An Underappreciated Ingredient in Tropical Cyclogenesis: Lower Tropospheric Warming by Subsidence in Mesoscale Convective Systems

A question central to tropical cyclogenesis (TC genesis) is how a deep tropospheric warm core develops, which marks the beginning of a tropical cyclone connecting its dynamical core to the warm ocean. The warm core induces hydrostatic pressure surface falls in the developing TC. Previous studies have documented various physical mechanisms responsible for the development of the warm core. Deep convection likely plays a critical role; however, the role of the deep convection is not fully understood. The development of the warm core is often viewed in terms of latent heat release in convective systems. Especially in upper levels, the latent heating can contribute to system warming; however, the low levels are also subject to evaporative cooling of rainfall. This leads to a warm-aloft, cold-below (i.e., cold-cored) thermodynamic structure. The upper level warm anomaly is counter-acted by the low-mid level cool anomaly, resulting in the cold-core structure with little significant hydrostatic surface pressure fall. The question is where the additional warming needed to induce the initial drop in surface pressure comes from.

This study uses in-situ observation from the Impact of Typhoons on the Ocean in the Pacific (ITOP) field campaign in August – October 2010. The GPS dropsondes deployed from the Air Force C-130 during several TC genesis missions conducted at ~350 hPa are used to investigate the lower tropospheric warming prior to TC genesis. This high altitude deployment allows the dropsondes to resolve the thermodynamic structure at high vertical resolution through a deep layer of the troposphere. The dropsondes were deployed ~100 km apart in square spiral patterns, which can resolve the broad, disturbance-scale (~500-1000 km) conditions. Most dropsondes fell into the high TPW clear air in between the convective systems. The missions were in to the precursors of Tropical Storm Malou, Typhoon Fanapi, and Typhoon Megi. There were three Fanapi flights: as a weak, non-INVEST disturbance; as an INVEST system tracked by JTWC; and as a tropical depression. Furthermore, Malou was a weak system, and Megi was a tropical depression. Thus, the systems were sampled throughout the process of TC genesis from an early pre-TC stage to a tropical depression.

The dropsonde data reveal that the warmest profiles at the low-mid levels are often associated with “onion” shape Skew-T profiles, suggesting that the air had a history of mesoscale descent. The onion profiles seen during ITOP were less pronounced (RH ~ 70-80%) than previously observed in the wake of strong tropical squall lines (RH as low as 40-50%, which would be detrimental to TC genesis). These profiles were ubiquitous to the disturbance, not just in active mesoscale convective systems. The “onion” profile can linger after the mesoscale systems have dissipated, and would be reinforced by subsequent organized mesoscale systems. Based on hydrostatic pressure calculations, the persisting low-level warming from air with a history of mesoscale descent can significantly contribute to the initial surface pressure falls on the order of ~1-2 hPa during TC genesis, especially when it is superimposed with the upper-level warming. A high-resolution (1.3 km) model study of pre-Fanapi also confirms the importance of low-mid warming outside of the convective core updrafts. In summary, the role of subsidence warming may be an underappreciated aspect of the TC genesis problem.