N. Brough

Transport and mixing between airmasses in cold frontal regions during Dynamics and Chemistry of Frontal Zones (DCFZ)

The passage of two cold front systems over the United Kingdom are compared and contrasted, using the results of a detailed aircraft and ground-based study. The measurements are interpreted by means of three-dimensional, reverse-domain-filling trajectories using both global models and limited-area mesoscale models. This method provides a three-dimensional picture of the interleaving air-masses in the frontal zone as defined by their Lagrangian histories. The two systems studied differ in that the first is associated with an intense surface low in January and the second is associated with a relatively weak surface low in April. In the intense surface low case the trajectory study suggests that a dry intrusion with stratospheric characteristics penetrated deep into the troposphere along the upper level front. Measurements indeed revealed an unsaturated layer with anomalously high ozone. This layer was intersected at four levels in the troposphere (at 8.5, 7.1, 5.2 and 3.7 km), and the lower the intersection, the lower the measured anomalous ozone and the higher the water vapor content. It is argued that this is best explained by the dry-intrusion layer becoming mixed with background air by three-dimensional turbulence, also encountered by the aircraft, along the upper level front. Evidence for this mixing is apparent on tracer-tracer scatterplots. In the weak surface low case, by contrast, the dry intrusion has a more complex structure, with up to three separate layers of enhanced ozone and low humidity. Strong evidence for mixing was apparent only in the lowest layer. The weaker system may therefore be much more efficient at transporting upper tropospheric/stratospheric ozone to the lower troposphere. The transport of boundary layer air to the upper troposphere in the warm conveyor belt (WCB), however, was found to be around 8 times stronger in the intense system. Sonde measurements suggested that the WCB was ventilated by convection from the surface front in some regions to about 5-6 km, while it was stably stratified in other regions, suggesting layerwise long-range transport.

Particles and iodine compounds in coastal Antarctica

Aerosol particle number concentrations have been measured at Halley and Neumayer on the Antarctic coast, since 2004 and 1984, respectively. Sulphur compounds known to be implicated in particle formation and growth were independently measured: sulphate ions and methane sulphonic acid in filtered aerosol samples and gas phase dimethyl sulphide for limited periods. Iodine oxide, IO, was determined by a satellite sensor from 2003 to 2009 and by different ground-based sensors at Halley in 2004 and 2007. Previous model results and midlatitude observations show that iodine compounds consistent with the large values of IO observed may be responsible for an increase in number concentrations of small particles. Coastal Antarctica is useful for investigating correlations between particles, sulphur, and iodine compounds, because of their large annual cycles and the source of iodine compounds in sea ice. After smoothing all the measured data by several days, the shapes of the annual cycles in particle concentration at Halley and Neumayer are approximated by linear combinations of the shapes of sulphur compounds and IO but not by sulphur compounds alone. However, there is no short-term correlation between IO and particle concentration. The apparent correlation by eye after smoothing but not in the short term suggests that iodine compounds and particles are sourced some distance offshore. This suggests that new particles formed from iodine compounds are viable, i.e., they can last long enough to grow to the larger particles that contribute to cloud condensation nuclei, rather than being simply collected by existing particles. If so, there is significant potential for climate feedback near the sea ice zone via the aerosol indirect effect.