Wave Propagation and Growth Dependency to Environmental Moisture: A Case of an Atlantic Tropical Cyclogenesis

In a recent model evaluation of the African Easterly Wave (AEW) that became Helene (pre-Helene; 2006) over the Atlantic, the wave was categorized as a mixed-off-equatorial moisture mode during tropical cyclogenesis as it evolved under weak temperature gradient balance. It was found that the simulated pre-Helene waves were more intense and overall slower than in ERA5, especially the wave that evolved in a more moisture-rich environment. The growth and propagation of the wave were related to the position of the convection with respect to the center of the wave vortex. The influence of environmental moisture on wave propagation before and during genesis remains an open question. Motivated by the recent findings, in this study, moisture sensitivity experiments are performed with a convection-permitting model to further evaluate the moisture dependency of the pre-Helene wave and later tropical cyclogenesis. The Model for Prediction Across Scales (MPAS) regional configuration is used to allow altering initial and lateral boundary conditions of relative humidity (RH) through the entire atmospheric column using ERA5 pressure-level data. Preliminary results reveal that over land the strength of the wave-trough meridional flow is related to mid-to-upper-level diabatic heating tendencies from clouds located in the northerly phase of the wave and to the lack of shallow convection within the vortex. In MOIST (RH x 1.2 experiment), the wave moves slower, yet organized convection propagates out of phase with the wave speed, ultimately weakening the wave and subsequent tropical cyclogenesis. In CONTROL, where the wave propagates faster, the phasing between wave and convection supports a stronger wave prior to genesis and ultimately genesis when compared to MOIST. A moister atmosphere (MOIST) favors a larger fraction of shallow convection (bottom heavy and weaker updrafts) at the center and ahead of the vortex, detraining the mid-troposphere and weakening the mid-tropospheric vorticity. This leads to a wave that weakens prior to genesis compared to CONTROL as well as a more abrupt decrease in speed prior to genesis. The lack of cloud microphysics heating tendencies in DRY (RH x 0.5 experiment) resulted in a weaker mid-to-upper-level circulation but stronger surface-to-low-level winds. The lack of moisture is detrimental to the simulated pre-Helene; however, a moister environment does not necessarily result in a more intense wave or tropical cyclogenesis event. A wave that propagates more slowly (‘moist wave’ versus ‘dry wave’), does not necessarily favor growth. For further growth, convection that is in phase with the vortex should be deep moist convection.