Investigating the Role of Dry Air Intrusion in the Initial Evolution of Hurricane Ian’s Precursor Using Model Simulations and Airborne Observations

The Joint Aeolus Tropical Atlantic Campaigns (JATAC) 2021 and 2022 deployed on the US Virgin Islands and Cabo Verde, respectively, with science objectives related to the life cycle of convective systems, the long-range transport of dust and its impact on air quality, and the satellite calibration/validation of current and the preparation of future ESA and NASA missions (Aeolus, EarthCARE, AOS, WIVERN). The NASA components of JATAC - the Convective Processes Experiment-Aerosols and Winds (CPEX-AW) and the CPEX–Cabo Verde (CPEX-CV) field campaigns - included a focus on the complex multi-scale processes and interactions that lead to convective development and its upscale growth: Understanding the environmental conditions supporting the development of tropical cyclones (TCs) remains a research and operational challenge, owing in part to limited observations of the lifecycle of convective activity that eventually become TCs. In the Atlantic basin, early stages of TC development favor the region off the west coast of Africa as African Easterly Waves move offshore and provide, at times, favorable conditions for TC development. CPEX-CV provided airborne measurements in this region, with a total of 13 research flights throughout September 2022. The payload included a triple-frequency precipitation radar, Doppler wind lidar, and dropsondes, among other remote sensing and in situ instrumentation, offering a rare 4-D look at tropical oceanic convective systems and their environment.

To support these campaigns, we developed the JPL CPEX-AW/CV portal (https://cpex-aw.jpl.nasa.gov), which integrates model forecasts with multi-parameter satellite and airborne observations from a variety of instruments. The portal provides an interactive system for multi-scale visualization and on-line analysis, allowing for the interrogation of a large number of variables in support of flight planning, execution and post-campaign analysis, providing the large-scale context of the detailed airborne observations.

The portal’s utility was demonstrated in an initial investigation into the evolution of a tropical wave observed on the 09/16/2022 flight (Hristova-Veleva et al., 2023). This flight targeted a growing convective system associated with a broad circulation, the wave structure itself, an Aeolus validation underflight, and dust over Mindelo in coordination with other JATAC measurements. The convection sampled on the wave’s western edge was intense and with lightening, although did not grow upscale into a large mesoscale convective system (MCS). Moreover, the wave appeared to die-off shortly after our flight. However, only a couple of days later, as the wave remnants approached South America, new convection developed slightly ahead of it, in association with low-level moisture convergence. The wave then grew and became hurricane Ian – a powerful Category 5 storm, the third-costliest weather disaster on record, the deadliest hurricane to strike the Florida since the 1935 Labor Day hurricane, and the strongest hurricane to make landfall in Florida since Michael in 2018.

Our questions are: i) Why did the Ian’s pre-cursor wave appear to die off on the 17^th-18^th? And ii) What helped its subsequent growth?

This presentation focuses on the first question as it relates to the difficulties in forecasting TC genesis.

To begin answering this question we focused our portal-based analysis on investigating gradients in environmental moisture, evolution of environmental wind shear in the vicinity of the precipitation, and the presence (or absence) of large-scale convergence as we suspected some combination of these factors limited the initial development of this convective system into a tropical cyclone. Indeed, using short-term forecasts from large-scale models (GEOS-5 and GFS) we found two unfavorable elements for wave development: i) Signatures of significant dry-air intrusion into the MCS region associated with the Saharan Air Layer (SAL) to the north and associated with a strong jet at 700mb; ii) Large-scale low-level divergence just ahead of the wave, observed at the time the wave was decaying (a day after the flight into the region of strong convection).

Furthermore, looking at the evolution of the satellite-observed structure of the precipitating complex of interest, we found that it evolved from a rather circular system into a highly elongated one, bearing the signatures of cold-pool forced convection. This poses the question: What was the role of the MCS we observed and its interaction with the SAL, in increasing the low-level divergence ahead of the system?

To investigate the role of the dry-air intrusion we then employ a high-resolution model forecast with realistic representation of the storm morphology and evolution (as compared to the satellite observations). We analyze backward and forward trajectories of parcels, initialized at the locations when and where the SAL appeared to interact with the MCS. The motivation is to see: i) whether the dry SAL entered the system (the backward trajectories); and ii) how it could have contributed to the cold pool development that leads to the increased divergence ahead of the system and its demise at the later time (the forward trajectories).

Looking at the evolution of Equivalent Potential Temperature and the height of the parcels, we find signatures of SAL being entrained into the system and hypothesize its possible contribution, through enhanced evaporative cooling, to the development of the simulated cold-pool, which then appears to be related with the enhanced divergence ahead of the wave and its initial demise.

Next, we design model sensitivity experiments to test this hypothesis and to explore further the role of the evaporative cooling due to dry air intrusion, and whether this mechanism was a key contributing factor to MCS’s dissipation, and its association with the wave’s initial demise.

Here we will present an overview of the earlier investigations and will focus on the results from the analyses of the high-resolution model simulations. Furthermore, we will compare the model simulations with the airborne observations to confirm their representativeness.

This research points to the role of the SAL in modulating the growth of the waves as they exit Africa.

Part of the work was performed at the Jet Propulsion Laboratory, California Institute of Technology, under contract with the National Aeronautics and Space Administration.