High-Latitude Atmospheric River Cross-Sections over the North Atlantic: Assessing Optimized Airborne Sounding Strategies

Regarding arctic amplification, meridional transports of moisture and heat from subpolar regions represent a crucial phenomenon. Among such intrusions, Atmospheric Rivers (ARs) are characterized by narrow and transient moisture flows, which are responsible for up to 90% of vertical integrated water vapour transport (IVT) into the Arctic. Moreover, they are relevant for meridional air mass transformations and precipitation events. To identify local sources and sinks of moisture associated with such AR pathways, the accurate determination of total IVT along the AR cross-sections is indispensable. However, since ARs primarily occur over ocean basins, e.g. the North Atlantic, there is a lack of measurements inside ARs. Spaceborne sensors struggle to profile the interior of AR cores, leading to a blind zone where the majority of water vapour is located.

Conversely, airborne released dropsondes currently provide the most detailed insights on ARs. The frequency of dropsonde releases is, however, technically limited, so that uncertainties in the calculated total IVT of the AR transect may be significant. In particular, when the IVT within the AR core has high lateral variability, unresolved AR-IVT characteristics can constrain the moisture budget analysis. During the North Atlantic Waveguide and Downstream Impact Experiment (NAWDEX), conducted in autumn 2016, the High Altitude and LOng- Range research aircraft (HALO) performed several flight segments along high-latitude AR cross-sections. From these North Atlantic ARs associated with strong meridional water vapour transport, we exemplarily present high-resolution measurements and sounding profiles in the interior of AR cross-sections. We focus on a polar case (research flight RF10, 13^th October 2016) and include simulations from the cloud-resolving model ICON-2km, to investigate the lateral AR-IVT variability.  

Comparing dropsonde IVT values with the simulations from ICON-2km, the model shows a valid representation of the AR. Therefore, we use the high-resolution simulations to generate additional synthetic observations. They allow us to identify major sources of error for observational representation of IVT variability in AR cross-sections. Analysing the vertical profile of water vapour transport, we find that specific humidity and wind speed contribute to lateral IVT variability at different heights. With regard to the total cross-section IVT, we derive across-track sounding resolutions required for typical arctic AR-IVT characteristics. The considered AR shows the presence of a low-level jet, a pre-cold-frontal strong wind corridor below 1000 m, resulting from the temperature gradient across the cold front. Since maximum values and increasing lateral variability of IVT appear close to this low-level jet, our results emphasize the need of high-resolution, i.e frequent sonde releases, around the low-level jet to calculate the cross-section total IVT. Our findings aim at optimizing observational airborne strategies for future campaigns, e.g. HALO-AC³ in 2022, in order to lower the uncertainties of IVT in high-latitude and arctic ARs.