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Top-down and Bottom-up aerosol-cloud-closure: towards understanding sources of uncertainty in deriving cloud radiative flux

Abstract : Top-down and bottom-up aerosol-cloud-radiative flux closures were conducted at the Mace Head atmospheric research station in Galway, Ireland in August 2015. This study is part of the BACCHUS (Impact of 15 Biogenic versus Anthropogenic emissions on Clouds and Climate: towards a Holistic UnderStanding) European collaborative project, with the goal of understanding key processes affecting aerosol-cloud-radiative flux closures to improve future climate predictions and develop sustainable policies for Europe. Instrument platforms include ground-based, unmanned aerial vehicles (UAV) 1 , and satellite measurements of aerosols, clouds and meteorological variables. The ground-based and airborne measurements of aerosol size distributions and cloud condensation nuclei (CCN) 20 concentration were used to initiate a 1D microphysical aerosol-cloud parcel model (ACPM). UAVs were equipped for a specific science mission, with an optical particle counter for aerosol distribution profiles, a cloud sensor to measure cloud extinction, or a 5-hole probe for 3D wind vectors. UAV cloud measurements are rare and have only become possible in recent years through the miniaturization of instrumentation. These are the first UAV measurements at Mace Head. ACPM simulations are compared to in-situ cloud extinction measurements from UAVs to quantify 25 closure in terms of cloud radiative flux. Two out of seven cases exhibit sub-adiabatic vertical temperature profiles within the cloud, which suggests that entrainment processes affect cloud microphysical properties and lead to an overestimate of simulated cloud radiative flux. Including an entrainment parameterization and explicitly calculating the entrainment fraction in the ACPM simulations both improved cloud-top radiative closure. Entrainment reduced the difference between simulated and observation-derived cloud-top radiative flux (RF) by between 30 W m-2 and 30 40 W m-2. After accounting for entrainment, satellite-derived cloud droplet number concentrations (CDNC) were within 30% of simulated CDNC. In cases with a well-mixed boundary layer, RF is less than 25 W m-2 after accounting for cloud-top entrainment, compared to less than 50 W m-2 when entrainment is not taken into account. In cases with a decoupled boundary layer, cloud microphysical properties are inconsistent with ground-based aerosol measurements, as expected, and RF is as high as 88 W m-2 , even after accounting for cloud-top entrainment. This 35 1 The regulatory term for UAV is Remotely Piloted Aircraft (RPA).
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Submitted on : Monday, July 24, 2017 - 2:42:57 PM
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K Sanchez, G C Roberts, R Calmer, K Nicoll, E Hashimshoni, et al.. Top-down and Bottom-up aerosol-cloud-closure: towards understanding sources of uncertainty in deriving cloud radiative flux. Atmospheric Chemistry and Physics Discussions, 2017, pp.1 - 32. ⟨10.5194/acp-2017-201⟩. ⟨meteo-01567780⟩



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