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1. How Does Horizontal Diffusion Influence the Intensification and Maximum Intensity of Numerically Simulated Tropical Cyclones?

   https://doi.org/10.1175/JAS-D-22-0014.1  

   Fei, Rong ; Wang, Yuqing ( March 2023 , Journal of the Atmospheric Sciences)

   Recent studies have demonstrated the sensitivity of simulated tropical cyclone (TC) intensity to horizontal diffusion in numerical models. It is unclear whether such sensitivity comes from the horizontal diffusion in or above the boundary layer. To address this issue, both an Ooyama-type model and a full-physics model are used to conduct sensitivity experiments with reduced or enlarged horizontal mixing length (l_h) in the boundary layer and/or in the free atmosphere. Results from both models show that enlarging (reducing)l_hthroughout the model domain considerably reduces (increases) the TC intensification rate and quasi-steady intensity. A new finding is that changingl_habove the boundary layer imposes a much greater influence than that in the boundary layer. Largel_habove the boundary layer is found to effectively reduce the radial gradient of tangential wind inside the radius of maximum tangential wind and thus the inward flux of absolute vorticity, reducing the positive tangential wind tendency and the TC intensification rate and the steady-state intensity. In contrast, although largerl_hin the boundary layer reduces the boundary layer tangential wind tendency, it also leads to the more inward-penetrated inflow and thus enhances the inward flux of absolute vorticity, which offsets part of the direct negative contribution by horizontal diffusion, making the net change in tangential wind tendency not obvious. Results from three-dimensional simulations also show that the resolved eddies contribute negatively to TC spinup whenl_his small, while its effect weakens whenl_his enhanced either in or above the boundary layer.

    

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2. How Frequently Does Rapid Intensification Occur after Rapid Contraction of the Radius of Maximum Wind in Tropical Cyclones over the North Atlantic and Eastern North Pacific?

   https://doi.org/10.1175/MWR-D-21-0322.1  

   Li, Yuanlong ; Wang, Yuqing ; Tan, Zhe-Min ( July 2022 , Monthly Weather Review)

   Abstract The phenomenon that rapid contraction (RC) of the radius of maximum wind (RMW) could precede rapid intensification (RI) in tropical cyclones (TCs) has been found in several previous studies, but it is still unclear how frequently and to what extent RC precedes RI in rapidly intensifying and contracting TCs in observations. In this study, the statistical relationship between RMW RC and TC RI is examined based on the extended best track dataset for the North Atlantic and eastern North Pacific during 1999–2019. Results show that for more than ∼65% of available TCs, the time of the peak contraction rate precedes the time of the peak intensification rate, on average, by ∼10–15 h. With the quantitatively defined RC and RI, results show that ∼50% TCs with RC experience RI, and TCs with larger intensity and smaller RMW and embedded in more favorable environmental conditions tend to experience RI more readily following an RC. Among those TCs with RC and RI, more than ∼65% involve the onset of RC preceding the onset of RI, on average, by ∼15–25 h. The preceding time tends to be longer with lower TC intensity and larger RMW and shows weak correlations with environmental conditions. The qualitative results are insensitive to the time interval for the calculation of intensification/contraction rates and the definition of RI. The results from this study can improve our understanding of TC structure and intensity changes. 

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3. The Mean Kinematic Structure of the Tropical Cyclone Boundary Layer and Its Relationship to Intensity Change

   https://doi.org/10.1175/MWR-D-21-0335.1  

   Zhang, Jun A. ; Rogers, Robert F. ; Reasor, Paul D. ; Gamache, John ( January 2023 , Monthly Weather Review)

   Abstract This study investigates the relationship between the azimuthally averaged kinematic structure of the tropical cyclone boundary layer (TCBL) and storm intensity, intensity change, and vortex structure above the BL. These relationships are explored using composites of airborne Doppler radar vertical profiles, which have a higher vertical resolution than typically used three-dimensional analyses and, therefore, better capture TCBL structure. Results show that the BL height, defined by the depth of the inflow layer, is greater in weak storms than in strong storms. The inflow layer outside the radius of maximum tangential wind speed (RMW) is deeper in intensifying storms than in nonintensifying storms at an early stage. The peak BL convergence inside the RMW is larger in intensifying storms than in nonintensifying storms. Updrafts originating from the TCBL are concentrated near the RMW for intensifying TCs, while updrafts span a large radial range outside the RMW for nonintensifying TCs. In terms of vortex structure above the BL, storms with a quickly decaying radial profile of tangential wind outside the RMW (“narrow” vortices) tend to have a deeper inflow layer outside the RMW, stronger inflow near the RMW, deeper and more concentrated strong updrafts close to the RMW, and weaker inflow in the outer core region than those with a slowly decaying tangential wind profile (“broad” vortices). The narrow TCs also tend to intensify faster than broad TCs, suggesting that a key relationship exists among vortex shape, the BL kinematic structure, and TC intensity change. This relationship is further explored by comparisons of absolute angular momentum budget terms for each vortex shape. 

