The role of ascent timescale for WCB moisture transport into the UTLS

Abstract. Warm conveyor belts (WCBs) are coherent ascending airstreams in extratropical cyclones. They are a major source of moisture for the extratropical upper troposphere and lower stratosphere (UTLS), where moisture acts as a potent greenhouse gas and WCB-associated cirrus contribute to cloud radiative forcing. However, the processes controlling WCB moisture transport and cloud properties are poorly characterised. Furthermore, recent studies have revealed (embedded) convection as a ubiquitous feature of WCBs, highlighting the importance of understanding their updraft and microphysical structure. We present a Lagrangian investigation of WCB moisture transport for a case from the WISE (Wave-driven ISentropic Exchange) campaign based on a convection-permitting simulation. Lagrangian non-dimensional metrics of the moisture budget suggest that the ascent timescale (τ₆₀₀) strongly controls the end-of-ascent total moisture content, which is largest for slowly ascending trajectories (τ₆₀₀ > 20 h, ~30 % of all WCB trajectories). This is due to relatively warm end-of-ascent temperatures and the strong temperature control on transported water vapor. Deviations from equilibrium water vapor – condensate partitioning are largest for slow trajectories due to faster glaciation and lower ice crystal numbers. A local moisture transport minimum at intermediate tau₆₀₀ results from a shift towards a riming dominated precipitation formation pathway and decreasing outflow temperatures with decreasing τ₆₀₀. The fastest trajectories (τ₆₀₀ < 5 h, ~5 % of all WCB trajectories) transport the largest condensate mass to the UTLS due to less efficient condensate loss, and produce the longest-lived outflow cirrus. Models that parameterise convection may under-represent these processes, potentially impacting weather forecasts and climate predictions.