FY 2024

Air-sea heat fluxes associated with convective cold pools

Wills, S., M.F. Cronin, and D. Zhang

J. Geophys. Res., 128(20), e2023JD039708, doi: 10.1029/2023JD039708, View open access article at AGU/Wiley (external link) (2023)

High-resolution air-sea observations collected by a suite of remotely-piloted uncrewed surface vehicles (USV) provide new insight on surface latent and sensible heat fluxes associated with convective cold pools over the eastern Pacific Intertropical Convergence Zone (ITCZ). Convective cold pools are linked to isolated, heavily precipitating cells over the ocean that yield downdrafts of evaporatively-cooled air that spreads out horizontally in all directions and are characterized by strong air temperature gradients and horizontal wind speeds. Cold pool frontal signatures are identified based on air temperature depression criteria of −1.5°C in 10 min or less, resulting in a total of 276 events observed within the eastern Pacific ITCZ across 590 total seadays of data from 10 drones over the course of 3 years. Composite analysis reveals enhanced latent and sensible heat fluxes from the ocean to the atmosphere immediately following the frontal passage, and a decomposition of the bulk flux formulas indicates that variations in latent heat flux are largely influenced by the anomalous wind field (i.e., the cold pool gust front) acting on the mean background air-sea moisture gradient, while variations in sensible heat flux are largely influenced by the mean background wind acting on the anomalous air-sea temperature gradient (i.e., the cold pool temperature front). High-frequency variations in wind speed and velocity are further explored in the context of “gustiness” and averaging intervals used in bulk flux algorithms. A case study is also presented for a convective cold pool event captured by a traveling mesoscale network, or mesonet, of USVs.

Plain Language Summary. New 1-min measurements of oceanic and atmospheric variables collected by automated observing platforms, Saildrone uncrewed surface vehicles, are used to explore convective cold pools over the stormy region of the northern hemisphere eastern tropical Pacific ocean. Convective cold pools are domes of cold air near the surface that form underneath intensely raining thunderstorms. The cold pool boundary (or front) separates the cold, dry air within the dome from the warm, moist air of the surrounding environment, and thus qualifying events are identified based on temperature drops of −1.5°C in 10 min or less in the 1-min saildrone data. Results based on 276 cold pool events observed between 10 drones over 3 years reveal that increasing wind speed and decreasing air temperature associated with the cold pool front largely contribute to enhanced air-sea heat fluxes (i.e., exchanges of energy) from the ocean to the atmosphere, highlighting the importance of high-resolution (i.e., 1-min) observations to capture variations on small time and space scales. A cold pool event observed by multiple saildrones is also explored, and wind measurements from a single saildrone are further investigated in the context of a “gustiness” parameter used in the bulk flux equations.