Transport of Formaldehyde to the Upper Troposphere In Deep Convective Storms During the 2012 DC3 Study

Abstract:

The Deep Convective Clouds and Chemistry (DC3) campaign in the summer of 2012 provided an opportunity to study the impacts of deep convection on reactive and soluble precursors of ozone and HO_x radicals, including CH₂O, in the upper troposphere and lower stratosphere (UTLS) over North America. Formaldehyde measurements were acquired in the inflow and outflow of numerous storms on the NASA DC-8 and NSF/NCAR GV-aircraft employing fast, sensitive, and accurate difference frequency generation infrared absorption spectrometers. Since our Fall 2013 AGU Meeting poster, we have developed an improved methodology based upon 3 independent approaches, to determine the amount of CH₂O that is scavenged by deep convective storms. The first approach is based upon WRF-Chem model simulations, which provides greater confidence in the determination of CH₂O scavenging efficiencies and allows the estimation of CH₂O ice retention factors.The second approach is a modified mixing model employing 4 non-reactive passive tracers (n,i-butane, n,i-pentane) to estimate altitude-dependent lateral entrainment rates. This information is coupled with time-dependent measurements in the outflow of various storms, which when extrapolated to time zero in the storm core, results in estimates of CH₂O scavenging efficiencies. This analysis includes estimates of photochemically produced CH₂O in the storm core. A third approach is based upon CH₂O/n-butane ratio comparisons in both the storm inflow and outflow.

Results from various storms over Oklahoma, Colorado, and Alabama will be presented. However, the analysis will primarily focus on the May 29, 2012 supercell storm in Oklahoma. During this storm, the 4 passive tracers produced a very consistent lateral entrainment rate of 0.083 ± 0.008 km^-1, a value that broadly agrees with entrainment rates determined previously from analyzing moist static energy profiles (Luo et al., Geophys. Res. Lett., 2010). For this storm, the 3-independent approaches give CH₂O scavenging efficiencies in the 49-55% range. Although somewhat higher than previous determinations, there is still sufficient transport of CH₂O to the UTLS, thus providing an important source to the HO_x budget in convective outflow regions.