Centimeter-scale-resolution airborne temperature measurements in clouds and in marine surface layer during EUREC4A

<p><span>Characterization of small-scale temperature structure of convective clouds and their environment is crucial to understand turbulent entrainment, mixing and its effect on cloud dynamics and microphysics. A newly constructed ultra-fast thermometer UFT2, developed from the former UFT-M, allowing for temperature measurements in clouds with the resolution better than few centimeters, was deployed on the British Antarctic Survey Twin-Otter research aircraft in the course of the EUREC4A research campaign. The goal was to perform first ever fine-scale temperature characterization of subtropical marine warm cumulus clouds.</span></p><p><span>The prototype instrument worked relatively well and allow to collect data from 7 of 17 research flights, including hundreds of cloud penetrations and segments of flights in the marine surface layer. Data, collected with 20 kHz sampling rate, after filtering and averaging allowed to achieve physical resolution of ~3cm at ~60m/s true air speed of the aircraft.</span></p><p><span>Performance of the UFT-2 sensor and its calibration will be discussed. The discussion will be illustrated with examples of multi-scale temperature records collected in cloud interiors, cloud edges, cloud shells at various altitudes as well as in the marine surface layer ~30 m above the sea level.</span></p>

Influence of Swell on Marine Surface-Layer Structure

The influence of swell on turbulence and scalar profiles in a marine surface layer and underlying physics is examined in this study through diagnosis of large-eddy simulations (LES) that explicitly resolve the surface layer and underlying swell. In general, under stable conditions, the mean wind and scalar profiles can be significantly modified by swell. The influence of swell on wind shear, turbulence structure, scalar profiles, and evaporation duct (ED) characteristics becomes less pronounced in a more convective boundary layer, where the buoyancy production of turbulence is significant. Dynamically, swell has little direct impact on scalar profiles. Instead it modifies the vertical wind shear by exerting pressure drag on the wave boundary layer. The resulting redistribution of vertical wind shear leads to changes in turbulence production and therefore turbulence mixing of scalars. Over swell, the eddy diffusivities from LES systematically deviate from the Monin–Obukhov similarity theory (MOST) prediction, implying that MOST becomes invalid over a swell-dominated sea. The deviations from MOST are more pronounced in a neutral or stable boundary layer under relatively low winds and less so in a convective boundary layer.

Laser ablation–accelerator mass spectrometry reveals complete bomb 14C signal in an otolith with confirmation of 60-year longevity for red snapper (Lutjanus campechanus)

Bomb-produced 14C has been used to make valid estimates of age for various marine organisms for 25 years, but fish ages that lead to birth years earlier than the period of increase in 14C lose their time specificity. As a result, bomb 14C dating is limited to a minimum age from the last year of prebomb levels because the temporal variation in 14C in the marine surface layer is negligible for decades before c. 1958. The longevity of red snapper (Lutjanus campechanus) in the Gulf of Mexico remains unresolved despite various forms of support for ages near 50–60 years. Although the age and growth of red snapper have been verified or validated to a limited extent, some scepticism remains about longevity estimates that exceed 30 years. In this study, red snapper otoliths were analysed for 14C using a novel laser ablation–accelerator mass spectrometry technique to provide a continuous record of 14C uptake. This approach provided a basis for age validation that extends beyond the normal limits of bomb 14C dating with confirmation of a 60-year longevity for red snapper in the Gulf of Mexico.

Large-Eddy Simulations and Observations of Atmospheric Marine Boundary Layers above Nonequilibrium Surface Waves

Winds and waves in marine boundary layers are often in an unsettled state when fast-running swell generated by distant storms propagates into local regions and modifies the overlying turbulent fields. A large-eddy simulation (LES) model with the capability to resolve a moving sinusoidal wave at its lower boundary is developed to investigate this low-wind/fast-wave regime. It is used to simulate idealized situations with wind following and opposing fast-propagating waves (swell), and stationary bumps. LES predicts momentum transfer from the ocean to the atmosphere for wind following swell, and this can greatly modify the turbulence production mechanism in the marine surface layer. In certain circumstances the generation of a low-level jet reduces the mean shear between the surface layer and the PBL top, resulting in a near collapse of turbulence in the PBL. When light winds oppose the propagating swell, turbulence levels increase over the depth of the boundary layer and the surface drag increases by a factor of 4 compared to a flat surface. The mean wind profile, turbulence variances, and vertical momentum flux are then dependent on the state of the wave field. The LES results are compared with measurements from the Coupled Boundary Layers Air–Sea Transfer (CBLAST) field campaign. A quadrant analysis of the momentum flux from CBLAST verifies a wave age dependence predicted by the LES solutions. The measured bulk drag coefficient CD then depends on wind speed and wave state. In situations with light wind following swell, CD is approximately 50% lower than values obtained from standard bulk parameterizations that have no sea state dependence. In extreme cases with light wind and persistent swell, CD < 0.