Improved Analysis and Prediction of Tropical Storm Hermine (2022) over the Eastern Tropical Atlantic Using CPEX-CV Observations

In September 2022, NASA conducted the Convective Processes Experiment-Cabo Verde (CPEX-CV) campaign over the data-sparse eastern Atlantic. Over this region, CPEX-CV collected a suite of dense observations to aid in the study of convective systems. Tropical Storm (TS) Hermine formed in late September and had an unusual northward trajectory. Hermine was sampled by two consecutive flights prior to becoming a TS, which provided an opportunity to improve Hermine’s forecast via data assimilation and to verify model forecasts. With an improved forecast after assimilating CPEX-CV data, model data can be used to study the processes controlling Hermine’s intensity and evolution more accurately. Two experiments were conducted. One experiment assimilated CPEX-CV observations (WCPEX) while the other did not (WoCPEX). Compared to WoCPEX, the assimilation of CPEX-CV observations in the WCPEX analysis produced a stronger Saharan air layer, more intense dry air intrusion, easterly wind bias corrections, and a stronger mid-level circulation associated with the pre-Hermine disturbance. Forecast results show that the strengthening of Hermine into a TS in WoCPEX was delayed for 12 hours due to the storm’s large vertical tilt and weaker mid-level vorticity; it also had a westward track bias. In WCPEX, while convection near the storm was weaker at earlier forecast times due to a more intense dry air intrusion, stronger, more organized mid-level vorticity and better vertical alignment of the storm improved Hermine’s intensity and track forecast. Additional sensitivity tests revealed assimilating only CPEX-CV remote sensing observations improved Hermine’s forecast nearly as much as assimilating all CPEX-CV observations.