5.4      Scavenging of ozone precursors in convective clouds observed during a SEAC4RS case study.

Cuchiara, Gustavo C., NCAR and University of Colorado, Mary C. Barth, NCAR, Alan Fried, University of Colorado

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 extends the analysis of severe convection studied during the 2012 DC3 campaign to less severe airmass storms encountered during the 2013 SEAC4RS campaign. The vertical transport of the soluble trace gases, formaldehyde, hydrogen peroxide, methyl hydrogen peroxide was studied during various storm intercepts encountered by the NASA DC-8 aircraft on September 2, 2013. Cloud-resolving simulations with the Weather Research and Forecasting model with Chemistry (WRF-Chem) are used in high resolution (Dx = 1.3 km) to understand the processes affecting these soluble trace gas precursors of ozone, such as entrainment, scavenging, and ice physics processes. In this work, we first evaluated the vertical structure of convective storms comparing NEXRAD observed and WRF-Chem simulated radar reflectivities. These 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, which were found to be in agreement with those derived from observations. The WRF-Chem predicted scavenging efficiencies (SE) were compared to the SEs deduced from observations in order to estimate the ice retention factors (rf = fraction of trace gas retained in an ice particle when cloud drops freeze) for the five aforementioned gases. We find that the ice retention factor for formaldehyde is 0.3-0.6 to have SE of 40-60%, hydrogen peroxide is rf  0.2 to have SE of 80-90%, and methyl hydrogen peroxide is not sensitive to rf and has a SE of 5-25%. These retention factors are different than those determined for severe convection studied during the DC3 campaign, and potentially can have an effect on ozone production in convective outflow regions.