Assessment of the LMDZ Model to the Dynamic and Thermodynamic Properties of Cyclogenesis in the Tropical Atlantic Ocean and on the West African Coast

The study's primary objective is to evaluate the LMDZ model's capacity to simulate the cyclogenesis process, interannual variability of cyclone activity, and associated processes in the tropical Atlantic, focusing on the West African coasts to the central tropical Atlantic region. Two main approaches are used. Firstly, the model's ability to capture the interannual variability in Atlantic cyclogenesis activity is examined through seasonal mean. These seasonal average conditions were identified based on ERAI, along with years characterized by strong and low cyclonic activities. Secondly, a more descriptive approach is undertaken, involving the spatiotemporal monitoring of the Hurricane Karl, which originated near the Cape Verdean coasts on September 16th, 2004, until its dissipation. Horizontal sections of the tropospheric layers most sensitive to the cyclonic phenomenon are used to comprehensively track its progress.

The results show a significant variability of cyclonic activity in the tropical Atlantic at different time scales, indicating that the period from July to September and the region along the Intertropical Convergence Zone (ITCZ) are favorable for tropical cyclogenesis. It also revealed that the years of high cyclonic activity are mainly characterized by low Sea level pressure, strong 850 hPa relative vorticity, high 700 hPa relative humidity, and strong 200 hPa divergence anomalies, whereas the opposite is observed during the low activity years. The LMDZ model performs well in reproducing cyclonic parameters from the surface to the upper troposphere with Mean Absolute Errors being less important from the surface (11%) to the high troposphere (17%). At the synoptic scale, the model accurately replicates hurricane characteristics, including intensity categories, spatial distribution, and trajectories. However, it falls short in accurately representing the genesis phase, such as tropical depression.

Key words: Cyclogenesis, interannual variability, cyclonic activity, tropical depression.