Effects of Midlevel Dry Air on Development of the Axisymmetric Tropical Cyclone Secondary Circulation

Abstract

Idealized experiments conducted with an axisymmetric tropical cyclone (TC) model are used to assess the effects of midlevel dry air on the axisymmetric TC secondary circulation. Moist entropy diagnostics of convective parcels are used to determine how midlevel dry air affects the distribution and strength of convection. Analyzing upward and downward motions in the Eulerian radius–height coordinate system shows that the moistest simulation has stronger vertical motions and a wider overturning circulation compared to drier simulations. A Lagrangian entropy framework further analyzes convective motions by separating upward higher-entropy streams from downward lower-entropy streams. Results show that the driest simulation has a weaker mean overturning circulation with updrafts characterized by lower mean entropy compared to moister simulations. Turbulent entrainment of dry air into deep convection at midlevels is small, suggesting that the influence of midlevel dry air on convective strength and the structure of the secondary circulation are through modification of the inflow layer. Backward trajectories show low-entropy air subsiding into the subcloud layer from low to midlevels of the atmosphere between radii of 200 and 400 km. Surface fluxes increase the entropy of these parcels before they rise in convective updrafts, but the increased recovery time, combined with descending motion closer to the inner core, decreases the width of the TC secondary circulation in the driest simulation.

Upper‐level trajectories in the prototype problem for tropical cyclone intensification

“…(2016) and Alland et al . (2017). It may be interesting to note that upper‐tropospheric inflow layers were found in both of these studies, but were not remarked upon.…”

“…Further analysis of the effects of diffusion in the mid-/upper troposphere in these simulations would be possible using the isentropic compositing approach of Pauluis and Mrowiec (2013), but this has not been attempted in view of the effort involved in relation to the likely additional insights. Such analyses have been employed in the tropical cyclone context by Mrowiec et al (2016) and Alland et al (2017). It may be interesting to note that upper-tropospheric inflow layers were found in both of these studies, but were not remarked upon.…”

Impact of Dry Midlevel Air on the Tropical Cyclone Outer Circulation

“…Recent studies also showed that the cyclone can be influenced by its own midlevel dry air via similar pathways as found in our simulations. For example, Alland et al (2017) showed that the low-u e air from the midtroposphere can subside into the boundary layer between the radius of 200 and 400 km in an axisymmetric TC model. Another example is given by Corsaro and Toumi (2017) who found that the idealized TC with b drift can recirculate low-entropy air back into the tropical cyclone from upper and middle levels.…”

“…Braun et al (2012) briefly mentioned the inhibiting effect of dry midlevel air on TC growth, if the dry air is inserted before the initial spinup and close to the core. Alland et al (2017) specifically studied the effects of dry midlevel air on the development of TC secondary circulation with an axisymmetric model. They showed that the simulation with a moister midtroposphere has a wider overturning circulation.…”

Basinwise Statistical Analysis of Factors Limiting Tropical Storm Formation From an Initial Tropical Circulation

“…Some studies found that moderate shear or easterly shear is favorable for developing cases (Tuleya & Kurihara, 1981, 1982; Zehr, 1992; Zeng et al, 2010). In contrast, enhanced vertical wind shear misaligns the storm, which sometimes leads to ventilation, thereby disrupting the intensification process (Alland et al, 2017; Davis & Ahijevych, 2012; Riemer et al, 2010; Tang & Emanuel, 2010; Wang et al, 2012). Other studies showed that abundant low‐level convergence and enhanced mass flux through organized outflow is associated with sustained deep convection in developing cases, with an anticyclonic circulation in the upper levels (Lee, 1989a, 1989b; McBride & Zehr, 1981; Sears & Velden, 2014; Tuleya, 1991; Zehr, 1992; Zhang & Zhu, 2012).…”