How entrainment and mixing scenarios affect droplet spectra in cumulus clouds

The EMPM (Explicit Mixing Parcel Model) predicts the evolving in-cloud variability due to entrainment and finite-rate turbulent mixing using a 1D representation of a rising cloudy parcel. The 1D formulation allows the model to resolve fine-scale variability down to the smallest turbulent scales (about 1 mm). The EMPM calculates the growth of thousands of individual cloud droplets based on each droplet's local environment.

We used the EMPM to investigate the impact on droplet spectra evolution in cumulus clouds of the following aspects of entrainment and mixing: (1) single versus multiple entrainment events, (2) isobaric versus ascending parcel trajectories, and (3) entrainment of CCN.

We were motivated by aircraft measurements of the joint pdf of cloud droplet number concentration (CDNC) and mean volume radius (MVR), averaged over 10-m intervals, that were analyzed by Burnet and Brenguier (2006).

Burnet and Brenguier (2006) proposed that isobaric entrainment and mixing, combined with buoyancy sorting, can explain the distributions of CDNC and MVR in cumulus clouds observed during the Small Cumulus Microphysics Study (SCMS). However, our results suggest that ascent of entrained air and entrainment of CCN should also be considered.