Shallow cumulus convection: A validation of large‐eddy simulation against aircraft and Landsat observations

Abstract

AbstractLarge‐eddy simulation (LES) results of shallow cumulus convection are directly evaluated against in‐cloud aircraft measurements, as made during the Small Cumulus Microphysics Study (SCMS). To this purpose an LES case is first constructed, based on available observations (during SCMS). Then the simulations are directly compared with the in‐cloud measurements by using conditionally sampled fields. An advantage of the SCMS dataset is the combination of a range of different surface measurements, in‐cloud measurements by an aircraft at many levels in the cloud layer, and the availability of high‐resolution Landsat images.The results show that given the correct initial and boundary conditions the LES concept is capable of realistically predicting the bulk thermodynamic properties of temperature, moisture and liquid‐water content of the cumulus‐cloud ensemble as observed in SCMS. Furthermore, the vertical component of the in‐cloud turbulent kinetic energy and the cloud size distribution in LES were in agreement with the observations. These results support the credibility of cloud statistics as produced by LES in general, and encourage its use as a tool for testing hypotheses and developing parametrizations of shallow cumulus cloud processes.Several hypotheses which make use of conditionally sampled fields were tested on the SCMS data. The magnitudes and the decrease with height of the bulk entrainment rate following from the SCMS data confirm the typical values of the order of magnitude of 10−3 m−1 as reported in several other recent LES studies. An alternative formulation of the lateral entrainment rate as a function of the liquid‐water content and the mean lapse rate agrees well with the original form based on the conserved variables. Applying an often‐used simplified equation for the cloud vertical velocity to the aircraft measurements results in a reasonably closed budget. Copyright © 2003 Royal Meteorological Society