Abstract
AbstractThe 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.
Highlights
In currently operational weather forecasting and climate models, the effect exerted by convective clouds on large-scale atmospheric flow is most commonly represented using a mass flux approach (Arakawa, 2004)
We have presented a first attempt to measure the cloud-base mass flux of shallow cumuli and the corresponding probability distributions that are directly comparable to the cloud-base mass flux distributions simulated using idealized large-eddy simulation (LES)
The observations were collected at the Barbados Cloud Observatory (BCO) during 2017
Summary
In currently operational weather forecasting and climate models, the effect exerted by convective clouds on large-scale atmospheric flow is most commonly represented using a mass flux approach (Arakawa, 2004) By this approach, the convective transport of conserved atmospheric quantities is modelled by independent buoyant plumes, with the intensity of the vertical transport set by the value of the mass flux at the cloud base. The early studies of Ogura and Cho (1973) and Nitta (1975) diagnosed the statistics of cumulus cloud populations from observed large-scale meteorological conditions using the theoretical models of Yanai et al (1973) and Arakawa and Schubert (1974) It remained unclear from these studies whether the mass flux distribution was determined by the large-scale conditions and processes and if so, what the main control parameters were. Direct observational evidence of the probability distributions of the mass flux in cumulus clouds is needed
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