The Role of Eyewall Turbulent Transport in the Pathway to Intensification of Tropical Cyclones

Abstract

In a tropical cyclone (TC), turbulence not only exists in the planetary boundary layer (PBL) but also can be generated above the PBL by the cloud processes in the eyewall and rainbands. It is found that the Hurricane Analysis and Forecast System (HAFS), a new multi‐scale operational model for TC prediction, fails to capture the intense turbulent mixing in eyewall and rainband clouds due to a poor estimation of static stability in clouds. The problem is fixed by including the effects of multi‐phase water in the stability calculation. Simulations of 21 TCs and tropical storms in the North Atlantic basin of 2016–2019 hurricane seasons totaling 118 forecast cycles show that the stability correction substantially improves HAFS's skill in predicting storm track and intensity. Analyses of HAFS's simulations of Hurricane Michael (2018) show that the positive tendency of vortex's tangential wind resulting from the radially inward transport of absolute vorticity dominates the eddy correlation tendencies induced by the model‐resolved asymmetric eddies and serves as a main mechanism for the rapid intensification of Michael. The sub‐grid scale (SGS) turbulent transport above the PBL in the eyewall plays a pivotal role in initiating a positive feedback among the eyewall convection, mean secondary overturning circulation, vortex acceleration via the inward transport of absolute vorticity, surface evaporation, and radial convergence of moisture in the PBL. Without the SGS transport above the PBL, the model‐resolved vertical transport alone may not be sufficient in initiating the positive feedback underlying the rapid intensification of TCs.

On the Lateral Entrainment Instability in the Inner Core Region of Tropical Cyclones

“…Airborne radar observations show that large TKEs are generated in the eyewalls and rainbands by cloud processes aloft (e.g., Marks et al 2008;Lorsolo et al 2010, Zhang and Montgomery 2012, and Zhu et al 2019). The resultant turbulent transport above the boundary layer plays an important role in the intensification of TCs (Zhu et al, 2019(Zhu et al, , 2021. In this study, (2015) by HWRF, we show that the moat air if entrained into the eyewalls and rainbands will meet the instability criterion, leading to the potential unstable convective downdraft.…”

Fine-Scale Structures in the Mid-Level Eyewall of Super Typhoon Rammasun (2014) Simulated With the WRF-LES Framework

“…this study should be useful for revealing the structures of finescale structures associated with strong TKE numerically. Future research effort should be continuously devoted to confirming the existence of mid-level horizontal rolls and improving in-cloud turbulent-mixing parameterization given the importance of the fine-scale systems on TC structure changes and intensity (Zhu et al, 2019(Zhu et al, , 2021.…”

“…Intensity change is currently one of the most difficult challenges in tropical cyclone (TC) forecast (Rogers et al, 2006;Rogers et al, 2013). While the strong turbulence in the traditional intensity theories is usually regarded as a flow feature pertaining to the planetary boundary layer (PBL) (Charney and Eliassen, 1964;Ooyama, 1964;Ooyama, 1969;Emanuel, 1986;Emanuel, 1995), a few studies suggested that intense turbulent mixing generated by cloud processes also exists above the PBL in the eyewall and rainbands of a TC (Lorsolo et al, 2010;Rogers et al, 2012;Zhu et al, 2019;Chen and Bryan, 2021;Zhu et al, 2021). Using airborne Doppler measurements, Lorsolo et al (2010) and Rogers et al (2012) found strong turbulence in the convective eyewall throughout the troposphere mainly within the radius of maximum wind.…”

Vertical Eddy Diffusivity in the Tropical Cyclone Boundary Layer during Landfall

“…These describe turbulent mixing and related fluxes in terms of the vertical gradient of the mean quantities, which control the feedbacks involving momentum, moisture, and heat between the surface and atmosphere. Numerical and observational studies have sought to constrain the turbulent mixing parameterizations in a numerical weather prediction (NWP) model for forecasting both TC track and intensification events over the ocean [6][7][8][9][10][11][12][13][14][15][16]. For example, Nolan et al [17], Kepert [18], and Zhang et al [19] found that improved estimations of the eddy diffusivity coefficient in the hurricane boundary layer are beneficial for hurricane prediction over the ocean.…”