Using cloud resolving model simulations of tropical deep convection to study turbulence in anvil cirrus

University essay from Uppsala universitet/Luft-, vatten- och landskapslära

Abstract: Identifying the dynamical processes that are active in tropical cirrus clouds is important for understanding the role of cirrus in the tropical atmosphere. This study focuses on analyzing turbulent motions inside tropical anvil cirrus with the use of a Cloud Resolving Model. Convection in the transition from shallow to deep convection has been simulated with Colorado State University Large Eddy Simulator/Cloud Resolving Model System for Atmospheric Model (SAM 6.3) in a high resolution three-dimensional simulation and anvil cirrus formed in the end of this simulation has been analyzed. For model set up, data gathered during the Tropical Rainfall Measuring Mission Large-Scale Biosphere-Atmosphere (TRMM LBA) field experiment in Amazonas, Brazil have been used as large scale forcing. 31 anvil clouds have been localized from a single time step of the simulation, “a snapshot”, of the entire simulated cloud field consisting of convective clouds of different scales and subsequently divided into three categories that represent different stages of the anvil lifetime; growing, mature and dissipating anvil stages. The classification is based on in-cloud properties such as cloud condensate content and vertical velocities. The simulated anvils have been analyzed both individually and as groups to examine the transition from isotropic three-dimensional turbulence in the convective core of the thunderstorm to stratified two-dimensional turbulence in the anvil outflow. A dimensionless number F is derived and used as a measure of the “isotropic” behavior of the turbulence inside the cloud. F is expressed as the ratio between the horizontal part of TKE and the total (horizontal + vertical) Experiments show that SAM 6.3 clearly can resolve turbulent structures and that the transition from isotropic three-dimensional turbulence to stratified two-dimensional turbulence occurs in the middle layers of the mature and dissipating anvil stages.

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