Mesoscale simulations of tropical cyclone Enawo (March 2017) and its impact on TTL water vapor

Abstract. In early March 2017, tropical cyclone (TC) Enawo formed north of Réunion Island and moved westward toward Madagascar. Enawo evolved from a tropical depression on 2 March to an intense TC on 6 March. This study explores the water vapor transport into the tropical tropopause layer (TTL) throughout TC Enawo's development. High-resolution (2 km) mesoscale simulations using the Meso-NH model were performed to cover TC Enawo's lifecycle over the ocean for the period 2–7 March 2017. The simulated convective cloud field agrees with geostationary satellite infrared observations. Compared to the Global Precipitation Measurements (GPM) and Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observation (CALIPSO) satellite observations, the simulation seems to reproduce well both location and amplitude of the observed precipitation. Simulated and observed ice water content have similar ranges in the upper troposphere but simulated ice above the tropopause is overestimated by a factor 10. Balloon-borne measurements of water vapor, temperature and horizontal winds are also used to validate the Meso-NH simulations in the upper-troposphere and TTL regions. The simulations reveal that the maximum water vapor transport into the TTL occurred on 4 March, when deep (cold) convective clouds were observed. As a result, the lower stratospheric water vapor is increased by ~50 % when compared to pre-storm conditions. An increase of ~2 ppmv in water vapor mixing ratio was simulated in the lower stratosphere within a 700-km region surrounding Enawo's center. Our simulation of TC Enawo suggests that TCs over the Southwest Indian Ocean (0–30° S, 30–90° E) could produce a moistening of 0.4 ppmv. We extended our results to the global tropics (30° S–30° N) using the estimates from published work (Allison et al., 2018; Preston et al., 2019) and by calculating statistics on TC numbers and durations using the International Best Track Archive for Climate Stewardship (IBTrACS) dataset. We estimated a global impact of TC induced tropical lower stratospheric moistening of 0.3 to 0.5 ppmv. Our results suggest that TCs may play an important role in the moistening of the TTL/lower stratosphere via direct injection of ice particles and subsequent sublimation.