Journal cover Journal topic
Atmospheric Chemistry and Physics An interactive open-access journal of the European Geosciences Union
Atmos. Chem. Phys., 17, 9797-9814, 2017
https://doi.org/10.5194/acp-17-9797-2017
© Author(s) 2017. This work is distributed under
the Creative Commons Attribution 3.0 License.
Research article
22 Aug 2017
Top-down and bottom-up aerosol–cloud closure: towards understanding sources of uncertainty in deriving cloud shortwave radiative flux
Kevin J. Sanchez1,2, Gregory C. Roberts1,2, Radiance Calmer2, Keri Nicoll3,4, Eyal Hashimshoni5, Daniel Rosenfeld5, Jurgita Ovadnevaite6, Jana Preissler6, Darius Ceburnis6, Colin O'Dowd6, and Lynn M. Russell1 1Scripps Institution of Oceanography, University of California, San Diego, CA, USA
2Centre National de Recherches Météorologiques, Météo-France & CNRS UMR3589, Toulouse, France
3Department of Meteorology, University of Reading, Reading, UK
4Department of Electronic and Electrical Engineering, University of Bath, Bath, UK
5Institute of Earth Sciences, The Hebrew University of Jerusalem, Jerusalem, Israel
6School of Physics and Centre for Climate and Air Pollution Studies, National University of Ireland Galway, Galway, Ireland
Abstract. Top-down and bottom-up aerosol–cloud shortwave 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 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 shortwave radiative flux closures to improve future climate predictions and develop sustainable policies for Europe. Instrument platforms include ground-based unmanned aerial vehicles (UAVs)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) concentration were used to initiate a 1-D 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 five-hole probe for 3-D 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 closure in terms of cloud shortwave 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 shortwave 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 shortwave radiative flux (δRF) by between 25 and 60 W m−2. After accounting for entrainment, satellite-derived cloud droplet number concentrations (CDNCs) were within 30 % of simulated CDNC. In cases with a well-mixed boundary layer, δRF is no greater than 20 W m−2 after accounting for cloud-top entrainment and up to 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 δRF is as high as 88 W m−2, even high (> 30 W m−2) after accounting for cloud-top entrainment. This work demonstrates the need to take in situ measurements of aerosol properties for cases where the boundary layer is decoupled as well as consider cloud-top entrainment to accurately model stratocumulus cloud radiative flux.

1The regulatory term for UAV is remotely piloted aircraft (RPA).


Citation: Sanchez, K. J., Roberts, G. C., Calmer, R., Nicoll, K., Hashimshoni, E., Rosenfeld, D., Ovadnevaite, J., Preissler, J., Ceburnis, D., O'Dowd, C., and Russell, L. M.: Top-down and bottom-up aerosol–cloud closure: towards understanding sources of uncertainty in deriving cloud shortwave radiative flux, Atmos. Chem. Phys., 17, 9797-9814, https://doi.org/10.5194/acp-17-9797-2017, 2017.
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Short summary
Unmanned aerial vehicles are equipped with meteorological sensors to measure cloud properties. The measurements are used to calculate the amount of solar radiation reflected by the clouds and compared to simulation results. The uncertainties related to radiative forcing in the simulations are from the lack of mixing in the boundary layer and mixing of dry air into the cloud top. Conservative variables are used to calculate the amount of air mixed into cloud top to minimize these uncertainties.
Unmanned aerial vehicles are equipped with meteorological sensors to measure cloud properties....
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