The Relationship between Storm Motion, Vertical Wind Shear, and Convective Asymmetries in Tropical Cyclones

Abstract

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.

Comparing the Spatial Patterns of Rainfall and Atmospheric Moisture among Tropical Cyclones Having a Track Similar to Hurricane Irene (2011)

“…However, the strengthening ridge located in an anomalous position farther north and east ( Figure 3b,c) forces the systems to move north-northeast along the coastline of the U.S. rather than northeast and out to sea. TCs in the current study have translational speeds that exceed the averages of 5.2 and 6 ms −1 for Atlantic Basin TCs as reported by [24,48]. The average translational speed of the 25 TCs (including Irene) as they entered the study region was 10 ms −1 , while it was 16 ms −1 upon exit (Table 1).…”

Tropical Cyclone Track and Structure Sensitivity to Initialization in Idealized Simulations: A Preliminary Study

“…Here, however, the near-core is defined as 1.5 ≤ R/R m ≤ 3.5 in the 3 km experiment, and 2 ≤ R/R m ≤ 4 in the coarser ensemble, TCs had appeared to be outliers in those previous studies. Even the fastest moving cyclones in these experiments had only moderate motions compared to observations (e.g., Shea and Gray 1973;Franklin et al 1996;Corbosiero and Molinari 2003), but it should again be recalled that there is no imposed large-scale steering here.…”

“…1). At the simulation midpoint, translation speeds varied between 2 and 7 m s -1 , spanning the range of the most common motion speeds in Corbosiero and Molinari's (2003) study, although it should be borne in mind that these idealized TCs had no truly independent environmental steering. By hour 60, however, speed variation among the bogussed model storms had become fairly small and more dependent on α 0 than R m0 (Fig.…”

Reevaluating the Role of the Saharan Air Layer in Atlantic Tropical Cyclogenesis and Evolution

“…Whether and by which means storms are detrimentally impacted by vertical shear on their periphery has not been well established. In most previous studies of vertical wind shear impacts on tropical cyclones, the ''detrimental'' shear was thought to be that existing over the core (the center and out to some specified radius) of the storm (Marks et al 1992;Franklin et al 1993;Reasor et al 2000;Black et al 2002;Corbosiero andMolinari 2002, 2003;Rogers et al 2003;Chan et al 2004;Braun et al 2006;Braun and Wu 2007;Chen et al 2006) and was often assumed to be horizontally uniform in modeling studies (Jones 1995;Frank andRitchie 1999, 2001;Wong and Chan 2004). For this study, we assume that the presence of an AEJ near the periphery of a storm is not necessarily detrimental to storm development.…”