Repository logo

Environmental influences on hurricane intensification




Merrill, Robert T., author
Department of Atmospheric Science, Colorado State University, publisher

Journal Title

Journal ISSN

Volume Title


Though qualitatively similar in structure, different hurricanes can attain different peak intensities during their lifetimes. Forecasters and empiricists relate the intensity to the sea surface temperature and the "effectiveness" of the upper tropospheric outflow, but offer no clear explanation of how the latter operates. Numerical modelers usually ignore the surrounding flow and emphasize interaction between the convective and vortex scales exclusively. This paper examines more closely the observed upper-tropospheric environmental flow differences between hurricanes which intensify and those which fail to do so, and combines them with previously published empirical and modeling results into a general conceptual model of environmental influences on hurricane intensification. Upper tropospheric wind observations (from satellite cloud tracking. aircraft reports, and rawinsondes) are composited for 28 hurricanes according to intensity tendency. A rotated coordinate system based on the outflow jet location is used so that the asymmetric flow structure is preserved. Little difference is observed in total outflow on the synoptic scale. However, intensifying hurricanes have a less constricted outflow with evidence or lateral connections with the surrounding flow. The asymmetric flow consists of a wave thought to be associated with barotropic instability or the anticyclonic flow above the hurricane and the juxtaposition of surrounding flow features. A quasi-equilibrium balance between hurricane convection and the upper tropospheric environment is proposed. The moist-neutral stratification of the vortex core is a balance between the convection which acts to increase stability and the outflow which acts to reduce it. Reduce the outflow layer cooling and the core stabilizes convective buoyancy is reduced, and a new balance with less vigorous convection is established. If vertically sheared, the environmental flow can also regulate intensity by inducing asymmetric convective structure. Vortex-convection feedbacks are considered to be important mainly in the stages of tropical cyclone development prior to eye formation, which is seen as the first indication that the stabilization process is occurring. Several observational and numerical tests for this conceptual model are then proposed.


December, 1985.
Includes bibliographical references (pages 139-149).

Rights Access


Hurricane modification
Adaptation (Biology)
Nature and nurture


Associated Publications