DC3 Science Goals and Hypotheses

Sample active convection to investigate a) storm dynamics and physics, b) lightning and its production of nitrogen oxides, c) cloud hydrometeor effects on wet scavenging of species, and d) chemistry in the anvil.

1. Quantify tracer transport from the boundary layer (BL) to the upper troposphere (UT)
   
   Inert tracers are transported primarily to the upper troposphere within 3-5 km of the tropopause in shear-driven storms, such as those found in Colorado and Oklahoma, and can be used to determine the maximum outflow altitude, which will be different than cloud top height, the level of neutral buoyancy, and the maximum ice content altitude. These same inert tracers are transported throughout the free troposphere in airmass thunderstorms, more common in the southeastern U.S. This implies that shear-driven thunderstorms contribute more to UTLS chemistry, ozone production, and cross tropopause transport than airmass thunderstorms.
2. Estimate scavenging of soluble tracer species
   
   In the anvil and near the convective cores, soluble species, e.g. HNO3, H2O2 and CH2O, will be depleted compared to their background UT mixing ratios because ice scavenges the dissolved species in cloud water within the convective core. Furthermore, because of the short time an air parcel is in contact with liquid water and the high updraft speeds, transport of soluble species to the UT will be more efficient in the high plains (Colorado) storms compared to the storms in northern Alabama. The warmer cloud bases and greater moisture contents in Oklahoma and Alabama have larger liquid water regions resulting in more efficient scavenging of soluble species.
3. Production of nitrogen oxides (NOx) from lightning
   
   The contribution of lightning to NOx concentrations in the anvil, and subsequently in the upper troposphere, depends on overall flash rates and aggregate channel lengths at heights extending from just above the melting level to the uppermost region of the convective core. The amount of NOx produced by a cloud-to-ground flash is on average roughly equivalent to that produced by an intracloud flash.
4. Lightning flash rate correlations with storm parameters
   
   The flash rates of a storm are proportional to the volume of updrafts greater than 10 m/s in the -10 ̊C to -40 ̊C layer and to storm graupel echo volume. Cloud-to-ground lightning occurrence usually follows the occurrence of precipitation in the 0 ̊C to -10 ̊C layer after graupel has appeared in this region or higher regions. Conversely, cloud-to-ground lightning occurrence is inhibited in storms that produce little precipitation.
5. Inverted polarity lightning
   
   Storms that produce inverted-polarity IC flashes in the upper part of storms and inverted- polarity CG flashes are those in which a large fraction of the adiabatic liquid water profile is realized as cloud liquid in the mixed phase region.
6. Chemistry in the anvil
   
   The chemical composition of the convective outflow within and near the visible anvil will be stratified into a top layer with high radiation fluxes accelerating radical chemistry, and a lower layer with low radiation fluxes and near nighttime-like radical chemistry.

Quantify the changes in chemistry and composition in the upper troposphere after active convection.

1. 12-48 hours after convection
   
   In sampling the convective plume 12-48 hours after convection, we expect to find that 8-12 ppbv ozone will be produced per day due to high NOx and enhanced concentrations of HOx precursor species. The ozone production will vary in a complex nonlinear fashion depending on the NOx and VOC abundance transported to the anvil from the boundary layer and the amount of NOx produced by lightning.
2. Seasonal transition of the UT chemical composition
   
   Survey flights at the end of June from the central U.S. to the northern Caribbean will find the greatest UT ozone and NOx mixing ratios above the Gulf of Mexico and Florida. Daily ozonesonde/lidar profiles from Huntsville, Alabama will document the seasonal buildup and decay of the UT ozone enhancement from May to September.