Comparing parameterized versus measured microphysical properties of tropical convective cloud bases during the ACRIDICON–CHUVA campaign

Abstract. The objective of this study is to validate parameterizations that were recently developed for satellite retrievals of cloud condensation nuclei supersaturation spectra, N_CCN(S), at cloud base alongside more traditional parameterizations connecting N_CCN(S) with cloud base updrafts and drop concentrations. This was based on the HALO aircraft measurements during the ACRIDICON–CHUVA campaign over the Amazon region, which took place in September 2014. The properties of convective clouds were measured with a cloud combination probe (CCP), a cloud and aerosol spectrometer (CAS-DPOL), and a CCN counter onboard the HALO aircraft. An intercomparison of the cloud drop size distributions (DSDs) and the cloud water content (CWC) derived from the different instruments generally shows good agreement within the instrumental uncertainties. To this end, the directly measured cloud drop concentrations (N_d) near cloud base were compared with inferred values based on the measured cloud base updraft velocity (W_b) and N_CCN(S) spectra. The measurements of N_d at cloud base were also compared with drop concentrations (N_a) derived on the basis of an adiabatic assumption and obtained from the vertical evolution of cloud drop effective radius (r_e) above cloud base. The measurements of N_CCN(S) and W_b reproduced the observed N_d within the measurements uncertainties when the old (1959) Twomey's parameterization was used. The agreement between the measured and calculated N_d was only within a factor of 2 with attempts to use cloud base S, as obtained from the measured W_b, N_d, and N_CCN(S). This underscores the yet unresolved challenge of aircraft measurements of S in clouds. Importantly, the vertical evolution of r_e with height reproduced the observation-based nearly adiabatic cloud base drop concentrations, N_a. The combination of these results provides aircraft observational support for the various components of the satellite-retrieved methodology that was recently developed to retrieve N_CCN(S) under the base of convective clouds. This parameterization can now be applied with the proper qualifications to cloud simulations and satellite retrievals.