Lightning plays an important role in atmospheric chemistry and in the initiation of wildfires, but the impact of global warming on lightning rates is poorly constrained. Here we propose that the lightning flash rate is proportional to the convective available potential energy (CAPE) times the precipitation rate. Using observations, the product of CAPE and precipitation explains 77% of the variance in the time series of total cloud-to-ground lightning flashes over the contiguous United States (CONUS). Storms convert CAPE times precipitated water mass to discharged lightning energy with an efficiency of 1%. When this proxy is applied to 11 climate models, CONUS lightning strikes are predicted to increase 12 ± 5% per degree Celsius of global warming and about 50% over this century.

The influence of the direction of storm motion on the azimuthal distribution of electrified convection in 35 Atlantic basin tropical cyclones from 1985 to 1999 was examined using data from the National Lightning Detection Network. In the inner 100 km, flashes most often occurred in the front half of storms, with a preference for the right-front quadrant. In the outer rainbands (r ϭ 100-300 km), flashes occurred predominantly to the right of motion, although the maximum remained in the right-front quadrant. The results are shown to be consistent with previous studies of asymmetries in rainfall, radar reflectivity, and vertical motion with respect to tropical cyclone motion. The motion effect has been attributed to the influence of asymmetric friction in the tropical cyclone boundary layer. The authors previously found a strong signature in the azimuthal distribution of lightning with respect to vertical wind shear. Because both effects show clearly, vertical wind shear and storm motion must themselves be systematically related. It was found that more than three-quarters of 12-hourly periods contained a storm motion vector that was left of (i.e., counterclockwise from) the shear vector. These results support the importance of a downshear shift in the upper anticyclone, which produces motion left of shear for all directions of shear. The results are further broken down by direction of shear, and it is shown that the beta effect also plays a significant role in the relationship between motion and vertical wind shear. These results also suggest that substantial downshear tilt of the cyclonic part of the tropical cyclone vortex is uncommon, because that alone produces motion right of shear. The relative importance of asymmetric friction and vertical wind shear on the azimuthal asymmetry of convection was determined by examining circumstances in which the two effects would place maximum lightning in different quadrants. Without exception, the influence of vertical wind shear dominated the distribution. Although asymmetric friction creates vertical motion asymmetries at the top of the boundary layer, these apparently do not produce deep convection if vertical wind shear-induced circulations oppose them.

The objective of this study is to understand how interactions with upper-tropospheric troughs affect the intensity of tropical cyclones. The study includes all named Atlantic tropical cyclones between 1985 and 1996. To minimize other factors affecting intensity change, times when storms are over subcritical sea surface temperatures (Յ26ЊC) or near landfall are removed from the sample. A trough interaction is defined to occur when the eddy momentum flux convergence calculated over a 300-600-km radial range is greater than 10 (m s Ϫ1) day Ϫ1. The trough interaction cases are separated into four composites: (i) favorable superposition [tropical cyclone intensifies with an upper-tropospheric potential vorticity (PV) maximum within 400 km of the tropical cyclone center], (ii) unfavorable superposition, (iii) favorable distant interaction (upper PV maximum between 400 and 1000 km from the tropical cyclone center), and (iv) unfavorable distant interaction. For comparison, two additional composites are created: (v) favorable no trough, and (vi) unfavorable no trough. Tropical cyclones over warm water and away from land are more likely to intensify than weaken after an interaction with an upper-level trough; 78% of superposition cases and 61% of distant interaction cases deepened. In the favorable superposition composite, intensification begins soon after a small-scale upper-tropospheric PV maximum approaches the storm center. As in previous studies, the PV maximum subsequently weakens, most likely due to diabatic heating, and never crosses the center and reverses the deepening. In the favorable distant interaction composite, the upper PV maximum remains well to the west of the tropical cyclone center, and intensification is not due to superposition. Strong upper-level divergence occurs downshear of the center, and an upper-level jet is located poleward of the maximum divergence. The center of the intensifying tropical cyclone is located in the right entrance region of the jet, where upward motion is favored. It is argued that the tropical cyclone and upper-level jet develop in a coupled fashion. In the unfavorable distant interaction composite, weakening is attributed to a slightly larger and stronger upper PV maximum than occurs in the favorable distant interaction composite, which induces about 5 m s Ϫ1 more vertical wind shear over the tropical cyclone center. The fairly subtle PV changes that bring about this increase in vertical shear may help account for the difficulty in forecasting tropical cyclone intensity change during distant trough interactions. The no-trough composites have dramatically smaller azimuthal asymmetries than those involving trough interactions. The major distinguishing factor between deepening and filling storms in the no-trough composites is the magnitude of the vertical wind shear.

