Fine-Scale Turbulence, Cloud Microphysics, and Radiative Cooling Rate Measurements in the Entrainment Interface Layer of Marine Stratocumulus

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Although much progress has been achieved in the general understanding ofmixing and radiative processes of Stratocumulus (Sc), the representation of feedback processes of Sc clouds causes significant uncertains in climate projections. These uncertainties are partly caused by problems in understanding and realistically describing fine-scale mixing mainly at cloud top. Cloud top cooling (entrainment, radiative and evaporative cooling) and dynamic/turbulent processes are strongly linked. Cloud top cooling results in a buoyancy reduction which induces vertical motion within the clouds. Vertical motion subsequently generates turbulence that increases entrainment processes and associated evaporative cooling. In order to investigate these processes we suggest the following project objectives: (a) to improve the understanding of the fine-scale structure of the entrainment interface layer (EIL), (b) to quantify the influence of the EIL on the entrainment in Sc clouds, and (c) to evaluate the role of radiative heating and cooling rates in cloud entrainment and convection processes. In order to reach these goals observations with a modified version of the established, helicopter--borne, combined sensor package ACTOS (Airborne Cloud Turbulence Observation System) and SMART--HELIOS (Spectral Modular Airborne Radiation measurement sysTem) will be performed in an observational campaign at the Azores. Both instrumental platforms are carried as external cargo by a helicopter with low true airspeed.The unique sensor package enables in-situ measurements of dynamic, thermodynamic, cloud microphysical, and radiative parameters with a high spatial resolution (mostly in the decimeter range). No other measurement platform world-wide can achieve such a high resolution which is, however, crucial to achieve the general aims of this proposal. This is particular true considering that the EIL can have thickness as small as 10 m, therefore, high measurement resolution is critical.