Intense tropical cyclones often possess relatively little convection around their cores. In radar composites, this surrounding region is usually echo-free or contains light stratiform precipitation. While subsidence is typically quite pronounced in this region, it is not the only mechanism suppressing convection. Another possible mechanism leading to weak-echo moats is presented in this paper. The basic idea is that the strain-dominated flow surrounding an intense vortex core creates an unfavorable environment for sustained deep, moist convection. Strain-dominated regions of a tropical cyclone can be distinguished from rotationdominated regions by the sign of S . In a tropical cyclone, an approximately 30-kmwide annular region can exist just outside the radius of maximum tangential wind, where fil is less than 30 min and even as small as 5 min. This region is defined as the rapid filamentation zone. Since the time scale for deep moist convective overturning is approximately 30 min, deep convection can be significantly distorted and even suppressed in the rapid filamentation zone. A nondivergent barotropic model illustrates the effects of rapid filamentation zones in category 1-5 hurricanes and demonstrates the evolution of such zones during binary vortex interaction and mesovortex formation from a thin annular ring of enhanced vorticity.

Two hurricane eye features that have yet to be adequately explained are the clear-air moat that forms at the outer edge of the eye and the hub cloud that forms near the circulation centre. To investigate whether these features can be explained by the spatial distribution of the subsidence field, we have derived an analytical solution of the Sawyer-Eliassen transverse circulation equation for a three-region approximation with an unforced central eye region of intermediate or high inertial stability, a diabatically-forced eyewall region of high inertial stability, and an unforced far-field of low inertial stability. This analytical solution isolates the conditions under which the subsidence is concentrated near the edge of the eye. The crucial parameter is the dimensionless dynamical radius of the eye, defined as the physical radius of the eye divided by the characteristic Rossby length in the eye. When this dimensionless dynamical radius is less than 0.6, there is less than 10% horizontal variation in the subsidence rate across the eye; when it is greater than 1.8, the subsidence rate at the edge of the eye is more than twice as strong as at the centre of the eye. When subsidence is concentrated at the edge of the eye, the largest temperature anomalies occur near there rather than at the vortex centre. This warm-ring structure, as opposed to a warm-core structure, is often observed in the lower troposphere of intense hurricanes.

The 2004 North American Monsoon Experiment (NAME) provided an unprecedented observing network for studying the structure and evolution of the North American monsoon. This paper focuses on multiscale characteristics of the flow during NAME from the large scale to the mesoscale using atmospheric sounding data from the enhanced observing network.The onset of the 2004 summer monsoon over the NAME region accompanied the typical northward shift of the upper-level anticyclone or monsoon high over northern Mexico into the southwestern United States, but in 2004 this shift occurred slightly later than normal and the monsoon high did not extend as far north as usual. Consequently, precipitation over the southwestern United States was slightly below normal, although increased troughiness over the Great Plains contributed to increased rainfall over eastern New Mexico and western Texas. The first major pulse of moisture into the Southwest occurred around 13 July in association with a strong Gulf of California surge. This surge was linked to the westward passages of Tropical Storm Blas to the south and an upper-level inverted trough over northern Texas. The development of Blas appeared to be favored as an easterly wave moved into the eastern Pacific during the active phase of a Madden-Julian oscillation.On the regional scale, sounding data reveal a prominent sea breeze along the east shore of the Gulf of California, with a deep return flow as a consequence of the elevated Sierra Madre Occidental (SMO) immediately to the east. Subsidence produced a dry layer over the gulf, whereas a deep moist layer existed over the west slopes of the SMO. A prominent nocturnal low-level jet was present on most days over the northern gulf. The diurnal cycle of heating and moistening (Q 1 and Q 2 ) over the SMO was characterized by deep convective profiles in the mid-to upper troposphere at 1800 LT, followed by stratiform-like profiles at midnight, consistent with the observed diurnal evolution of precipitation over this coastal mountainous region. The analyses in the core NAME domain are based on a gridded dataset derived from atmospheric soundings only and, therefore, should prove useful in validating reanalyses and regional models.

The aim of this study was to develop a reliable and valid measure of hurricane risk perception. The utility of such a measure lies in the need to understand how people make decisions when facing an evacuation order. This study included participants located within a 15-mile buffer of the Gulf and southeast Atlantic U.S. coasts. The study was executed as a three-wave panel with mail surveys in 2010-2012 (T baseline N = 629, 56%; T retention N = 427, 75%; T retention N = 350, 89%). An inventory based on the psychometric model was developed to discriminate cognitive and affective perceptions of hurricane risk, and included open-ended responses to solicit additional concepts in the T survey. Analysis of the T data modified the inventory and this revised item set was fielded at T and then replicated at T . The resulting scales were assessed for validity against existing measures for perception of hurricane risk, dispositional optimism, and locus of control. A measure of evacuation expectation was also examined as a dependent variable, which was significantly predicted by the new measures. The resulting scale was found to be reliable, stable, and largely valid against the comparison measures. Despite limitations involving sample size, bias, and the strength of some reliabilities, it was concluded that the measure has potential to inform approaches to hurricane preparedness efforts and advance planning for evacuation messages, and that the measure has good promise to generalize to other contexts in natural hazards as well as other domains of risk.

The interaction between two tropical cyclones with different core vorticities and different sizes is studied with the aid of a nondivergent barotropic model, on both the f plane and the sphere. A classification of a wide range of cases is presented, using the Dritschel-Waugh scheme, which subdivides vortex interactions into five types: elastic interaction, partial straining out, complete straining out, partial merger, and complete merger. The type of interaction for a vortex pair on the f plane, and the same pair on the sphere, was the same for 77 out of 80 cases studied. The primary difference between the results on the f plane and those on the sphere is that the vorticity centroid of the pair is fixed on the f plane but can drift a considerable distance poleward and westward on the sphere. In the spherical case, the interaction between the cyclone pair and the associated ␤induced cyclonic and anticyclonic circulations can play an important role. The ''partial merger'' regime is studied in detail. In this regime the interaction between vortices can lead to episodic exchanges of vorticity, with both vortices surviving and entering a stage of continued but weaker interaction. With the aid of passive tracers, it is found that the exchange of vorticity is restricted to the vortex periphery even when the vorticity field within each vortex is flat, so that the vortex core is the last region to be eroded. It is hypothesized that some observed interacting tropical cyclones actually do undergo this partialmerger process.