The influence of vertical wind shear on the azimuthal distribution of cloud-to-ground lightning in tropical cyclones was examined using flash locations from the National Lightning Detection Network. The study covers 35 Atlantic basin tropical cyclones from 1985-99 while they were over land and within 400 km of the coast over water. A strong correlation was found between the azimuthal distribution of flashes and the direction of the vertical wind shear in the environment. When the magnitude of the vertical shear exceeded 5 m s Ϫ1 , more than 90% of flashes occurred downshear in both the storm core (defined as the inner 100 km) and the outer band region (r ϭ 100-300 km). A slight preference for downshear left occurred in the storm core, and a strong preference for downshear right in the outer rainbands. The results were valid both over land and water, and for depression, storm, and hurricane stages. It is argued that in convectively active tropical cyclones, deep divergent circulations oppose the vertical wind shear and act to minimize the tilt. This allows the convection maximum to remain downshear rather than rotating with time. The downshear left preference in the core is stronger for hurricanes than for weaker tropical cyclones. This suggests that the helical nature of updrafts in the core, which is most likely for the small orbital periods of hurricanes, plays a role in shifting the maximum lightning counterclockwise from updraft initiation downshear. The downshear right maximum outside the core resembles the stationary band complex of Willoughby et al. and the rain shield of Senn and Hiser. The existence and azimuthal position of this feature appears to be controlled by the magnitude and direction of the vertical wind shear.

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A large-amplitude mixed Rossby-gravity wave packet is identified in the western Pacific using 6-10-day bandpass-filtered winds. Individual disturbances of 2300-3000-km wavelength propagated westward as the packet moved slowly eastward. The packet first appeared, and subsequently amplified, within a region of active convection associated with the Madden-Julian oscillation (MJO), which was isolated by low-pass-filtered outgoing longwave radiation. The packet lasted about 5 weeks, then rapidly dispersed as the active MJO moved away from it to the east. West of 150ЊE, individual disturbances within the packet turned northwestward away from the equator, indicating an apparent transition from mixed Rossby-gravity waves to off-equatorial tropical depression (TD)type disturbances. Cyclones filled with cloud and anticyclones cleared during the transition. Nevertheless, convective structure consistent with mixed Rossby-gravity waves remained outside the circulation centers, and three tropical cyclones formed on the edges of three consecutive cyclonic gyres as they moved off the equator. Although the expected Rossby-Kelvin wave structure was present in the background winds within the active MJO, tropical cyclone genesis did not occur within the trailing Rossby gyres, but 2500 km to the west and north. This case study provides evidence that equatorial modes, under the right conditions, can supply precursor disturbances for repeated formation of tropical cyclones. It is argued based on previous work in the literature that this sequence of events is not uncommon.

Cloud-to-ground lightning flash locations were examined for nine Atlantic basin hurricanes using data from the National Lightning Detection Network. A common radial distribution in ground flash density was evident: a weak maximum in the eyewall region, a clear minimum 80-100 km outside the eyewall, and a strong maximum in the vicinity of outer rainbands (210-290-km radius). These results are consistent with the authors' previous study of Hurricane Andrew. None of the storms showed this characteristic radial structure during prehurricane stages. The results support the division of precipitation in the hurricane into three distinct regimes. The eyewall is a unique phenomenon but shares some attributes with deep, weakly electrified oceanic monsoonal convection. The region outside the eyewall and under the central dense overcast has characteristics of the trailing stratiform region of mesoscale convective systems, including a relatively high fraction of positive polarity flashes. The outer bands, with mean maximum flash density at the 250-km radius, contain the vast majority of ground flashes in the storms. Eyewall lightning, defined as that within 40 km of the center, was examined for four moderate-to-strong hurricanes. Such lightning occurred episodically during hurricane stage, with 93% of hourly intervals containing no detected flashes. Eyewall lightning outbreaks over water always occurred at the beginning of or during times of intensification, but often were indicative of the imminent end of deepening. It is proposed that the existence of such inner core lightning might reveal the presence of an eyewall cycle. For the one storm with available aircraft reconnaissance data, eyewall cycles were reliably identified by the occurrence of inner core lightning, and inner core lightning appeared only during such cycles. Suggestions are made as to how eyewall flashes in existing hurricanes might be used to help predict hurricane intensity change.

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