More Like this

1. Asymmetric Rainband Processes Leading to Secondary Eyewall Formation in a Model Simulation of Hurricane Matthew (2016)

   https://doi.org/10.1175/JAS-D-20-0061.1  

   Yu, Chau-Lam; Didlake, Anthony C.; Zhang, Fuqing; Nystrom, Robert G. (January 2021, Journal of the Atmospheric Sciences)

   Abstract The dynamics of an asymmetric rainband complex leading into secondary eyewall formation (SEF) are examined in a simulation of Hurricane Matthew (2016), with particular focus on the tangential wind field evolution. Prior to SEF, the storm experiences an axisymmetric broadening of the tangential wind field as a stationary rainband complex in the downshear quadrants intensifies. The axisymmetric acceleration pattern that causes this broadening is an inward-descending structure of positive acceleration nearly 100 km wide in radial extent and maximizes in the low levels near 50 km radius. Vertical advection from convective updrafts in the downshear-right quadrant largely contributes to the low-level acceleration maximum, while the broader inward-descending pattern is due to horizontal advection within stratiform precipitation in the downshear-left quadrant. This broad slantwise pattern of positive acceleration is due to a mesoscale descending inflow (MDI) that is driven by midlevel cooling within the stratiform regions and draws absolute angular momentum inward. The MDI is further revealed by examining the irrotational component of the radial velocity, which shows the MDI extending downwind into the upshear-left quadrant. Here, the MDI connects with the boundary layer, where new convective updrafts are triggered along its inner edge; these new upshear-left updrafts are found to be important to the subsequent axisymmetrization of the low-level tangential wind maximum within the incipient secondary eyewall. 

   more » « 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. (August 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 ( I 2 ) at those radii for nonintensifying hurricanes. Differences in tangential wind structure (and I 2 ) 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 ( θ e ) and conditional stability are highest inside the RMW of nonintensifying storms, which is potentially related to TC intensity. At greater radii, inflow layer θ e 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. 

   more » « less
3. The Rapid Intensification of Hurricane Michael (2018): Storm Structure and the Relationship to Environmental and Air–Sea Interactions

   https://doi.org/10.1175/MWR-D-20-0145.1  

   Wadler, Joshua B.; Zhang, Jun A.; Rogers, Robert F.; Jaimes, Benjamin; Shay, Lynn K. (January 2021, Monthly Weather Review)

   null (Ed.)

   Abstract The spatial and temporal variation in multiscale structures during the rapid intensification of Hurricane Michael (2018) are explored using a coupled atmospheric–oceanic dataset obtained from NOAA WP-3D and G-IV aircraft missions. During Michael’s early life cycle, the importance of ocean structure is studied to explore how the storm intensified despite experiencing moderate vertical shear. Michael maintained a fairly symmetric precipitation distribution and resisted lateral mixing of dry environmental air into the circulation upshear. The storm also interacted with an oceanic eddy field leading to cross-storm sea surface temperature (SST) gradients of ~2.5°C. This led to the highest enthalpy fluxes occurring left of shear, favoring the sustainment of updrafts into the upshear quadrants and a quick recovery from low-entropy downdraft air. Later in the life cycle, Michael interacted with more uniform and higher SSTs that were greater than 28°C, while vertical shear imposed asymmetries in Michael’s secondary circulation and distribution of entropy. Midlevel (~4–8 km) outflow downshear, a feature characteristic of hurricanes in shear, transported high-entropy air from the eyewall region outward. This outflow created a cap that reduced entrainment across the boundary layer top, protecting it from dry midtropospheric air out to large radii (i.e., >100 km), and allowing for rapid energy increases from air–sea enthalpy fluxes. Upshear, low-level (~0.5–2 km) outflow transported high-entropy air outward, which aided boundary layer recovery from low-entropy downdraft air. This study underscores the importance of simultaneously measuring atmospheric and oceanographic parameters to understand tropical cyclone structure during rapid intensification. 

   more » « less
4. How does vertical wind shear influence updraft characteristics and hydrometeor distributions in supercell thunderstorms?

   https://doi.org/10.1175/MWR-D-23-0166.1  

   Mulholland, Jake P; Nowotarski, Christopher J; Peters, John M; Morrison, Hugh; Nielsen, Erik R (May 2024, Monthly Weather Review)

   Abstract Vertical wind shear is known to affect supercell thunderstorms by displacing updraft hydrometeor mass downshear, thereby facilitating the storms’ longevity. Shear also impacts the size of supercell updrafts, with stronger shear leading to wider, less dilute, and stronger updrafts with likely greater hydrometeor production. To more clearly define the role of shear across different vertical layers on hydrometeor concentrations and displacements relative to supercell updrafts, a suite of idealized numerical model simulations of supercells was conducted. Shear magnitudes were systematically varied across the 0–1 km, 1–6 km, and 6–12 km AGL layers while the thermodynamic environment was held fixed. Simulations show that as shear magnitude increases, especially from 1–6 km, updrafts become wider and less dilute with an increase in hydrometeor loading, along with an increase in the low-level precipitation area/rate and total precipitation accumulation. Even with greater updraft hydrometeor loading amid stronger shear, updrafts are more intense in stronger shear simulations due to larger thermal buoyancy owing to wider, less dilute updraft cores. Furthermore, downshear hydrometeor displacements are larger in environments with stronger 1–6 km shear. In contrast, there is relatively less sensitivity of hydrometeor concentrations and displacements to variations in either 0–1 km or 6–12 km shear. Results are consistent across free tropospheric relative humidity sensitivity simulations, which show an increase in updraft size and hydrometeor mass with increasing free tropospheric relative humidity owing to a reduction in entrainment-driven dilution for wider updrafts in moister environments. 

   more » « less
5. Updraft Maintenance and Axisymmetrization during Secondary Eyewall Formation in a Model Simulation of Hurricane Matthew (2016)

   https://doi.org/10.1175/JAS-D-21-0103.1  

   Yu, Chau-Lam; Didlake, Anthony C.; Zhang, Fuqing (April 2022, Journal of the Atmospheric Sciences)

   Abstract As a follow-on to a previous study on secondary eyewall formation (SEF) in a simulation of Hurricane Matthew (2016), this study investigates the emergence and maintenance of an asymmetric rainband updraft region that leads to an SEF event. Under moderate deep-layer environmental wind shear, the storm develops a quasi-stationary rainband complex with intense, persistent updrafts in its left-of-shear, downwind end. Using a budget of equivalent potential temperature θ E , it is demonstrated that the maintenance of the left-of-shear updraft is aided by a mesoscale cold pool induced by rainband stratiform cooling which interacts with the storm’s moist envelope of high- θ E air. An extended period of destabilization occurs through differential horizontal advection of θ E in the boundary layer, which continuously replenishes the moist instability that would otherwise be depleted by the updrafts. The initial lifting of the updraft is found to be the result of buoyancy advection resulting from the density contrast between the surface cold pool and the inner-core high- θ E air. A potential vorticity (PV) budget analysis shows that these left-of-shear updrafts generate low- to midlevel PV through diabatic heating and boundary layer processes, which shapes the local PV enhancement and propagates cyclonically downwind. Meanwhile, in the mid- to upper levels, eddy PV flux convergence and PV generation continue to occur in the stratiform precipitation extending downwind into the upshear quadrants, which substantially increases the azimuthal mean PV at the radius of the developing secondary eyewall and marks the occurrence of the axisymmetrization process. 

   more » « less