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1. Evaluating the impact of parameterized turbulent mixing and boundary layer structure on hurricane intensification

   Zhang, Jun A. ; Rogers, Robert ; Marks, Frank ; Tallapragada, Vijay ( June 2018 , 23 Symposium on Boundary Layers and Turbulence)

   This talk presents results from the authors’ recent work on evaluating the role of turbulence and boundary-layer parameterizations on hurricane intensification. We show that observation-based modification of these physical parameterizations significantly improved the HWRF intensity forecast. Turbulent mixing in both the vertical and horizontal directions are found to be crucial for hurricane spin-up dynamics in 3D numerical simulations and HWRF forecasts. Vertical turbulent mixing regulates the inflow strength and the location of boundary-layer convergence that in turns regulates the distribution of deep convection and the intensification of the whole hurricane vortex. Convergence of angular momentum in the boundary layer that is a key component of the hurricane spin-up theory is also found to be regulated by vertical turbulent mixing in connection to the boundary layer inflow. Horizontal turbulent mixing, on the other hand, mainly influences the eddy momentum flux inside the radius of the maximum wind speed in the angular momentum budget. The effect of horizontal turbulent mixing on the convergence of angular momentum is on smoothing the radial gradient of the angular momentum when the horizontal mixing length is large. In a sheared storm, both the vertical and horizontal turbulent mixing affect vortex and shear interaction in terms ofmore »the evolution of vortex tilt and boundary-layer recovery processes.« less
2. 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 TCmore »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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3. Intensification of Tilted Tropical Cyclones over Relatively Cool and Warm Oceans in Idealized Numerical Simulations

   https://doi.org/10.1175/JAS-D-21-0051.1  

   Schecter, David A. ( February 2022 , Journal of the Atmospheric Sciences)

   A cloud-resolving model is used to examine the intensification of tilted tropical cyclones from depression to hurricane strength over relatively cool and warm oceans under idealized conditions where environmental vertical wind shear has become minimal. Variation of the SST does not substantially change the time-averaged relationship between tilt and the radial length scale of the inner core, or between tilt and the azimuthal distribution of precipitation during the hurricane formation period (HFP). By contrast, for systems having similar structural parameters, the HFP lengthens superlinearly in association with a decline of the precipitation rate as the SST decreases from 30° to 26°C. In many simulations, hurricane formation progresses from a phase of slow or neutral intensification to fast spinup. The transition to fast spinup occurs after the magnitudes of tilt and convective asymmetry drop below certain SST-dependent levels following an alignment process explained in an earlier paper. For reasons examined herein, the alignment coincides with enhancements of lower–middle-tropospheric relative humidity and lower-tropospheric CAPE inward of the radius of maximum surface wind speedr_m. Such moist-thermodynamic modifications appear to facilitate initiation of the faster mode of intensification, which involves contraction ofr_mand the characteristic radius of deep convection. The mean transitional values ofmore »the tilt magnitude and lower–middle-tropospheric relative humidity for SSTs of 28°–30°C are respectively higher and lower than their counterparts at 26°C. Greater magnitudes of the surface enthalpy flux and core deep-layer CAPE found at the higher SSTs plausibly compensate for less complete alignment and core humidification at the transition time.

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4. Does the Rotating Convection Paradigm Describe Secondary Eyewall Formation in Idealized Three-Dimensional Simulations?

   https://doi.org/10.1175/JAS-D-21-0143.1  

   Persing, John ; Montgomery, Michael T. ( March 2022 , Journal of the Atmospheric Sciences)

   The formation of a plausible secondary eyewall is examined with two principal simulation experiments that differ only in the fixed value of rain fall speed, one with a value of 70 m s⁻¹(approaching the pseudo-adiabatic limit) that simulates a secondary eyewall, and one with a value of 7 m s⁻¹that does not simulate a secondary eyewall. Key differences are sought between these idealized three-dimensional simulations. A notable expansion of the lower-tropospheric tangential wind field to approximately 400-km radius is found associated with the precursor period of the secondary eyewall. The wind field expansion is traced to an enhanced vertical mass flux across the 5.25-km height level, which leads, in turn, to enhanced radial inflow in the lower troposphere and above the boundary layer. The inflow spins up the tangential wind outside the primary eyewall via the conventional spinup mechanism. This amplified tangential wind field is linked to a broad region of outwardly directed agradient force in the upper boundary layer. Whereas scattered convection is found outside the primary eyewall in both simulations, the agradient force is shown to promote a ring-like organization of this convection when boundary layer convergence occurs in a persistent, localized region of supergradient winds. Themore »results support prior work highlighting a new model of secondary eyewall formation emphasizing a boundary layer control pathway for initiating the outer eyewall as part of the rotating convection paradigm of tropical cyclone evolution.

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5. Evolution of Dynamic and Thermodynamic Structures before and during Rapid Intensification of Tropical Cyclones: Sensitivity to Vertical Wind Shear

   https://doi.org/10.1175/MWR-D-18-0173.1  

   Tao, Dandan ; Zhang, Fuqing ( April 2019 , Monthly Weather Review)

   This study explores the spatial and temporal changes in tropical cyclone (TC) thermodynamic and dynamic structures before, near, and during rapid intensification (RI) under different vertical wind shear conditions through four sets of convection-permitting ensemble simulations. A composite analysis of TC structural evolution is performed by matching the RI onset time of each member. Without background flow, the axisymmetric TC undergoes a gradual strengthening of the inner-core vorticity and warm core throughout the simulation. In the presence of moderate environmental shear (5–6 m s⁻¹), both the location and magnitude of the asymmetries in boundary layer radial flow, relative humidity, and vertical motion evolve with the tilt vector throughout the simulation. A budget analysis indicates that tilting is crucial to maintaining the midlevel vortex while stretching and vertical advection are responsible for the upper-level vorticity generation before RI when strong asymmetries arise. Two warm anomalies are observed before the RI onset when the vortex column is tilted. When approaching the RI onset, these two warm anomalies gradually merge into one. Overall, the most symmetric vortex structure is found near the RI onset. Moderately sheared TCs experience an adjustment period from a highly asymmetric structure with updrafts concentrated at the down-tilt sidemore »before RI to a more axisymmetric structure during RI as the eyewall updrafts develop. This adjustment period near the RI onset, however, is found to be the least active period for deep convection. TC development under a smaller environmental shear (2.5 m s⁻¹) condition displays an intermediate evolution between ensemble experiments with no background flow and with moderate shear (5–6 m s⁻¹).

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