Comment on “Cloud droplet spectral width relationship to CCN spectra and vertical velocity” by Hudson et al.

Abstract

Hudson et al. [2012, hereafter H12] have lately shown that the standard deviation (σ) of droplet size distributions was inversely related to cloud condensation nuclei (CCN) concentration at 1% supersaturation (NCCN) for the in situ aircraft measurements collected during the Rain in Cumulus over the Ocean (RICO) project. Using adiabatic parcel model simulations for various values of updraft velocity (w), they further reported a tendency for the σ -N CCN correlation to change signs from positive to negative as w increases beyond a certain value. The analysis of σ -N CCN correlation and its dependence on w certainly add new understanding of dispersion effect (e.g., Liu and Daum, 2002, LD02 hereafter; Liu et al., 2006b, LDY06 hereafter) and imply that like the more widely known aerosol effect on cloud droplet number concentration (N), dispersion effect may exhibit aerosol-limited and w-limited regimes such that dispersion effect can either diminish or enhance the cooling of number effect, depending on the regimes. However, there appears to be some misunderstanding/misinterpretation of LD02 and LDY06 concerning the use of relative dispersion as a measure of spectral width. Furthermore, our examination of the data reported in Table 1 of H12 shows that there is a positive correlation between NCCN and w, and thus it cannot be ruled out that the observed negative σ -N CCN correlation is a manifestation of the covariation in w, or arises from the indirect effect of aerosol on cloud dynamics, or a combination of both. Below these points are detailed.