Composited structure of non‐precipitating shallow cumulus clouds

• Published in: Quarterly journal of the Royal Meteorological Society Vol. 147; no. 738; pp. 2818 - 2833
• Main Authors: Gu, Jian‐Feng, Plant, Robert S., Holloway, Christopher E., Jones, Todd R.
• Format: Journal Article
• Language: English
• Published: Chichester, UK John Wiley & Sons, Ltd 01.07.2021; Wiley Subscription Services, Inc; Royal Meteorological Society
• Subjects: Clouds; Cumulus clouds; Downdraft; ENVIRONMENTAL SCIENCES; Inversion; Kinetic energy; Moisture; normalized cloud structure; Perturbations; Potential temperature; power-law distribution; Relative humidity; shallow cumulus clouds; Transition zone; Turbulent kinetic energy; Turbulent mixing; Updraft; Velocity; vertical flux representation; Vertical velocities; Vertical wind shear; Water vapor; Water vapour; Wind shear
• ISSN: 0035-9009; 1477-870X
• DOI: 10.1002/qj.4101

The normalized distributions of thermodynamic and dynamical variables both within and outside shallow clouds are investigated through a composite algorithm using large‐eddy simulations of oceanic and continental cases. The normalized magnitude is maximum near the cloud centre and decreases outwards. While relative humidity (RH) and cloud liquid water (ql) decrease smoothly to match the environment, the vertical velocity, virtual potential temperature (θv), and potential temperature (θ) perturbations have more complicated behaviour towards the cloud boundary. Below the inversion layer, θv′ becomes negative before the vertical velocity has turned from an updraft to a subsiding shell outside the cloud, indicating the presence of a transition zone where the updraft is negatively buoyant. Due to the downdraft outside the cloud and enhanced horizontal turbulent mixing across the edge, the normalized turbulent kinetic energy (TKE) and horizontal turbulent kinetic energy (HTKE) decrease more slowly from the cloud centre outwards than the thermodynamic variables. The distributions all present asymmetric structures in response to the vertical wind shear, with more negatively buoyant air, stronger downdrafts, and larger TKE on the downshear side. We discuss several implications of the distributions for theoretical models and parameterizations. Positive buoyancy near the cloud base is mostly due to the virtual effect of water vapour, emphasizing the role of moisture in triggering. The mean vertical velocity is found to be approximately half the maximum vertical velocity within each cloud, providing a constraint to achieve possible power‐law distributions for some models. Finally, the normalized distributions for different variables are used to estimate the vertical heat and moisture fluxes within clouds. The results suggest that distributions near the cloud edge and variability of maximum perturbations need careful treatment. The fluxes are underestimated in the inversion layer because cloud‐top downdrafts cannot be captured well. The distributions of thermodynamic and dynamical variables within the cloud and across the cloud boundaries for non‐precipitating shallow cumulus clouds over ocean and continent are demonstrated through compositing algorithms. The general features of the distributions are similar, but some variables have more complicated distributions towards the cloud edges. Our results suggest that distributions near the cloud edge and variability of maximum perturbations need careful treatment, if a convection scheme uses assumed distributions for the parameterization of vertical fluxes.