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

Abstract The distribution of the cloud-base mass flux is long thought to be independent of the large-scale forcing. However, recent idealized modelling studies reveal its dependence on some of the large-scale conditions. Such a dependence makes it possible to isolate the observed large-scale conditions similar to those in the 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 the 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 point-wise measurement procedure at 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 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 sub-sampling the measurements under the similar horizontal wind distribution and area-averaged surface Bowen ratio to those modelled in LES. This provides confidence in our observational method and also shows that LES produces realistic clouds that can be observed in nature under the same large-scale conditions as imposed in LES. We also confirm that in our case-study the stronger horizontal winds and higher Bowen ratios shift the distributions to higher mass flux values, which is coincident with clouds of larger horizontal areas and not with stronger updrafts. This article is protected by copyright. All rights reserved