Abstract
The large-scale environment and small-scale microphysics have been shown to play an integral role in the intensification of tropical cyclones (TCs). Here, we examine how the environment and microphysics impacts intensification by analyzing storms undergoing various rates of intensification. First, a 42-year climatology of all intensifying Atlantic TCs between 1980 and 2021 that undergo three distinct intensification rates: slow (SI; 5-10 kt 24 hr-1), moderate (MI; 15-25 kt 24 hr-1), and rapid (RI; ≥ 30 kt 24 hr-1) is analyzed from reanalysis. The distributions show that although low shear and high RH are more favorable for RI, there is still a significant probability that RI can occur in less favorable environments. Evaluating the shear and RH over the 48 hours prior to onset indicates various patterns in the progression of these variables to onset in the different annuli and intensification rates.
Using the reanalysis data collected from all RI and SI events, the climatologically averaged profiles of environmental wind, mixing ratio, and temperature are used to force the environment of an idealized model to further discriminate the structural and thermodynamic differences of RI and SI storms. On average, RI storms exhibit more rapid vertical alignment and more axisymmeterization at onset compared to SI storms which display more dry air ventilation. Despite these differences at onset, the RI and SI storms are not distinguishable just 24 hours prior, indicating that it is the storm’s evolution over time that influences the intensification rate and not a standalone value at a particular time.
To address microphysical uncertainties, the representation of graupel within the model is altered by adjusting the fall speed of the hydrometeor, or by removing it from the scheme. The graupel adjustments have the greatest impact on storms in the RI environment. They experienced delayed or limited intensification with less graupel due to decreased vertical mass flux, diabatic heating, and changes in tilt and symmeterization. The results demonstrate the importance of graupel in promoting RI.