Abstract
Abstract. In this study we compare long-term Doppler and Raman lidar observations against a full month of large eddy simulations of continental shallow cumulus clouds. The goal is to evaluate if the simulations can reproduce the mean observed vertical velocity and moisture structure of cumulus clouds and their associated subcloud circulations, as well as to establish if these properties depend on the size of the cloud. We propose methods to compare continuous chords of cloud detected from Doppler and Raman lidars with equivalent chords derived from 1D and 3D model output. While the individual chords are highly variable, composites of thousands of observed and millions of simulated chords contain a clear signal. We find that the simulations underestimate cloud size and fraction but successfully reproduce the observed structure of vertical velocity and moisture perturbations. There is a clear scaling of vertical velocity and moisture anomalies below the chords with chord size, but the moisture anomalies are only 1 %–2 % higher than the horizontal mean values. The differences between the observations and simulations are smaller than the difference in sampling the modeled chords in time or space. The shape of the vertical velocity and moisture anomalies from cloud chords sampled spatially from 3D model snapshots is almost perfectly symmetric. In contrast, the chords sampled temporally from the lidar observations and 1D model output have a marked asymmetry, with stronger updrafts and higher moisture anomalies occurring earlier on.
Highlights
Shallow cumulus cloud populations contain a wide range of spatial, temporal, and physical–dynamical variability
While the Doppler lidars cannot measure above the boundary layer or inside the cloud, they have sufficiently high temporal and spatial resolution to resolve the vertical velocity below shallow cumulus clouds
These goals were achieved by comparing cloud chords observed by Doppler and Raman lidars against chords derived from 1D and 3D model output
Summary
Shallow cumulus cloud populations contain a wide range of spatial, temporal, and physical–dynamical variability. Given the large role turbulence and individual thermals play in shallow cumulus development, many samples are needed to detect if a size dependence is present This is where upward-facing lidar observations excel. While the Doppler lidars cannot measure above the boundary layer or inside the cloud, they have sufficiently high temporal and spatial resolution to resolve the vertical velocity below shallow cumulus clouds. Bringing together LES experiments and observations on cumulus statistics, in particular detailed dependencies on size, requires large numbers of both observations and LES experiments This is especially true for continental shallow cumulus clouds that have a marked daily cycle and strong day-to-day variability. After accomplishing the main goals of this paper we use our large data set to quantify the sampling uncertainty of various observed chord properties as a function of lidar deployment days.
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