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Top-down and bottom-up aerosol–cloud shortwave radiative flux\udclosures were conducted at the Mace Head Atmospheric Research Station in\udGalway, Ireland, in August 2015. This study is part of the BACCHUS (Impact of\udBiogenic versus Anthropogenic emissions on Clouds and Climate: towards a\udHolistic UnderStanding) European collaborative project, with the goal of\udunderstanding key processes affecting aerosol–cloud shortwave radiative flux\udclosures to improve future climate predictions and develop sustainable\udpolicies for Europe. Instrument platforms include ground-based unmanned\udaerial vehicles (UAVs)1 and satellite measurements of aerosols, clouds and\udmeteorological variables. The ground-based and airborne measurements of\udaerosol size distributions and cloud condensation nuclei (CCN) concentration\udwere used to initiate a 1-D microphysical aerosol–cloud parcel model (ACPM).\udUAVs were equipped for a specific science mission, with an optical particle\udcounter for aerosol distribution profiles, a cloud sensor to measure cloud\udextinction or a five-hole probe for 3-D wind vectors. UAV cloud measurements are\udrare and have only become possible in recent years through the\udminiaturization of instrumentation. These are the first UAV measurements at\udMace Head. ACPM simulations are compared to in situ cloud extinction\udmeasurements from UAVs to quantify closure in terms of cloud shortwave\udradiative flux. Two out of seven cases exhibit sub-adiabatic vertical\udtemperature profiles within the cloud, which suggests that entrainment\udprocesses affect cloud microphysical properties and lead to an overestimate\udof simulated cloud shortwave radiative flux. Including an entrainment\udparameterization and explicitly calculating the entrainment fraction in the\udACPM simulations both improved cloud-top radiative closure. Entrainment\udreduced the difference between simulated and observation-derived cloud-top\udshortwave radiative flux (δRF) by between 25 and 60 W m−2. After accounting for entrainment, satellite-derived cloud droplet\udnumber concentrations (CDNCs) were within 30 % of simulated CDNC. In cases\udwith a well-mixed boundary layer, δRF is no greater than 20 W m−2\udafter accounting for cloud-top entrainment and up to 50 W m−2\udwhen entrainment is not taken into account. In cases with a decoupled\udboundary layer, cloud microphysical properties are inconsistent with\udground-based aerosol measurements, as expected, and δRF is as high as\ud88 W m−2, even high (> 30 W m−2) after accounting for\udcloud-top entrainment. This work demonstrates the need to take in situ\udmeasurements of aerosol properties for cases where the boundary layer is\uddecoupled as well as consider cloud-top entrainment to accurately model\udstratocumulus cloud radiative flux.\ud 1The regulatory term for UAV is remotely piloted aircraft (RPA)

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