Dynamics of the Stratiform Sector of a Tropical Cyclone Rainband

Abstract

Airborne Doppler radar documented the stratiform sector of a rainband within the stationary rainband complex of Hurricane Rita. The stratiform rainband sector is a mesoscale feature consisting of nearly uniform precipitation and weak vertical velocities from collapsing convective cells. Upward transport and associated latent heating occur within the stratiform cloud layer in the form of rising radial outflow. Beneath, downward transport is organized into descending radial inflow in response to two regions of latent cooling. In the outer, upper regions of the rainband, sublimational cooling introduces horizontal buoyancy gradients, which produce horizontal vorticity and descending inflow similar to that of the trailing-stratiform region of a mesoscale convective system. Within the zone of heavier stratiform precipitation, melting cooling along the outer rainband edge creates a midlevel horizontal buoyancy gradient across the rainband that drives air farther inward beneath the brightband. The organization of this transport initially is robust but fades downwind as the convection dissipates.The stratiform-induced secondary circulation results in convergence of angular momentum above the boundary layer and broadening of the storm's rotational wind field. At the radial location where inflow suddenly converges, a midlevel tangential jet develops and extends into the downwind end of the rainband complex. This circulation may contribute to ventilation of the eyewall as inflow of low-entropy air continues past the rainband in both the boundary layer and midlevels. Given the expanse of the stratiform rainband region, its thermodynamic and kinematic impacts likely help to modify the structure and intensity of the total vortex.

A Squall-Line-Like Principal Rainband in Typhoon Hagupit (2008) Observed by Airborne Doppler Radar

“…The dual-Doppler observation from 0038 to 0102 UTC of the first flight leg, which corresponds to the dual-Doppler analysis domain in Fig. 18 of Didlake and Houze (2009)]. The horizontal kinematic structures of the convective line are illustrated in Figs.…”

“…A convective-stratiform separation algorithm [appendix A of Didlake and Houze (2009)] was applied to the reflectivity field at 3-km altitude (the flight level). Each VCS is selected through the following process.…”

“…Willoughby et al (1984) first categorized different TC rainbands (TCRs) as either moving or quasi-steady rainbands. Recent studies have shown distinct differences in structures between inner and outer rainbands (Houze 2010;Li and Wang 2012), with the convection in inner rainbands more vertically confined with outwardtilting convective cores (Didlake and Houze 2013a) and the convection in outer rainbands more vertically developed with inward-tilting convective cores (Yu and Tsai 2013). The principal rainband is the most predominant rainband in a TC and is hypothesized to exist on the boundary between the vortex circulation and the environmental flow (Willoughby et al 1984;Willoughby 1988;Houze 2010).…”

“…The quasi-steady rainbands consisted of three different types: the principal rainband, secondary rainbands, and the connecting rainband. The transition plays an important dynamic role as it changes the convergence and associated vortex stretching from low to midlevels (Didlake and Houze 2013b). Wang (2009) further quantified this vortex-environment boundary as approximately 3 times the radius of maximum wind (RMW) and categorized TCRs as inner rainbands if their radial locations were inside this boundary, or outer rainbands conversely.…”

Passive Microwave Quantification of Tropical Cyclone Inner-Core Cloud Populations Relative to Subsequent Intensity Change

“…Tao and Jiang (2015) attempt to reach a similar goal using PR data; however, they rely solely on radar 20-dBZ height values to classify precipitation. However, this approach is unable to differentiate convective from stratiform clouds, which could plausibly appear in their category of ''shallow convection-precipitation'' (20-dBZ height ,6 km) based on numerous prior studies that show stratiform precipitation having typical a 20-dBZ height of 5-8 km within TCs (e.g., Black et al 1994Black et al , 1996Black et al , 2002Hence and Houze 2011;Didlake and Houze 2013). Thus, we seek to clarify the roles of cloud and precipitation within the TC inner-core using passive microwave observations and process-based radiative transfer modeling.…”