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 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.

Size dependence in chord characteristics from simulated and observed continental shallow cumulus

“…the TSI cloud fraction and by 60 % compared to the AR-SCL value-added product. This general underestimation is also present in the original LASSO simulations as shown by Gustafson et al (2020) and matches the experience of previous continental shallow cumulus LES studies over the ARM-SGP site (Zhang et al, 2017) and over Cabauw in the Netherlands (Schalkwijk et al, 2015). Recently, Fast et al (2019) showed that using a more realistic surface moisture distribution leads to larger and longer-lived clouds, indicating that our cloud underestimation could be due to the homogeneous surface conditions of the LASSO setup.…”

“…-Compared to observations, MicroHH in combination with the LASSO forcing leads to a lower cloud fraction (similar to Schalkwijk et al, 2015;Zhang et al, 2017;Gustafson et al, 2020) and slightly shorter cloud chords.…”

“…This is especially true for continental shallow cumulus clouds that have a marked daily cycle and strong day-to-day variability. But thanks to ongoing efforts to run semicontinuous LES experiments for observational super-sites in Europe (Neggers et al, 2012;Schalkwijk et al, 2015;van Laar et al, 2019) and the US (LASSO -LES ARM Symbiotic Simulation and Observation; Gustafson et al, 2018Gustafson et al, , 2020, we now have the ability to robustly compare observed and simulated cloud statistics, as done recently for cloud-base vertical velocity by Endo et al (2019). Maritime cumulus comparisons require fewer simulations, as shown by the analysis of cloud-base mass flux at the Barbados Cloud Observatory by Sakradzija and Klingebiel (2020).…”

A climatology of trade-wind cumulus cold pools and their link to mesoscale cloud organization

“…1 m s −1 also marks the upper tail of cloud-base averaged updraft vertical velocities at the BCO (see Fig. 4b of Sakradzija and Klingebiel, 2020). This suggests that the gust-front vertical velocity maxima are very relevant for triggering new convection in the trade cumulus regime.…”

“…The average cold-pool passage is characterized by a 0.9 K temperature drop, a 0.2 g kg −1 humidity increase just before and another 0.2 g kg −1 humidity increase right after the front onset, followed by a −0.4 g kg −1 humidity decrease at the end of the front, wind speed and pressure increases of 1.15 m s −1 and 9 Pa, and rain intensities of 0.9 mm h −1 . The vertical velocity at the sub-cloud layer top shows pronounced maxima of 1 m s −1 near the cold-pool onset, which lies in the upper tail of cloud-base averaged updraft velocities at the BCO (Sakradzija and Klingebiel, 2020) and thus is very relevant for triggering new convection. The second half of the front is marked by sub-cloud layer averaged downdrafts of −0.55 m s −1 .…”