Thermodynamic and cloud evolution in a cold air outbreak during HALO-(AC)³: Quasi-Lagrangian observations compared to the ERA5 and CARRA reanalyses

Abstract. Intense air mass transformations take place when cold, dry Arctic air masses move southward from the closed sea ice onto the much warmer ice-free Arctic ocean during marine cold air outbreaks (MCAOs). In spite of intensive research on MCAOs during recent years, the temporal rates of diabatic heating and moisture uptake relevant also for cloud formation/dissipation have not been measured along MCAO flows. Instead, reanalyses have typically been used for climatological investigations of MCAOs or to supply higher-resolution models with lateral boundary conditions and time-dependent forcings. Meanwhile, the uncertainties connected to those datasets remain unclear.

Here, we present height-resolved observations of diabatic heating rates, moisture uptake, and cloud evolution measured in a quasi-Lagrangian manner. The investigated specific MCAO was observed on 01 April 2022 during the HALO-(AC)³ airborne campaign that was conducted in spring 2022. Shortly after passing the ice edge, maximum diabatic heating rates larger than 6 K h⁻¹ and moisture uptake of more than 0.3 g kg⁻¹ h⁻¹ were measured close above the ocean surface. As the air mass continued its drift southwards, clouds started to form and vertical mixing within the steadily deepening boundary layer was intensified. The quasi-Lagrange observations are compared with reanalysis data from the European Centre for Medium-Range Weather Forecasts (ECMWF) latest global reanalysis ERA5 and the Copernicus Arctic Regional Reanalysis (CARRA). It was found that the mean absolute errors (MAEs) of ERA5 versus CARRA data are 60 % higher for air temperature over sea ice (1.4 K versus 0.9 K), and 70 % higher for specific humidity over ice-free ocean (0.12 g kg⁻¹ versus 0.07 g kg⁻¹ ). We relate these differences not only to issues with representations of the marginal ice zone and corresponding surface fluxes in ERA5, but also to the cloud scheme producing excess liquid-bearing clouds and precipitation, causing a too-dry marine boundary layer. Overall, the combination of CARRA’s high spatial resolution, an improved handling of cold surfaces, and the demonstrated higher fidelity towards the observations, make it a well-suited candidate for further investigations of Arctic air mass transformations.