Assessing the Impact of Dry Free-Tropospheric Air on TC Genesis Using a High-Resolution Idealized Modeling Approach

Dense, low-entropy downdrafts are understood to be detrimental to TC genesis for a plethora of reasons. Conventional understanding is that dry free-tropospheric air aids in the development of these downdrafts via large evaporative cooling. However, drier free-tropospheric air is also known to lead to a decrease in updraft hydrometeor mass, which could lead to less evaporative cooling within associated downdrafts. This leads us to hypothesize that evaporative cooling is not the primary mechanism leading to denser, low-entropy downdrafts in environments with low free-tropospheric environments, if such an environment even leads to stronger downdrafts relative to those in an environment with higher free-tropospheric RH. We present an analysis of thousands of trajectories of parcels participating in both updrafts and downdrafts of developing TCs to evaluate this hypothesis. Initial results suggest that little to no parcels participating in buoyant updrafts subsequently participate in deep, persistent downdrafts, which calls into question the assumption implicit in our hypothesis that many updraft parcels subsequently participate in downdrafts. Parcels with significant negative vertical velocities in the boundary layer are found to participate in brief periods of weak ascent at low-levels followed by (sometimes rapid) water-loading before descending. These results suggest that updraft qualities may have little connection to downdraft properties in developing TCs, but further analysis is needed and will be discussed.