Measuring meso-scale gradients in the Arctic during HALO-(AC)3

Moisture and aerosol exchange between the Arctic and the mid-latitudes plays a crucial role in the ongoing rapid warming of the Arctic climate. In particular the downward longwave radiation associated with liquid clouds as embedded in warm-air intrusions has been found to contribute significantly to enhanced melting of the ice sheet. The observed persistence of liquid clouds at high latitudes is not yet fully understood, but the transitions between cloud and cloud-free states are likely to be linked to large-scale dynamics. Recent Lagrangian Large-Eddy-Simulations (LES) following air masses into the Arctic have shown that the evolution of the mixed-layer depth is predominantly controlled by large-scale subsidence, and that relatively abrupt changes in subsidence rates can cause cloud collapse. This subsequently causes a drastic change in the surface energy budget. In order to support this modeling result with observations, simultaneous measurements of the large-scale subsidence and the mixed-phase cloud properties are necessary.

During the recent HALO-(AC)³ field campaign in Spring 2022, we used dropsonde patterns to measure divergence and other gradients across meso-scale areas. This approach has already been tested in the subtropics during the during the NARVAL and EUREC⁴A field experiments and has now been performed in the Arctic for the first time. Given the considerable differences in atmospheric dynamics between the subtropics and the Arctic, it is not clear a priori if this method can be equally successful. To find out, we present a comparison of independent measurements obtained during different flights and by two airplanes sampling the same area. Preliminary results show for the first time that the dropsonde method yields reliable estimates of mesoscale gradients in the Arctic, including subsidence and vorticity. Ongoing efforts to combine the wealth of HALO-(AC)³ data with targeted Lagrangian high resolution simulations will be briefly discussed.