| 01 Dec 2025
Effects of Model Grid Spacing for Warm Conveyor Belt (WCB) Moisture Transport into the Upper Troposphere and Lower Stratosphere (UTLS) – Part II: Eulerian Perspective
Abstract. Warm conveyor belts (WCBs) are important features of extratropical cyclones that transport water vapor and hydrometeors into the upper troposphere and lower stratosphere (UTLS), influencing Earth's radiative budget. Previous studies have demonstrated that the horizontal grid spacing of numerical weather prediction (NWP) models influences modeled WCB properties such as ascent rates and diabatic heating. This two-part study investigates how model grid spacing affects the transport of moisture. We analyze two ICON model simulations of one North Atlantic WCB case study: a convection-parameterizing run at ~ 13 km and a convection-permitting run at ~ 3.5 km approximate grid spacing. Here, present the Eulerian perspective to complement the Lagrangian perspective from the first part of this study. We determine that the convection-parameterizing simulation produces a more humid UTLS in the WCB outflow. This results from (i) larger and fewer ice crystals, slowing the depletion of supersaturation, and (ii) the convection parameterization scheme, which injects excess vapor into the UTLS, when compared to the convection permitting simulation. The convection-permitting simulation experiences larger vertical velocities, which allows for the formation of thicker clouds with more graupel. Cloud-top temperatures are similar, yet the convection permitting simulation produces more outgoing long-wave radiation, which we can attribute to differences in UTLS vapor. Our findings indicate that convection-parameterizing simulations likely misrepresent moisture and hydrometeor transport by WCBs. This has implications for how global climate models simulate the radiative impact of WCBs and their potential influence on upper-level flow.
In this manuscript, Schwenk and Miltenberger present a North Atlantic warm conveyor belt (WCB) case study comparing moisture transport, hydrometeor properties, and radiative effects between convection-permitting and convection-parameterizing simulations. A key contribution is a novel algorithm that combines Lagrangian trajectory information with moisture fields to construct three-dimensional WCB masks, which are then used for an Eulerian analysis of WCB outflow regions.
The studies findings -- more moisture, fewer but larger hydrometeors, and reduced outgoing longwave radiation in the WCB outflow of the convection-parameterizing simulation -- are highly relevant for understanding WCB-related biases in upper-level dynamics and radiative budgets in climate models.
I particularly commend the authors on their innovative approach to create WCB masks and the generally clear and well structured presentation.
The effort to diagnose and interpret the processes underlying the simulated differences is especially valuable, as it points toward potential pathways for improving convection parameterizations.
I look forward to seeing the study published after one major issue is addressed. I lay out this concern, along with additional general and my specific comments on individual sections and figures in the attached document.