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

Abstract 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 research flights prior to becoming a TS, which provided an opportunity to improve Hermine’s forecast via data assimilation and verify model forecasts. With an improved forecast after assimilating CPEX-CV observations, model data is used to study the processes controlling Hermine’s 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, more easterly wind bias corrections, and a stronger mid-level circulation within pre-Hermine. Forecasts show that the strengthening of pre-Hermine into a TS in WoCPEX was delayed by 12 hours due to the large vertical tilt of the vortex and weak mid-level vorticity; it also had a westward track bias. Compared to WoCPEX, in WCPEX, while convection near pre-Hermine was weaker at early forecast times due to a more intense dry-air intrusion, stronger, more organized mid-level vorticity and better vertical alignment of the vortex improved the intensity and track forecast. Additional sensitivity tests revealed that assimilating only CPEX-CV remote sensing observations improved Hermine’s forecast nearly as much as assimilating all CPEX-CV observations.