Southeast Pacific Stratocumulus Cloud Droplet Concentrations: Modeled, Retrieved, and Directly-observed Values

The prediction of droplet concentrations (N) within stratocumulus clouds is complicated by sensitivity to boundary layer meteorology and to the abundance of cloud condensation nuclei (CCN). Nevertheless, a consistent assessment of N is essential for understanding how stratocumulus, as well as other cloud types, influence weather and climate. Of particular interest is the interplay among CCN, cloud droplets and precipitation. Starting with airborne measurements acquired over the southeast Pacific, this work evaluates N using three complimentary methods: parcel modeling, remote sensing, and in-cloud measurement. Evaluations of the modeled, retrieved, and directly-observed concentrations are described. The directly-observed values are affected by processes not accounted for by the parcel model (e.g., precipitation scavenging). In this analysis the observed concentrations are 30% smaller than the modeled values and a comparable departure is documented for the retrieved vs. modeled comparison. Agreement among the comparisons is improved after accounting for the effects of precipitation scavenging and entrainment/mixing. Also, the widths of the concentration probability distribution functions are compared and are shown to agree better than in previous studies. Implications for N, and N variability, in numerical cloud models, are also evaluated.