The Role of African Easterly Waves North of the African Easterly Jet on Tropical Cyclogenesis

This study investigates the role of two merging African easterly waves on the genesis of Hurricane Danielle (2004) using observational data and a high resolution modeling framework. The first wave exists south of the African easterly jet (AEW_S) and has a maximum amplitude in the mid-troposphere in association with the jet. The second wave exists north of the African easterly jet (AEW_N) and has a maximum amplitude in the lower-troposphere in association with the semi-permanent Saharan heat low. These waves merge into one vortex over the West African coast. Within a few days, this vortex intensifies to become Hurricane Danielle.

The WRF Model is utilized to simulate this merging process. Results show that this modeling framework realistically simulates the merging between the AEW_N and AEW_S. A separate simulation that removes the AEW_N from the initial conditions does not produce a tropical cyclone, suggesting that the AEW_N, and its corresponding low-level cyclonic relative vorticity, is important in the development of Hurricane Danielle. A vorticity budget quantifies the importance of merging and diabatic processes in low-level vorticity generation associated with the merged vortex. The contributions from the merging (eddy vorticity flux) and diabatic (stretching and tilting) terms to the cyclonic relative vorticity generation at low-levels have comparable magnitudes, confirming that the relative vorticity associated with the AEW_N is at least as important as diabatic generation in increasing the low-level relative vorticity of the merged vortex. This study suggests that AEW_Ns are important to consider when forecasting tropical cyclogenesis.