downdraft ventilation typically occurs in rainbands that are organized on the side of the storm that is to the left of the shear direction. Radial ventilation occurs in horizontal currents that transport cooler, drier air from the environment around a storm closer toward the center. Such currents occur in the same rainbands that downdraft ventilation may be present in. Radial ventilation can also occur higher up in the atmosphere due to a misalignment of the vortex by the shear. Both downdraft and radial ventilation act to reduce the area of strong upward motion, thereby reducing the upward motion in the core, stunting storm development and intensification.

Third, we used our definitions of downdraft and radial ventilation to look for evidence of ventilation in observations. At the start of a rapid weakening period in Hurricane Sam (2021), downdraft ventilation was documented using instruments dropped from aircraft (i.e., dropsondes) and aircraft-mounted radar observations from flights into the storm. This data revealed deep, and sometimes intense, downward vertical motions (Figure 3; blue shading). These strong downward motions can transport cold and/or dry air diagnosed from the dropsondes downward, cutting off the storm's supply of warm and moist air. Radial ventilation was also observed occurring at mid-to-upper levels of the troposphere on the side of the storm from which the shear was coming from (upshear) and at low-to-mid levels of the troposphere on the opposite side (downshear). All these signatures are broadly consistent with ventilation pathway findings in idealized model simulations.

These newly discovered aspects of the impacts of ventilation on tropical cyclones have led to an improved understanding of physical processes affecting intensity change in tropical cyclones embedded in vertical wind shear. As tropical cyclones can cause large impacts on society and the economy, improved understanding, together with improvements in numerical weather prediction and forecast tools, can mitigate against these impacts. Observational analysis of ventilation can lead to real-time diagnostics that help with forecaster situational awareness. Such diagnostics can provide information to forecast intensity changes beyond simple metrics of vertical wind shear.

Last Modified: 06/29/2023
Modified by: Brian H Tang