On the hypothesized outflow control of tropical cyclone intensification

Abstract

We present idealized, three‐dimensional, convection‐permitting numerical experiments to evaluate the premise of the revised theory of tropical cyclone intensification proposed by Emanuel, . The premise is that small‐scale turbulence in the upper tropospheric outflow layer determines the thermal stratification of the outflow and, in turn, an amplification of the system‐scale tangential wind field above the boundary layer. The aim of our article is to test whether parametrized small‐scale turbulence in the outflow region of a developing storm is an essential process in the spin‐up of the maximum tangential winds. Compared with the control experiment, in which the small‐scale, shear‐stratified turbulence is parametrized in the usual way based on a Richardson number criterion, the vortex in a calculation without a parametrized representation of vertical mixing above the boundary layer has similar evolution of intensity. Richardson number near‐criticality is found mainly in the upper‐level outflow. However, the present solutions indicate that eddy processes in the eyewall play a significant role in determining the structure of moist entropy surfaces in the upper‐level outflow. In the three‐dimensional model, these eddy processes are largely realizations of asymmetric deep convection and are not obviously governed by any Richardson‐number‐based criterion. The experiments do not support the premise on which the new theory is based. The results would appear to have ramifications for recent studies that invoke the new theory.

Size and Structure of Dry and Moist Reversible Tropical Cyclones

“…The subgrid turbulences for vectors and scalars are parameterized for mesoscale modeling (cm1setup 5 2, ipbl 5 2 in the namelist) with the horizontal and asymptotic vertical mixing lengths (l h and l ' ) set to 1000 m (lhref1 5 lhref2 5 1000) and 50 m (l_inf 5 50), respectively. The values are comparable to those used in Montgomery et al (2019). Surface exchange coefficients for momentum c D and enthalpy c k are 0.002 and 0.0015, respectively.…”

“…Though the criticality of Ri in the outflow of an axisymmetric TC is generally found valid (Persing et al 2013;Peng et al 2018), its validity in a three-dimensional TC has not reached a consensus (Persing et al 2013;Tao et al 2019;Montgomery et al 2019). On the other hand, the role of Ri-related turbulent diffusion is found not essential to TC intensification (Montgomery et al 2019), not supporting the corresponding intensification theory (Emanuel 2012). In this study, we test the critical Ri assumption in more generalized three-dimensional drytype TCs.…”

Upper‐tropospheric inflow layers in tropical cyclones

“…Montgomery et al . (2019) provide further detailed justification for these selected parameter values.…”

“…Emanuel (2012) hypothesized that tropical cyclone intensification is controlled by small‐scale turbulent mixing in the upper‐tropospheric outflow and offered an analytical theory in which a parametrization of this mixing process is the sole positive term in an equation for the tendency of the maximum tangential wind (his equation (16)). Nevertheless, the physics of the intensification process, i.e., how in reality this mixing would lead to the required inward movement of the surfaces of absolute angular momentum at low levels for spin‐up, remain to be articulated (Montgomery et al ., 2019; Montgomery and Smith, 2019).…”

Lagrangian trajectories at the outflow of tropical cyclones

“…The dynamics of the outflow level of tropical cyclones (TCs) is less studied than the inflow and upflow branches of TC secondary circulation. In recent years, however, the outflow regions of TCs have received increasing attention in several aspects including their interaction with synoptic systems (Hanley, 2002; Rappin et al ., 2011; Dai et al ., 2017), tropopause height (Halverson et al ., 2006; Duran and Molinari, 2018), warm core structure (Durden, 2013; Cohen et al ., 2017; 2019) and far‐field storm structure (Emanuel and Rotunno, 2011; Ditchek et al ., 2017; Montgomery et al ., 2019; Peng et al ., 2019). The outflow region of TCs is known to develop strong, and highly asymmetric, outflow jets around the 250–150 hPa level (e.g., Black and Anthes, 1971; Rappin et al ., 2011; Molinari and Vollaro, 2014; Barrett et al ., 2016; Komaromi and Doyle, 2017; 2018).…”