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

It has been numerically demonstrated that the turbulence above the boundary is important to tropical cyclone intensification and rapid intensification, but the three-dimensional structures of the sub-grid-scale (SGS) eddy have not been revealed due to the lack of observational data. In this study, two numerical simulations of Super Typhoon Rammasun (2014) were conducted with the Advanced Weather Research and Forecast (WRF) model by incorporating the large-eddy simulation (LES) technique, in which the enhanced eyewall convection and the process of rapid intensification are captured. Consistent with previous observational studies, the strong turbulent kinetic energy (TKE) is found throughout the whole eyewall inside of the radius of maximum wind in both experiments. The simulations indicate that the strong TKE is associated with horizontal rolls with the horizontal extent of 2–4 km, which are aligned azimuthally in the intense eyewall convection. It is indicated that the three-dimensional structures of the SGS eddy can be simulated with the vertical grid spacing of ∼100 m when the horizontal grid spacing is 74 m. It is suggested that there is considerable turbulence associated with azimuthally-aligned horizontal rolls in the mid-level eyewall of tropical cyclone.

Intensity fluctuations in Hurricane Irma (2017) during a period of rapid intensification

Intensity fluctuations observed during a period of rapid intensification of Hurricane Irma (2017) between 04 September and 06 September were investigated in a detailed modelling study using an ensemble of Met Office Unified Model (MetUM) convection permitting forecasts. These intensity fluctuations consisted of alternating weakening and strengthening phases. During weakening phases the tropical cyclone temporarily paused its intensification. It was found that weakening phases were associated with a change in the potential vorticity structure, with a tendency for it to become more monopolar. Convection during strengthening phases was associated with isolated local regions of high relative vorticity and vertical velocity in the eyewall, while during weakening phases the storm became more azimuthally symmetric with weaker convection spread more evenly. The boundary layer was found to play an important role in the cause of the intensity fluctuations with an increase in the agradient wind within the boundary layer causing a spin--down just above the boundary layer during the weakening phases whereas during the strengthening phases the agradient wind reduces. This study offers new explanations for why these fluctuations occur and what causes them.

Observed Processes Underlying the Favorable Vortex Repositioning Early in the Development of Hurricane Dorian (2019).

Dorian’s evolution from a weak, disorganized tropical storm to a rapidly intensifying hurricane is documented through a unique multi-platform synthesis of NOAA’s P-3 tail-Doppler radar, airborne in situ data, and Meteo-France’s Martinique and Guadeloupe ground radar network. Dorian initially struggled to intensify with a misaligned vortex in moderate mid-tropospheric vertical wind shear that also allowed detrimental impacts from dry air near the inner core. Despite vertical wind shear eventually decreasing to less than 5 m/s and an increasingly symmetric distribution of stratiform precipitation, the vortex maintained its misalignment with asymmetric convection for 12 hours. Then, as the low-level circulation (LLC) approached St. Lucia, deep convection near the LLC’s center dissipated, the LLC broadened, and precipitation expanded radially outwards from the center temporally coinciding with the diurnal cycle. Convection then developed farther downtilt within a more favorable, humid environment and deepened appreciably at least partially due to interaction with Martinique. A distinct repositioning of the LLC towards Martinique is induced by spin-up of a mesovortex into a small, compact LLC.It is hypothesized that this somewhat atypical reformation event and the repositioning of the vortex into a more favorable environment, farther from detrimental dry mid-tropospheric air, increased its favorability for the rapid intensification that subsequently ensued. Although the repositioning resulted in tilt reducing to less than the scale of the vortex itself, the pre-existing broad mid-upper level cyclonic envelope remained intact with continued misalignment observed between the mid-level center and repositioned LLC even during the early stages of rapid intensification.

The implication of outflow structure for the rapid intensification of tropical cyclones under vertical wind shear

The implication of outflow structure for tropical cyclone (TC) rapid intensification (RI) is investigated via a climatological study using the best-track, reanalysis and infrared brightness temperature data during 1980–2019. Composite analyses are performed in a shear-relative framework for the RI events under different strengths of environmental shear. Results show that for the RI events under moderate (4.5–11 m s-1) or strong (> 11 m s-1) environmental shear the RI onset follows a significant increase of upper-level outflow upshear of the storm, which is intimately linked with the increasing active convection upshear. The intensified outflow blocks the upper-level environmental flow and thus decreases the local shear, building an environment favorable for RI. In contrast, the RI under weak environmental shear (< 4.5 m s-1) is found to be less attributed to this outflow-blocking mechanism. Comparison between the RI and non-RI cases under moderate or strong environmental shear reveals that the RI cases tend to have stronger outflow and convection in the upshear flank than the non-RI cases, confirming the importance of outflow blocking on the occurrence of RI. Statistical analysis further indicates that the 24-h future intensity change under moderate or strong shear is more negatively correlated with the local shear than with the environmental shear, implicating the potential of local shear and upshear outflow as predictors to improve the forecasting of TC intensity change and especially RI. Further analysis suggests that the environmental thermodynamic conditions may play an important role in modulating the upshear convection and thus outflow blocking.