Evolution of the upper tropospheric outflow in Hurricanes Iselle and Julio (2014) in the Navy Global Environmental Model (NAVGEM) analyses and in satellite and dropsonde observations

Abstract

Upper tropospheric outflow was examined during the life cycles of two hurricanes in the eastern and central Pacific Ocean. The outflow from Hurricanes Iselle and Julio was evaluated by using analyses from the Navy Global Environmental Model, which were very highly correlated with satellite atmospheric motion vector and NOAA G‐IV dropsonde observations. A synoptic overview provided the environmental context for the life cycles of both tropical cyclones (TCs). Then, the outflow magnitude and direction within 6 radial degrees of each TC center were analyzed in relation to TC intensity, the synoptic environment, and inertial stability, with the following results. In both Iselle and Julio, the azimuthally averaged outflow magnitude was maximized initially more than 4 radial degrees from the center, and that maximum moved steadily inward during a 4 day intensification period and reached a position radially inward of 2° within 6 h of the time of maximum surface winds. Furthermore, the direction of the outflow in both TCs was related to the evolution of the large‐scale upper tropospheric flow pattern, particularly the phasing of subtropical jet ridges and troughs moving eastward north of both TCs. Finally, outflow channels were consistently bounded by regions of lowest (highest) values of inertial stability counterclockwise (clockwise) from the maximum outflow azimuth, a pattern that persisted throughout the life cycles of both storms regardless of intensity, environmental flow, and the number and direction of outflow channels present.

On the Dynamics of Tropical Cyclone and Trough Interactions

“…Generally, a trough north or west of a TC will reduce I by reducing both z and y t , thereby creating an environment more favorable for the development of outflow. While the direction of positive y r is often broadly consistent with the direction of lower I, Barrett et al (2016) find the strongest outflow to occur 458-908 to the right of the minimum I. They attribute this phenomenon to the fact that the outflow is modifying its own environment by reducing (enhancing) the y t term in the equation for I counterclockwise (clockwise) of the region of strongest outflow.…”

“…TCs primarily interact with environmental flow at upper levels where I is weaker, as I is a measure of the resistance to motion in the radial direction (Schubert and Hack 1982;Shapiro and Willoughby 1982;Hack and Schubert 1986). Therefore, a reduction in I may allow stronger radial outflow to develop in the direction of low I (Holland and Merrill 1984;Rappin et al 2011;Molinari and Vollaro 2014;Barrett et al 2016). Molinari and Vollaro (2014) find positive radial wind y r to be maximized at the outer edge of the inertially unstable (I , 0) region and hypothesize that the length of time that the inertially unstable region persists may be related to the nature or length of the period of TC intensification.…”

“…A number of studies have examined the interaction of a TC and a trough for real cases using reanalyses, model analyses, or real-data simulations (e.g., Riehl 1954;Sadler 1976Sadler , 1978Holland and Merrill 1984;Vollaro 1989, 1990;Hanley et al 2001;Yu and Kwon 2005;Barrett et al 2016;Rios-Berrios et al 2016). Comparatively few studies exist in the literature that examine TC-trough interaction in a more fundamental manner such as in a modeling framework (Shi et al 1990;Kimball and Evans 2002;Ge et al 2008;Rappin et al 2011).…”

The Dependence of Northwest Pacific Tropical Cyclone Intensification Rates on Environmental Factors

“…Due to the asymmetric structure [36,37], low Richardson number [38,39], and weaker inertial instability [40][41][42][43] of the TC upper-level outflow, the outflow can easily interact with the upper-level larger-scale environment and the inner-core of a TC, thereby playing a mediating role between the environment and the storm core [41]. As such, it can affect the secondary circulation and, consequently, the TC intensity [37,41,[43][44][45][46][47][48][49].…”

A View of Tropical Cyclones from Above: The Tropical Cyclone Intensity Experiment

“…Recent research has further demonstrated that outflow tends to develop in regions where upper-tropospheric inertial stability is low, and stronger outflow tends to be associated with intensifying TCs (Rappin et al 2011;Barrett et al 2016;Komaromi and Doyle 2017). Synoptic-scale forcing has been found to further reduce upper-tropospheric inertial stability, which favors intensification (Rappin et al 2011).…”