Genesis of Pre–Hurricane Felix (2007). Part I: The Role of the Easterly Wave Critical Layer

Abstract

The formation of pre-Hurricane Felix (2007) in a tropical easterly wave is examined in a two-part study using the Weather Research and Forecasting (WRF) model with a high-resolution nested grid configuration that permits the representation of cloud system processes. The simulation commences during the wave stage of the precursor African easterly-wave disturbance. Here the simulated and observed developments are compared, while in Part II of the study various large-scale analyses, physical parameterizations, and initialization times are explored to document model sensitivities.In this first part the authors focus on the wave/vortex morphology, its interaction with the adjacent intertropical convergence zone complex, and the vorticity balance in the neighborhood of the developing storm. Analysis of the model simulation points to a bottom-up development process within the wave critical layer and supports the three new hypotheses of tropical cyclone formation proposed recently by Dunkerton, Montgomery, and Wang. It is shown also that low-level convergence associated with the ITCZ helps to enhance the wave signal and extend the ''wave pouch'' from the jet level to the top of the atmospheric boundary layer. The region of a quasi-closed Lagrangian circulation within the wave pouch provides a focal point for diabatic merger of convective vortices and their vortical remnants. The wave pouch serves also to protect the moist air inside from dry air intrusion, providing a favorable environment for sustained deep convection. Consistent with the authors' earlier findings, the tropical storm forms near the center of the wave pouch via system-scale convergence in the lower troposphere and vorticity aggregation. Components of the vorticity balance are shown to be scale dependent, with the immediate effects of cloud processes confined more closely to the storm center than the overturning Eliassen circulation induced by diabatic heating, the influence of which extends to larger radii.

The vorticity budget of developing typhoon Nuri (2008)

“…The high resolution simulation of the development of Atlantic hurricane Felix (2007) by Wang et al (2010a) and Wang et al (2010b) shows many features similar to those seen in the formation of Nuri. In particular, the low-level spinup due to vorticity convergence driven by convection in a protected core and the spinup at upper levels by the tilting term are reproduced.…”

An examination of two pathways to tropical cyclogenesis occurring in idealized simulations with a cloud-resolving numerical model

“…Other numerical modeling studies by H04, M06, Wang et al (2010) and Braun et al (2010), do not show development along pathway Two. The initial conditions used in the studies by N07 and herein, are idealized.…”

“…The critical layer of a tropical easterly wave, or "wave pouch", was hypothesized to be important to TC formation. The new cyclogenesis model is supported in part by observations of a developing Pacific easterly wave and high resolution cloud-representing numerical simulations in both real-case and idealized configurations (Zhang et al, 2011;Wang et al, 2010). Despite these encouraging scientific results, questions remain about the nature of the convective organization process within the wave pouch in the early stages of a tropical disturbance.…”

Genesis of Tropical Storm Debby (2006) within an African Easterly Wave: Roles of the Bottom-Up and Midlevel Pouch Processes

“…Emphasizing the role of AEWs in TCG, Dunkerton et al (2009) postulated that mesovortices and deep convection tend to be generated near the AEW critical latitude. Later, Montgomery et al (2010) and Wang et al (2010) elaborated on the concept of a ''marsupial pouch'' near the intersection of the AEW trough axis and critical latitude. Within this pouch region, TCG is favored as moist air parcels remain within a closed Lagrangian circulation of the AEW and, thus, are protected from adverse environmental conditions such as dry air in the Saharan air layer (SAL).…”

“…Within this pouch region, TCG is favored as moist air parcels remain within a closed Lagrangian circulation of the AEW and, thus, are protected from adverse environmental conditions such as dry air in the Saharan air layer (SAL). The wave pouch also facilitates vorticity aggregation and convective organization leading to TCG (Wang 2012(Wang , 2014.…”