Further examination of the thermodynamic modification of the inflow layer of tropical cyclones by vertical wind shear

Abstract

Abstract. Recent work has developed a new framework for the impact of vertical wind shear on the intensity evolution of tropical cyclones. A focus of this framework is on the frustration of the tropical cyclone's power machine by shearinduced, persistent downdrafts that flush relatively cool and dry (lower equivalent potential temperature, θ e ) air into the storm's inflow layer. These previous results have been based on idealised numerical experiments for which we have deliberately chosen a simple set of physical parameterisations. Before efforts are undertaken to test the proposed framework with real atmospheric data, we assess here the robustness of our previous results in a more realistic and representative experimental setup by surveying and diagnosing five additional numerical experiments. The modifications of the experimental setup comprise the values of the exchange coefficients of surface heat and momentum fluxes, the inclusion of experiments with ice microphysics, and the consideration of weaker, but still mature tropical cyclones.In all experiments, the depression of the inflow layer θ e values is significant and all tropical cyclones exhibit the same general structural changes when interacting with the imposed vertical wind shear. Tropical cyclones in which strong downdrafts occur more frequently exhibit a more pronounced depression of inflow layer θ e outside of the eyewall in our experiments. The magnitude of the θ e depression underneath the eyewall early after shear is imposed in our experiments correlates well with the magnitude of the ensuing weakening of the respective tropical cyclone. Based on the evidence presented, it is concluded that the newly proposed framework is a robust description of intensity modification in our suite of experiments.

Environmental influences on the intensity changes of tropical cyclones over the western North Pacific

“…11c), suggesting an amplified negative impact of dry air intrusion there. Although it remains unclear precisely how the drier air enters the eyewall updrafts and limits the strengthening of storms due to the coarser resolution of the GFS data we used, the apparent difference of DFX between the two groups demonstrates clear links between the environmental moisture content and TC intensity, which complements recent idealized studies of Riemer et al (2010Riemer et al ( , 2013.…”

“…An alternative manifestation of the ventilation effect is the outward eddy flux of warm core air by the shear-induced asymmetric flow in the upper levels, causing a decrease of the warm core from top to bottom and thus weakening the TC (Frank and Ritchie, 2001). An alternative hypothesis was recently presented in Riemer et al (2010Riemer et al ( , 2013: the cooler and drier (lower θ e) air could be brought into the boundary layer from above by shear-induced downdrafts flux, which significantly reduces eyewall θe values and subsequently constrains the storm intensity when the lower θe air enters the eyewall updrafts. Riemer and Montgomery (2011) further argued that the environmental vertical wind shear might promote or impede the interaction with the environmental air depending on the direction of the shear vector.…”

“…Previous studies suggested that the intruding drier air may negatively impact the intensity of TCs in the following pathways: (1) intrusion of the low-entropy air frustrates the conversion of heat into kinetic energy within the TC's power engine, as mentioned by Tang and Emanuel (2010); (2) entrainment of dry air into the convective cloud reduces buoyancy (James and Markowski, 2010); and (3) the drier and cooler air (low θ e) could serve as "anti-fuel" for the TC power when shear-induced downward-flux low θe air enters the inflow layer of the TC from above, especially at midlevels where mixing of low θ e air into the eyewall is thought to be particularly effective (Riemer et al, 2010(Riemer et al, , 2013. Consistent with findings from these idealized results, the current composite study with a large number of observed TCs over the WNP shows that a broad area of drier air is brought into the western half of the TC circulation that lowers the relative humidity and suppresses the moisture supply.…”

Microphysical Characteristics of an Asymmetric Eyewall in Major Hurricane Harvey (2017)

“…Hurricane Harvey (2017) was the first major hurricane landfall in the U.S. since Wilma (2005) and the first since the operational radar upgrade to dual polarization in 2013. The dynamical interaction between the vortex and VWS to produce the eyewall asymmetry has been well studied (Frank & Ritchie, 2001;Jones, 1995;Reasor et al, 2004;Riemer et al, 2010Riemer et al, , 2013, but the microphysical characteristics and their effect on eyewall structure are not well understood. The asymmetric structure of hurricane eyewalls is a well-known response to VWS (Black et al, 2002;Corbosiero & Molinari, 2003;Marks & Houze, 1987) and is often associated with intensity change (Nguyen & Molinari, 2012;Reasor et al, 2009;Rogers et al, 2016).…”

Tropical cyclone genesis potential across palaeoclimates

“…Wind shear, which is the vertical shear of the horizontal winds between the upper and lower troposphere, causes asymmetries in the developing cyclone which results in the ventilation of the upper-level warm core through the flushing of relatively cooler and drier air from the top (Frank and Ritchie, 2001). Stronger wind shear therefore influences inflow dynamics and weakens cyclone formation (Riemer et al, 2013). While noting caveats where such two-level vector differentials may be inadequate to describe the resultant wind shear in some scenarios (Velden and Sears, 2014), this study defines the wind shear as the difference between the 200 and 850 hPa winds given its ease of computation.…”