. The WWLLN cloud-to-ground detection efficiency is found to be strongly dependent on peak current and polarity, attaining values larger than 10% (35%) for currents stronger than ±35 kA (−130 kA) and values less than 2% for currents between 0 and −10 kA. The location accuracy is found to have a northward and westward bias, with average location errors of 4.03 km in the north-south and 4.98 km in the east-west directions, respectively. The WWLLN is shown to have strong limitations in capturing the diurnal cycle, missing both the timing of the maximum and minimum lightning activity (around 1600 and 0900 LT, respectively), and the amplitude of the cycle as well. It is found that in 3 h intervals, the number of flashes in the WWLLN has some proportionality to the number of flashes in the NLDN, suggesting that the WWLLN has strong potential for meteorological applications.

Lightning flash density in tropical cyclones (TCs) is investigated to identify whether lightning flashes provide information on TC intensity and/or intensity change, to provide further insight into TC asymmetric convective structure induced by vertical shear and storm motion, and to assess how well the World Wide Lightning Location Network (WWLLN) is suited for the observation of TCs. The 24 Atlantic basin TCs that came within 400 km of the United States from 2004 to 2007 are studied. The National Lightning Detection Network is used to analyze flash density as a function of peak current and to evaluate the WWLLN. Flash density is shown to be smaller for hurricanes than for tropical depressions and storms, with this reduction being gradually more pronounced as flash peak current increases. The results suggest that flash density in the inner core is a parameter with potential for distinguishing intensifying versus nonintensifying TCs, particularly in the weaker storm stages where flash densities are largest.Vertical wind shear produces a strong downshear left (right) asymmetry in the inner core (outer rainbands), whereas motion asymmetries are less clear. The unprecedented azimuthal resolution used in this study suggests that as shear strengthens, the azimuthal region of convection in the inner core is sharpened from a width of ;1308 to a width of ;608. The radial distribution of flash density is shown to exhibit a relatively narrow region of little activity (between 60 and 120 km from the eye), with increased activity in both regions closer to, and more distant from, the center (i.e., the eyewall and outer rainbands, respectively). Finally, it is shown that the WWLLN captures the convective activity in Atlantic basin TCs remarkably well, despite its low detection efficiency.

The authors conduct an analysis of the dynamics of secondary eyewall formation in two modeling frameworks to obtain a more complete understanding of the phenomenon. The first is a full-physics, three-dimensional mesoscale model in which the authors examine an idealized hurricane simulation that undergoes a canonical eyewall replacement cycle. Analysis of the mesoscale simulation shows that secondary eyewall formation occurs in a conditionally unstable environment, questioning the applicability of moist-neutral viewpoints and related mathematical formulations thereto for studying this process of tropical cyclone intensity change. The analysis offers also new evidence in support of a recent hypothesis that secondary eyewalls form via a progressive boundary layer control of the vortex dynamics in response to a radial broadening of the tangential wind field. The second analysis framework is an axisymmetric, nonlinear, time-dependent, slab boundary layer model with radial diffusion. When this boundary layer model is forced with the aforementioned mesoscale model's radial profile of pressure at the top of the boundary layer, it generates a secondary tangential wind maximum consistent with that from the full-physics, mesoscale simulation. These findings demonstrate that the boundary layer dynamics alone are capable of developing secondary wind maxima without prescribed secondary heat sources and/or invocation of special inertial stability properties of the swirling flow either within or above the boundary layer. Finally, the time-dependent slab model reveals that the simulated secondary wind maximum contracts inward, as secondary eyewalls do in mesoscale models and in nature, pointing to a hitherto unrecognized role of unbalanced dynamics in the eyewall replacement cycle.

An analysis is presented of two high‐resolution hurricane simulations of Katrina and Rita (2005) that exhibited secondary eyewall formation (SEF). The results support the notion of vortex Rossby waves (VRWs) having an important role in SEF and suggest that VRW activity is a defining aspect of the moat. SEF occurs at a radius of ∼65 (80) km in Katrina (Rita), close to the hypothesized stagnation radius of VRWs. VRW activity appears to be the result of eye‐eyewall mixing events, themselves a product of the release of barotropic instability. The convection in the radial region that becomes the moat is mainly in the form of VRWs propagating radially outward from the primary eyewall until the negative radial gradient of potential vorticity is no longer conducive for their propagation. These convectively coupled waves, originating and being expelled from the eyewall, are rotation dominated and have the coherency necessary to survive their passage through the strain‐dominated region outside the eyewall.

As global lightning detection has become more reliable, many studies have analyzed the characteristics of lightning in tropical cyclones (TCs); however, very few studies have examined flashes in eastern North Pacific (ENP) basin TCs. This study uses lightning detected by the World Wide Lightning Location Network (WWLLN) to explore the relationship between lightning and sea surface temperatures (SSTs), the diurnal cycle, the storm motion and vertical wind shear vectors, and the 24-h intensity change in ENP TCs during 2006–14. The results are compared to storms in the North Atlantic (NA). Higher flash counts were found over warmer SSTs, with 28°–30°C SSTs experiencing the highest 6-hourly flash counts. Most TC lightning flashes occurred at night and during the early morning hours, with minimal activity after local noon. The ENP peak (0800 LST) was slightly earlier than the NA (0900–1100 LST). Despite similar storm motion directions and differing vertical wind shear directions in the two basins, shear dominated the overall azimuthal lightning distribution. Lightning was most often observed downshear left in the inner core (0–100 km) and downshear right in the outer rainbands (100–300 km). A caveat to these relationships were fast-moving ENP TCs with opposing shear and motion vectors, in which lightning peaked downmotion (upshear) instead. Finally, similar to previous studies, higher flash densities in the inner core (outer rainbands) were associated with nonintensifying (intensifying) TCs. This last result constitutes further evidence in the efforts to associate lightning activity to TC intensity forecasting.