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

Abstract In September 2022, National Aeronautics and Space Administration (NASA) conducted the Convective Processes Experiment–Cabo Verde (CPEX-CV) campaign over the data-sparse eastern Atlantic Ocean. 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 are 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 midlevel circulation within pre-Hermine. Forecasts show that the strengthening of pre-Hermine into a TS in WoCPEX was delayed by 12 h due to the large vertical tilt of the vortex and weak midlevel 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 midlevel 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. Significance Statement In September 2022, NASA conducted a field campaign whose goal was to collect meteorological observations over the data-sparse eastern Atlantic Ocean. Field observations of Tropical Storm Hermine, which occurred during the campaign, were used in a weather forecast model to assess the effects of the observations on Hermine’s forecast. The modeling results showed that the additional observations from the field campaign improved the forecast of both the track and the strength of Hermine. The modeling results also showed that with the additional observations, the processes that contributed to Hermine’s formation were better represented.