Aerosol–Cloud Closure Study on Cloud Optical Properties using Remotely Piloted Aircraft Measurements during a BACCHUS Field Campaign in Cyprus

Abstract

In the framework of the EU-FP7 BACCHUS project, an intensive field campaign was performed in Cyprus (2015/03).Remotely Piloted Aircraft System (RPAS), ground-based instruments, and remote-sensing observations were operating in parallel to provide an integrated characterization of aerosol-cloud interactions. Remotely Piloted Aircraft (RPA) were equipped with a 5-hole probe, pyranometers, pressure, temperature and humidity sensors, and measured updraft velocity at cloud base and cloud optical properties of a stratocumulus layer. Ground-based measurements of dry aerosol size distributions and cloud 5 condensation nuclei spectra, and RPA observations of vertical wind velocity and meteorological state parameters are used here to initialize an Aerosol-Cloud Parcel Model (ACPM) and compare the in situ observations of cloud optical properties measured by the RPA to those simulated in the ACPM. Two different cases are studied with the ACPM, including an adiabatic case and an entrainment case, in which the in-cloud temperature profile from RPA is taken into account. Adiabatic ACPM simulation yields cloud droplet number concentrations at cloud base (ca. 400 cm -3 ) that are similar to those derived from a Hoppel minimum 10 analysis. Cloud optical properties have been inferred using the transmitted fraction of shortwave radiation profile measured by downwelling and upwelling pyranometers mounted on a RPA, and the observed transmitted fraction of solar radiation is then compared to simulations from the ACPM. ACPM simulations and RPA observations show better agreement when associated with entrainment compared to that of an adiabatic case. The mean difference between observed and adiabatic profiles of transmitted fraction of solar radiation is 0.12, while this difference is only 0.03 between observed and entrainment profiles. 15A sensitivity calculation is then conducted to quantify the relative impacts of two-fold changes in aerosol concentration, and updraft velocity to highlight the importance of accounting for the impact of entrainment in deriving cloud optical properties, as well as the ability of RPAs to leverage ground-based observations for studying aerosol-cloud interactions.Atmos. Chem. Phys. Discuss., https://doi.

Measured Constraints on Cloud Top Entrainment to Reduce Uncertainty of Nonprecipitating Stratocumulus Shortwave Radiative Forcing in the Southern Ocean

“…CDNCs are also always lower than the adiabatic simulations (Table 1; Figure 2) while D v is consistent with adiabatic simulations, suggesting entrainment of dry air is reducing LWC via the evaporation of cloud droplets. These results are consistent with previous observations in the SO (Yum et al, 1998), the tropics (Rauber et al, 2007; Roberts et al, 2008), and midlatitudes (Calmer et al, 2019; Sanchez et al, 2017). Consequently, the SWCF and CDNCs above cloud base are significantly and consistently overestimated compared to adiabatic conditions (Table 1).…”

“…For decades now, the IPCC has reported an uncertainty in the anthropogenic‐induced cloud forcing of approximately 1.5 W m −2 , which represents nearly 75% of the uncertainty related to the total anthropogenic radiative forcing (IPCC, 2014). This study in the SO extends beyond earlier studies in the midlatitudes (Calmer et al, 2019; Sanchez et al, 2016) to show that the reduction in LWP owing to entrainment is a major contributor to uncertainty in SWCF for stratocumulus clouds globally. Improving the parameterizations of entrainment using the mixing line techniques presented here has the potential to significantly improve GCM representation of cloud optical properties and, ultimately, the Earth's energy budget.…”

“…ENT ML utilizes the linear relationship of two conservative variables (total water content [q t ] and θ E ) to derive in-cloud q t from the measured in-cloud θ E to calculate the entrainment-induced reduction in LWP. This method was first shown in pristine and polluted midlatitude clouds (Calmer et al, 2019;Sanchez et al, 2017) and is now reproduced here with in situ CDP measurements for clouds in the SO. ENT ML only requires measurements of pressure, temperature, and relative humidity (PTU) in and above the cloud as well as the vertical extent of the cloud to estimate the subadiabatic LWC profile.…”

“…With the mixing line, the in‐cloud q t is a function of θ E , which is derived from in‐cloud temperature, pressure, and q v (see Text S3 and Figure S3). The measured thermodynamic properties of the above‐cloud air (shown as red circles in Figure 1) and cloud base air are then used to estimate subadiabatic LWC profile (Calmer et al, 2019; Sanchez et al, 2017). In this analysis, a line is made between a point at the cloud base and each point above the cloud that is identified as “entrained air” (red‐bounded gray markers in Figures 1 and S4).…”

Cloud droplet number closure for tropical convective clouds during the ACRIDICON–CHUVA campaign

“…Under such conditions, entrainment of additional aerosol particles from air masses surrounding the cloud could explain the observed trend in N d,m which would not be captured by the model. Depending on the conditions, entrainment has been shown to lead also to the opposite trend, i.e., to the decrease of N d (Calmer et al, 2019). While also different hygroscopicities of particles activated at different w might explain the trends in Figure 3a-c, it seems unlikely that during the same flight, aerosol populations with very different hygroscopicity were collected.…”