The Structure and Variability of Large-scale Atmospheric Vertical Motion in the Trades

Abstract:

We use measurements from the Elucidating the Role of Clouds-Circulation Coupling in Climate (EUREC⁴A) campaign to characterise the large-scale vertical motion in the atmosphere and analyse its variability across time-scales ranging from a few hours to a month (the entire campaign period from 19th January - 15th February, 2020). Previous campaigns have demonstrated the reliability of estimating large-scale area-averaged divergence using dropsonde measurements launched in a circular flight path (~200 km in diameter), something that was carried out extensively during the EUREC⁴A campaign — 85 circles over 19 flight-days in the North Atlantic trade-wind region. We find that the mean pressure velocity (ω) above the cloud layer does not vary a lot in the vertical when averaged over the entire campaign period and stays around 1-1.5 hPa/h, which agrees well with the ~1.5 K/day cooling rate that have been estimated for these regions. However, there is significant intra- and inter-day variability between ω profiles, in terms of the magnitudes, ranging from -7 to 6 hPa/h as well as in terms of the vertical structure of these profiles. We also investigate associations of ω with significant atmospheric levels, such as cloud-base, inversion layers and the occurrence of elevated moisture layers in the free troposphere. Such a rich characterisation of atmospheric large-scale dynamics is a first for observational studies. Our findings can give insight into how the atmospheric state varies over short time-scales, as well as their impact on cloudiness, thus providing clues about a critical question in climate science — the clouds-circulation coupling.