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.