Wet scavenging of soluble gases in DC3 deep convective storms using WRF‐Chem simulations and aircraft observations

Abstract

We examine wet scavenging of soluble trace gases in storms observed during the Deep Convective Clouds and Chemistry (DC3) field campaign. We conduct high‐resolution simulations with the Weather Research and Forecasting model with Chemistry (WRF‐Chem) of a severe storm in Oklahoma. The model represents well the storm location, size, and structure as compared with Next Generation Weather Radar reflectivity, and simulated CO transport is consistent with aircraft observations. Scavenging efficiencies (SEs) between inflow and outflow of soluble species are calculated from aircraft measurements and model simulations. Using a simple wet scavenging scheme, we simulate the SE of each soluble species within the error bars of the observations. The simulated SEs of all species except nitric acid (HNO3) are highly sensitive to the values specified for the fractions retained in ice when cloud water freezes. To reproduce the observations, we must assume zero ice retention for formaldehyde (CH2O) and hydrogen peroxide (H2O2) and complete retention for methyl hydrogen peroxide (CH3OOH) and sulfur dioxide (SO2), likely to compensate for the lack of aqueous chemistry in the model. We then compare scavenging efficiencies among storms that formed in Alabama and northeast Colorado and the Oklahoma storm. Significant differences in SEs are seen among storms and species. More scavenging of HNO3 and less removal of CH3OOH are seen in storms with higher maximum flash rates, an indication of more graupel mass. Graupel is associated with mixed‐phase scavenging and lightning production of nitrogen oxides (NOx), processes that may explain the observed differences in HNO3 and CH3OOH scavenging.

The influence of deep convection on HCHO and H<sub>2</sub>O<sub>2</sub> in the upper troposphere over Europe

“…Our results partly contradict the analysis of DC3 measurements Bela et al, 2016;Barth et al, 2016) that yielded much higher scavenging efficiencies. If we assume an average scavenging efficiency of 50 % for HCHO, as reported by Fried et al (2016), the measured HCHO mixing ratio of 1.45 ppbv in the outflow corresponds to a minimum inflow mixing ratio of 3 ppbv, neglecting entrainment and photochemical processing in the outflow area.…”

“…From in situ observa- tions in DC3, H 2 O 2 scavenging efficiencies between 79 and 97 % were deduced . As has been shown by a number of model studies (Marie et al, 2000(Marie et al, , 2003Barth et al, 2001;Bela et al, 2016) the H 2 O 2 scavenging efficiency strongly depends on the fate of H 2 O 2 during freezing of cloud particles. Incomplete retention can lead to degassing of H 2 O 2 from the droplets and reduce scavenging efficiencies.…”

“…Unfortunately, differences in the storm dynamics and microphysics between DC3 and HOOVER cannot be investigated since these details are not available for HOOVER. Differences to DC3 might be due to degassing from evaporating hydrometeors since the HOOVER measurements were performed in cloud-free air at considerable distance from the convective core, while the DC3 observations were made in the anvil Bela et al, 2016;Barth et al, 2016).…”

“…While insoluble species are efficiently transported to the UT via convection, soluble species are scavenged by cloud and rain droplets, with subsequent removal by precipitation (Wang and Crutzen, 1995;Crutzen and Lawrence, 2000). Nevertheless, recent observations have shown that highly soluble species can also reach the UT via deep convection, most likely due to incomplete removal of these species during pre-cipitation events (Crutzen and Lawrence, 2000;Marie et al, 2000, Barth et al, 2001Yin et al, 2002;Borbon et al, 2012;Bela et al, 2016;Fried et al, 2016). Considerable uncertainty remains with respect to the processes that control the concentrations of soluble species in the outflow of deep convection.…”

“…Stickler et al (2006) pointed out that increased NO concentrations due to lightning enhances the HCHO production in the convective outflow. Recently, results from the Deep Convective Clouds and Chemistry (DC3) field campaign (Barth et al, 2015) indicate that, to reproduce the observations, zero ice retention has to be assumed for HCHO and H 2 O 2 , leading to their incomplete removal by precipitation Bela et al, 2016;.…”