Deconvolution of Boundary Layer Depth and Aerosol Constraints on Cloud Water Path in Subtropical Stratocumuli

The liquid water path (LWP) adjustment due to aerosol-cloud interactions in marine stratocumuli remains a considerable source of uncertainty for climate sensitivity estimates. An unequivocal attribution of LWP changes to changes in aerosol concentration from climatology remains difficult due to the considerable covariance between meteorological conditions alongside changes in aerosol concentrations. Here, we show that LWP susceptibility in marine boundary layers (BLs) inferred from climatological relationships, triples in magnitude from −0.1 to −0.33 as the BL deepens. We further find deep BLs to be underrepresented in pollution track, process modelling and in-situ studies of aerosol-cloud interactions in marine stratocumuli. Susceptibility estimates based on these approaches are skewed towards shallow BLs of moderate LWP susceptibility. Therefore, extrapolating LWP susceptibility estimates from shallow BLs to the entire cloud climatology, may underestimate the true LWP adjustment within sub-tropical stratocumuli, and thus overestimate the effective aerosol radiative forcing in this region. Meanwhile, LWP susceptibility estimates inferred from climatology in deep BLs are still poorly constrained. While susceptibility estimates in shallow BLs are found to be consistent with process modelling studies, they are overestimated as compared to pollution track estimates.

On Estimating the Surface Wind Stress over the Sea

Our study analyzes measurements primarily from two Floating Instrument Platform (FLIP) field programs and from the Air–Sea Interaction Tower (ASIT) site to examine the relationship between the wind and sea surface stress for contrasting conditions. The direct relationship of the surface momentum flux to U2 is found to be better posed than the relationship between and U, where U is the wind speed and is the friction velocity. Our datasets indicate that the stress magnitude often decreases significantly with height near the surface due to thin marine boundary layers and/or enhanced stress divergence close to the sea surface. Our study attempts to correct the surface stress estimated from traditional observational levels by using multiple observational levels near the surface and extrapolating to the surface. The effect of stability on the surface stress appears to be generally smaller than errors due to the stress divergence. Definite conclusions require more extensive measurements close to the sea surface.

Simulation of Mesoscale Cellular Convection in Marine Stratocumulus. Part I: Drizzling Conditions

This study uses eddy-permitting simulations to investigate the mechanisms that promote mesoscale variability of moisture in drizzling stratocumulus-topped marine boundary layers. Simulations show that precipitation tends to increase horizontal scales. Analysis of terms in the prognostic equation for total water mixing ratio variance indicates that moisture stratification plays a leading role in setting horizontal scales. This result is supported by simulations in which horizontal mean thermodynamic profiles are strongly nudged to their initial well-mixed state, which limits cloud scales. It is found that the spatial variability of subcloud moist cold pools surprisingly tends to respond to, rather than determine, the mesoscale variability, which may distinguish them from dry cold pools associated with deeper convection. Simulations also indicate that moisture stratification increases cloud scales specifically by increasing latent heating within updrafts, which increases updraft buoyancy and favors greater horizontal scales.

Temperature and sunlight controls of mercury oxidation and deposition atop the Greenland ice sheet

We conducted the first ever mercury speciation measurements atop the Greenland ice sheet at Summit Station (Latitude 72.6° N, Altitude 3200 m) in the Spring and Summer of 2007 and 2008. These measurements were part of the GSHOX campaigns investigating the importance of halogen chemistry in this remote environment. Significant levels of BrO (1–5 pptv) in the near surface air were often accompanied by depletions of gaseous elemental mercury (GEM) below background levels, and in-situ production of reactive gaseous mercury (RGM). While halogen (i.e. Br) chemistry is normally associated with marine boundary layers, at Summit, Greenland, far from any marine source, we have conclusively detected bromine and mercury chemistry in the near surface air. We suggest that the fate of the formed mercury-bromine radical (HgBr) is further oxidation to stable RGM (HgBr2, HgBrOH, HgBrCl, etc.), or thermal decomposition. These fates appear to be controlled by the availability of Br, OH, Cl, etc. to produce RGM (Hg(II)), verses the lifetime of HgBr by thermal dissociation. At Summit, the availability of Br appears to be controlled by J(Br2), requiring a sun angle of > 5 degrees, while the formation of RGM from HgBr requires a temperature < −15 °C. The majority of the deposited RGM is readily photoreduced and re-emitted to the air as GEM. However, a very small fraction becomes buried at depth. Extrapolating to the entire Greenland ice sheet, we calculate an estimated net annual sequestration of ~ 13 metric tons Hg per year, buried long-term under the sunlit photoreduction zone.