17 Jan 2022
17 Jan 2022
Status: a revised version of this preprint is currently under review for the journal ACP.

Quantifying vertical wind shear effects in shallow cumulus clouds over Amazonia

Micael Amore Cecchini1,5, Marco de Bruine2, Jordi Vilà-Guerau de Arellano3,4, and Paulo Artaxo1 Micael Amore Cecchini et al.
  • 1Institute of Physics, University of São Paulo, São Paulo, Brazil
  • 2Institute for Marine and Atmospheric Research, Utrecht University, Utrecht, the Netherlands
  • 3Meteorology and Air Quality Section, Wageningen University, Wageningen, Netherlands
  • 4Atmospheric Chemistry Department, Max Planck Institute for Chemistry, Mainz, Germany
  • 5Deparment of Atmospheric Science, Colorado State University, Fort Collins, United States of America

Abstract. This study analyses and quantifies the effects of vertical wind shear (VWS) on the properties of shallow cumulus cloud fields over Central Amazonia. We perform idealized simulations with high resolution (50 m horizontally, 20 m vertically) using the Dutch Atmospheric Large Eddy Simulation (DALES) model, changing the initial conditions and large scale forcing of VWS. The resulting cloud field is analysed with by applying a cloud tracking algorithm to generate Lagrangian datasets of the lifecycle of individual clouds as well as their time-varying core and margins dimensions. The reference run has no wind speed or directional shear and represents a typical day in the local dry season. Numerical experiments with moderate and high wind speed shear are simulated by adding linear increases in the wind speed of 1.2 m s−1 km−1 and 2.4 m s−1 km−1, respectively. Three additional runs are made by adding 90° wind rotation between the surface and the top of the domain (5 km) on top of the three wind speed shear conditions. We find that clouds developing in a sheared environment have horizontal equivalent diameter increased by up to 100 m on average, but the cloud depth is reduced. Our quantification shows that VWS tends to increase the size of the cloud cores, but reduce its relative area, volume, and mass fractions compared to the overall cloud dimensions. The addition of 2.4 m s−1 km−1 of VWS decreases the relative core area by about 0.03 (about 10 % of the overall average) and its volume and mass ratios by about 0.05 (10 %–25 % in relative terms). Relevant for the cloud transport properties is that the updraught speed (w) and the liquid water content (LWC) are lower within the cores, and consequently the upward mass flux. All quantifications of cloud properties point to the inhibition of convective strength by VWS, therefore hampering the shallow-to-deep transition. However, open questions still remain given that a the individually deepest clouds were simulated under high environmental shear, even though they occur in small numbers. This could indicate other indirect effects of VWS that have opposite effects on cloud development if found to be significant in the future.

Micael Amore Cecchini et al.

Status: final response (author comments only)

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse
  • RC1: 'Comment on acp-2021-1060', Anonymous Referee #1, 11 Feb 2022
  • RC2: 'Comment on acp-2021-1060', Anonymous Referee #2, 18 Feb 2022
  • AC1: 'Response to Referee Comments to acp-2021-1060', Micael Amore Cecchini, 18 May 2022

Micael Amore Cecchini et al.

Micael Amore Cecchini et al.


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Short summary
Shallow clouds (vertical extent up to 3 km height) are ubiquitous throughout the Amazon and are responsible for redistributing the solar heat and moisture vertically and horizontally. They are a key component of the water cycle because they can grow past the shallow phase to contribute significantly to the precipitation formation. However, they need favorable environmental conditions to grow. In this study, we analyze how changing wind patterns affect the development of such shallow clouds.