Upper‐tropospheric inflow layers in tropical cyclones

Abstract

Three‐dimensional numerical simulations of tropical cyclone intensification with sufficient vertical resolution have shown the development of a layer of strong inflow just beneath the upper‐tropospheric outflow layer as well as, in some cases, a shallower layer of weaker inflow above the outflow layer. Here we provide an explanation for these inflow layers in the context of the prototype problem for tropical cyclone intensification, which considers the evolution of a vortex on an f‐plane in a quiescent environment, starting from an initially symmetric, moist, cloud‐free vortex over a warm ocean. We attribute the inflow layers to a subgradient radial force that exists through much of the upper troposphere beyond a certain radius. An alternative explanation that invokes classical axisymmetric balance theory is found to be problematic. We review evidence for the existence of such inflow layers in recent observations. Some effects of the inflow layers on the storm structure are discussed.

Contribution of mean and eddy momentum processes to tropical cyclone intensification

“…Wang et al . (2020) have carried out a more thorough investigation of these inflow jets in a separate study using this framework in which the vertical resolution in their model is increased in the upper troposphere at the expense of that in low levels within the frictional boundary layer. In particular, Wang et al .…”

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

“…Between 1 and 16 km, the vertical grid spacing is stretched smoothly from 100 to 500 m. For the present calculations, the output data were interpolated to a new fine grid in a region 800 km square in the horizontal and 20 km in the vertical using bicubic splines. This new grid has a horizontal grid spacing of 1 km and a vertical grid spacing of 100 m. For reference, Figure 1 shows the azimuthally averaged radial and vertical velocity fields from the model simulation of Wang et al (2020) at 60 and 74 h. At 60 h (panel (a)), the vortex is near the end of its period of rapid intensification with a maximum azimuthally averaged tangential wind speed of about 60 m⋅s −1 (see Wang et al, 2020, Figure 1a). The inflow layer beneath the upper-level outflow extends to the outer radius shown (300 km) with the maximum inflow at height of about 11 km.…”

Solutions of the Eliassen balance equation for inertially and/or symmetrically stable and unstable vortices

“…The second set of calculations, Calc‐B, relate to the balanced secondary circulation at 60 hr of the simulation in Wang et al . (2020). In this simulation, the horizontal grid spacing is 1 km and there are 78 vertical levels from 0 to 25 km.…”

“…As a first step in verifying the robustness of their findings, Wang et al . (2020) used an independent multigrid solution method for solving the Eliassen equation. The purpose of this study is threefold: (a) to document the details of the computational methods used in Wang et al .…”

“…The purpose of this study is threefold: (a) to document the details of the computational methods used in Wang et al . (2020), (b) to explore the sensitivity of the solutions to the particular method used, and (c) to assess the robustness of conclusions based on the SOR method. A specific question addressed is whether a convergent solution of the Eliassen equation for a high‐resolution simulation can be obtained using a multigrid method when the straightforward SOR method fails.…”