Vertical Transport, Entrainment, and Scavenging Processes Affecting Trace Gases in Modeled and Observed SEAC4RS Case Studies

In order to determine the processes responsible for vertical transport, entrainment, and scavenging of soluble trace gases, an analysis of storms combining observations and modeling is needed. This study examines the vertical transport of soluble trace gases in convective storms using two case studies that occurred in September 2013 during the SEAC⁴RS campaign. Cloud-resolving simulations with the Weather Research and Forecasting model with Chemistry (WRF-Chem) are used to understand the processes affecting soluble trace gases, such as entrainment, scavenging, and ice physics processes. In the first analysis, we evaluate the vertical structure of convective storms using a contour frequency by altitude diagram to compare NEXRAD observed and WRF-Chem simulated radar reflectivities. The results show that the model satisfactorily represented the vertical structure of the observed clouds, with slight differences in the timing, intensity, and location of the convection, as expected. Thus, modeled storms were selected based on the best representation of convection initiation time, the vertical development, and the height and time of the outflow region to compare with the clouds sampled by the NASA DC-8 aircraft. We coupled the model with tracers to estimate entrainment rates in the selected simulated clouds and compare to other methods that derive entrainment rates using observed chemical species. In agreement with previous studies, the tracer experiment results reveal that the inflow/outflow regions are responsible for the highest entrainment of air, showing that the WRF-Chem altitude-dependent entrainment rate estimates provide complementary information to methods that derive entrainment rates (that are assumed constant with altitude) from airborne measurements. In the final analysis, we examined the performance of the wet scavenging scheme in the WRF-Chem model in representing the wet removal of formaldehyde, hydrogen peroxide, and methyl hydrogen peroxide, and estimate the fraction of these gases retained in ice. For this analysis, sensitivity simulations with different ice retention factors were examined to determine the most representative ice retention fraction for each species. These newly derived ice retention fractions will be discussed in the context of previous studies that made estimates for other types of convection and that are determined from wind tunnel experiments.