UC Davis

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 during the field campaign in late September and had an unusual northward trajectory. Hermine was sampled by two consecutive research flights prior to becoming a TS, which provided the opportunity to improve Hermine’s forecast via data assimilation and to provide additional observations for forecast verification. In addition, with a better forecast after assimilating CPEX-CV data, model data can be used to capture the processes controlling Hermine’s intensity and evolution. 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 with enhanced absolute vorticity 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 caused by a stronger African easterly jet. 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.