Aerosols-cloud microphysics-thermodynamics-turbulence: evaluating supersaturation in a marine stratocumulus cloud

Abstract

This work presents a unique combination of aerosol, cloud microphysical, thermodynamic and turbulence variables to characterize supersaturation fluctuations in a turbulent marine stratocumulus (SC) layer. The analysis is based on observations with the helicopter-borne measurement platform ACTOS and a detailed cloud microphysical parcel model following three different approaches: (1) From the comparison of aerosol number size distributions inside and below the SC layer, the number of activated particles is calculated as 435&plusmn;87 cm⁻³ and compares well with the observed median droplet number concentration of <span style="border-top: 1px solid #000; color: #000;"><i>N</i></span>_d = 464 cm⁻³. Furthermore, a 50% activation diameter of <i>D</i>_p50≈115 nm was derived, which was linked to a critical supersaturation <i>S</i>_crit of 0.16% via Köhler theory. From the shape of the fraction of activated particles, we estimated a standard deviation of supersaturation fluctuations of &sigma;_<i>S'</i> = 0.09%. (2) These estimates are compared to more direct thermodynamic observations at cloud base. Therefore, supersaturation fluctuations (<i>S'</i>) are calculated based on highly-resolved thermodynamic data showing a standard deviation of <i>S'</i> ranging within 0.1%≤&sigma;_<i>S'</i>≤0.3 %. (3) The sensitivity of the supersaturation on observed vertical wind velocity fluctuations is investigated with the help of a detailed cloud microphysical model. These results show highest fluctuations of <i>S'</i> with &sigma;_<i>S'</i>=0.1% at cloud base and a decreasing &sigma;_<i>S'</i> with increasing liquid water content and droplet number concentration. All three approaches are independent of each other and vary only within a factor of about two.