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4. Observed Kinematic and Thermodynamic Structure in the Hurricane Boundary Layer during Intensity Change

   https://doi.org/10.1175/MWR-D-18-0380.1  

   Ahern, Kyle ; Bourassa, Mark A. ; Hart, Robert E. ; Zhang, Jun A. ; Rogers, Robert F. ( July 2019 , Monthly Weather Review)

   The axisymmetric structure of the inner-core hurricane boundary layer (BL) during intensification [IN; intensity tendency ≥20 kt (24 h)−1, where 1 kt ≈ 0.5144 m s−1], weakening [WE; intensity tendency <−10 kt (24 h)−1], and steady-state [SS; the remainder] periods are analyzed using composites of GPS dropwindsondes from reconnaissance missions between 1998 and 2015. A total of 3091 dropsondes were composited for analysis below 2.5-km elevation—1086 during IN, 1042 during WE, and 963 during SS. In nonintensifying hurricanes, the low-level tangential wind is greater outside the radius of maximum wind (RMW) than for intensifying hurricanes, implying higher inertial stability (I2) at those radii for nonintensifying hurricanes. Differences in tangential wind structure (and I2) between the groups also imply differences in secondary circulation. The IN radial inflow layer is of nearly equal or greater thickness than nonintensifying groups, and all groups show an inflow maximum just outside the RMW. Nonintensifying hurricanes have stronger inflow outside the eyewall region, likely associated with frictionally forced ascent out of the BL and enhanced subsidence into the BL at radii outside the RMW. Equivalent potential temperatures (θe) and conditional stability are highest inside the RMW of nonintensifying storms, which is potentially related to TC intensity. At greater radii, inflow layer θe is lowest in WE hurricanes, suggesting greater subsidence or more convective downdrafts at those radii compared to IN and SS hurricanes. Comparisons of prior observational and theoretical studies are highlighted, especially those relating BL structure to large-scale vortex structure, convection, and intensity.

    

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5. Effects of Sea Spray on the Simulated Tropical Cyclone Development: Dependence on Surface Drag Coefficient Parameterization

   https://doi.org/10.1029/2022JD037299  

   Duan, Ziqiang ; Wang, Yuqing ; Li, Yuanlong ( January 2023 , Journal of Geophysical Research: Atmospheres)

   Wave breaking under strong wind conditions in tropical cyclones (TCs) can generate sea spray droplets, which, during their suspension in air, release sensible heat due to the air‐sea temperature difference while absorb sensible heat from the environment when they evaporate and release latent heat to the environment. Since the spray mass flux is a function of surface drag coefficient (C_D), the effect of spray on TC evolution should depends on C_Dparameterization, while this has not been addressed so far. This study examines the effects of sea spray on the simulated TC evolution with two different C_Dparameterizations (the Weather Research and Forecasting (WRF) default scheme and the Donelan scheme). Results show that during the primary intensification stage, the TC with spray effect becomes stronger than that without spray when the WRF C_Dscheme is used, but becomes weaker when the Donelan C_Dscheme is used. This occurs because C_Dis maximum outside the radius of maximum wind (RMW) with the Donelan scheme, which produces relatively large spray‐mediated latent heat flux outside the RMW, which is unfavorable for TC intensification. The difference is enlarged by a feedback between spray and TC intensification involving the inertial stability and surface friction‐induced radial inflow. However, in the mature stage, the simulated TCs with spray become stronger no matter which C_Dscheme is used. In addition, the spray effect on the TC inner‐core size evolution also weakly depends on the drag parameterization. When C_Dis relatively greater outside the RMW, the inclusion of the spray effect would lead to the inner‐core size increase.

    

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