Comparing ground-based observations and a large-eddy simulation of shallow cumuli by isolating the main controlling factors of the mass flux distribution

The distribution of mass flux at the cloud base has long been thought to be independent of large-scale forcing. However, recent idealized modelling studies have revealed its dependence on some large-scale conditions. Such dependence makes it possible to isolate the observed large-scale conditions, which are similar to those in large-eddy simulations (LES), in order to compare the observed and modelled mass flux distributions. In this study, we derive for the first time the distribution of the cloud-base mass flux among individual shallow cumuli from ground-based observations at the Barbados Cloud Observatory (BCO) and compare it with the Rain In Cumulus over the Ocean (RICO) LES case study. The procedure of cloud sampling in LES mimics the pointwise measurement procedure at the BCO to provide a mass flux metric that is directly comparable with observations. We find a difference between the mass flux distribution observed during the year 2017 at the BCO and the distribution modelled by LES that is comparable to the seasonal changes in the observed distribution. This difference between the observed and modelled distributions is diminished and an extremely good match is found by subsampling the measurements under a similar horizontal wind distribution and area-averaged surface Bowen ratio to those modelled in LES. This provides confidence in our observational method and shows that LES produces realistic clouds that are comparable to those observed in nature under the same large-scale conditions. We also confirm that the stronger horizontal winds and higher Bowen ratios in our case study shift the distributions to higher mass flux values, which is coincident with clouds of larger horizontal areas and not with stronger updrafts