We compare Venus dayside cloud top winds from Akatsuki UVI images to contemporaneous winds in the lower clouds retrieved on the night side using IRTF/SpeX 2.3 µm images.
From Nov 19 to Dec 6, 2018, we obtained daily images of the night side of Venus with SpeX at 1.74, 2.3 and 5.1 µm, and spectral images cubes of the disk from 0.7 to 2.5 µm. On Nov 26, 27, and Dec 6, Akatsuki obtained high resolution UV images of the dayside cloud tops at 283 and 365 nm. Side-by-side, these simultaneous UVI and SpeX images cover up to 240° of longitude centered on the morning terminator. UVI 365 nm images show upper cloud features at about 70 km altitude, while the SpeX 2.3 µm images show the lower and middle clouds in silhouette, at about 55 km. The cloud tops exhibit 100-120 m/s zonal winds near the equator, while zonal winds in the lower cloud are 55-65 m/s. Both are periodic on timescales of (Earth) days, and zonal winds generally decrease with latitude both at the top of and within the cloud (except for mid-latitude jets).
The vertical wind shear beneath Venus’ cloud tops has been well documented by descent probes (Schubert et al., 1980), and is readily replicated by Venus GCMs (Lebonnois et al., 2016). However, it cannot be a global feature of the atmosphere, because deep instability at mid-high latitudes produces vigorous convection (Ando et al., 2020) that would locally weaken the shear within the clouds. We use Akatsuki UVI image triplets, each image taken two hours apart, to derive the zonal wind field at the cloud tops from -75° to +75° latitude. The typical duration of morning Venus observations at the IRTF was about 3.5 hours, so we similarly used 3 images, each about 90 minutes apart. The IRTF images were acquired at high cadence with 0.25 second integration times. For this analysis, we constructed several image stacks for each morning, each consisting of 10-22 of the sharpest images. With an improvement of about a factor of 4 in signal-to-noise, we found that the processed image stacks were suitable for tracking cloud features. With typical seeing conditions on Mauna Kea, the 2.3 µm IRTF/SpeX images of Venus have a resolution of about 100 km. For a 3.5 hour baseline, zonal winds at the lower clouds can be retrieved with a precision of approximately 16%, or about ± 4 m/s.
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