Inflow layer θe values (color) and upward motion at low levels above the inflow layer (contours) in  vertically-sheared tropical cyclones in idealized numerical experiments. Shear-induced downdrafts cause a significant depression of θe values (O(15K)). The dashed white arrow indicates the inward spiraling of air parcels with depressed θe values. The black arrow highlights the convective asymmetry outside of the eyewall. The shear vector is from the east. (Fig. 7 from Riemer et al., 2013, modified)

An important roadblock to improved intensity forecasts for tropical cyclones (TCs) is our incomplete understanding of the interaction TCs with the environmental flow. Vertical shear of the environmental winds is one of the main contributors to intensity changes. Previous hypotheses for the impact of vertical shear on TC intensity have primarily focused on processes above the inflow layer. We advocate a paradigm shift and argue that vertical wind shear has a profound impact on the thermodynamic properties of the inflow layer. Shear-induced, vortex-scale downdrafts flush cooler and drier (low-θe) air into the inflow layer from above and thus frustrate the TC power machine at its most vulnerable part. These downdrafts occur in association with a distinct convective asymmetry outside of the eyewall, reminiscent of a convective pattern that has previously been termed “stationary band complex”. One forcing mechanism for this convective asymmetry is frictional convergence underneath the outer parts of the tilted vortex. The tilt of the vortex can be interpreted as a standing wave-number 1 vortex Rossby wave structure. Our results thus point to an important connection between the thermodynamic impact in the near-core inflow layer and the asymmetric balanced dynamics governing the TC vortex evolution. Interestingly, we found that the timescale of the weakening of a sheared TC in our experiments is consistent with the axisymmetric spindown due to a partial shutdown of the eyewall convection. Our new framework is based on idealized numerical experiments but turns out to be quite insensitive to modifications of the experimental setup. We advocate further tests of this framework with case studies and real atmospheric data.

(For more comprehensive information please refer to Riemer et al. 2010 and Riemer et al. 2013)

Distribution of θe above the inflow layer at 2 km height (color) and upward motion at low levels above the inflow layer (contours). The horizontal scale denotes the distance from the center. The TC’s “moist envelope” is distorted by the storm-relative flow and exhibits a pronounced wave number 1 asymmetry. Dry environmental air can approach the storm center very closely to the south of the center. The spiraling streamline denotes a flow separatrix, i.e. a streamline that separates steady flow in distinct regions. Evidently, this streamline plays an important role in our experiment to organize the θe distribution. (Fig. 4(a) from Riemer and Montgomery, 2011)

Besides the direct dynamic and thermodynamic impact on a TC vertical wind shear has a profound impact on the flow topology in the vicinity of the storm also. A TC is a strong atmospheric vortex embedded in environmental flow. Vertical wind shear ensures that at some height levels the environmental flow is distinct from the motion of the TC and considerable storm-relative flow arises at such levels. This storm-relative flow transports environmental air towards the storm center. At the same time the swirling winds tend to deflect the environmental air around the TC. Such a flow configuration may lead to a distinct asymmetry in the distribution of air masses around the TC. In our numerical experiments, the low-level storm-relative flow above the inflow layer is from the west (upshear). Following the flow topology, the cooler and drier (low-θe) environmental air approaches the center within 80 km to the south of the storm (downshear-left). To the north (downshear-right), in contrast, moist and warm air is found radially outwards to approx. 150 km. Evidently, the TC’s "moist envelope" is distorted considerably by the vertical wind shear. Shear-induced, vortex-scale downdrafts occur most prominently in the downshear-left quadrant suggesting a link between the air mass distribution governed by the vortex - environmental flow interaction and the formation of  strong, persistent downdrafts. A simple kinematic model based on a weakly divergent point vortex in background flow captures the essence of many salient features of the flow topology in our idealized experiment. A regime diagram representing realistic values of TC intensity and vertical wind shear can be constructed for this simple model. The results indicate that distinct scenarios of environmental interaction depend on the ratio of storm intensity and shear magnitude. Furthermore, it is indicated that the thermodynamic resiliency of a storm, i.e. its ability to isolate itself from adverse thermodynamic interaction, increases with the size and the intensity of the storm. (For more comprehensive information please refer to Riemer and Montgomery 2011)