TOPIC 2.5 INTENSITY CHANGE: EXTERNAL INFLUENCES

Abstract

This report focuses on new advances regarding tropical cyclone (TC) intensity change induced by external influences. It briefly recalls the framework and main areas of progress identified during the last IWTC. It then highlights the major operational improvements in TC intensity forecasting since then and describes the research studies conducted to augment our understanding of the processes governing TC intensity changes under external forcing. Important shortcomings that remain are pointed out and suggestions are given to initiate discussion at the workshop on how to further progress on these matters. 2.5.0 Introduction The refinement of our knowledge regarding environmental control on TC intensity was highlighted during the IWTC-VII. Several advances were reported on the four dominant synoptic-scale factors influencing TC intensity: (i) vertical wind shear (VWS), (ii) relative humidity (RH), (iii) sea surface temperature (SST) and ocean heat content (OHC), as well as (iv) outflow-layer interactions with the environment. A spectrum of shear values and TC intensities that permit continued intensification was identified from observational and modeling studies. Tang and Emanuel (2010) underlined two mechanisms for inner-core ventilation, defined as the entrainment of low-entropy air from the environment (Simpson and Riehl 1958) under vertical shear: inward eddy fluxes (e.g., through vortex Rossby waves) and downward fluxes through convective downdrafts. Riemer et al. (2010) demonstrated how asymmetries resulting from the relative shear flow help import low entropy environmental air towards the TC core at mid levels where it excites downdrafts (by ice melting and evaporative cooling) that flush the inflow layer with low-entropy air, which is then taken into downstream updrafts; these processes reduce the eyewall buoyancy and convection and thus the storm intensity (Figure 1). There was disagreement on the role of the Saharan Air Layer in the Atlantic basin on TC intensity change. A composite study indicated that the rate of TC intensification was only weakly dependent on environmental conditions, suggesting that rapid intensification (RI; maximum wind speed increasing by at least 30 kt in 24 h) would be more influenced by internal dynamical processes, provided that a preexisting favorable environment exists (Hendricks et al. 2010). Despite some additional guidance and considering that intensity change forecasts were still challenging, IWTC-VII encouraged continued research towards improving the understanding of the impact of various environmental parameters on TC structure and intensity. These encompassed (i)-to-(iv), as well as water vapor advection, dust, topography, and boundary-layer fluxes. IWTC-VII also recommended research into predictability limits for intensity forecasting.