Radiative dynamic interactions in clouds

 

Support by NASA


With graduate student Clint Schmidt, we are using numerical models to explore how the deposition of terrestrial radiation within clouds drives cloud motions. We are showing that predicting how a cloud will evolve can be distilled to some surprisingly simple ingredients:


  1. Atmospheric static stability

  2. Cloud condensate density

  3. Cloud width

  4. Cloud temperature

  5. Below cloud humidity


Simple non-dimensional numbers can be made representing the relative likelihood of cloud spreading, isentropic lifting and mixed layer generation.


One prediction that was made was that mammatus clouds should be expected to form in the model in cirrus anvils that were wide, dense, and with low below-cloud  atmospheric humidity. The below is a simulation for this case, passed through a 3D-radiative transfer solver (SHDOM)  (Thanks to Céline Cornet of LOA). The perspective is looking up at cloud base, at one quarter of an initially cylindrical cloud. The weird black trangle at first is just an artifact of the 3D radiative transfer simulations when dealing with an initially flat surface. Turbulence develops at cloud base in response to radiative heating from below. The growing turbulent mixed layer simultaneously adjusts to its clear-sky surroundings by spreading outwards in density currents. Mammatus behavior was only observed in the simulations when the lower atmosphere was relatively dry. This is because a dry atmosphere is more radiatively transparent, which allows for a greater radiative temperature contrast between the lower atmosphere and cloud base. It is this temperature contrast that drives flows of energy into the cloud and creates the turbulence.


The movie below can take a while to load in a browser. If it doesn’t do anything remarkable it can be downloaded here.





























Nicolas Ferlay; T. J. Garrett; Fanny Minvielle, 2012: Upside-down mammatus at the top of a large frontal system. J. Atmos. Sci., (submitted).


Schmidt, C. T.* and T. J. Garrett, 2012: A simple framework for the dynamic response of cirrus clouds to local diabatic radiative heating (submitted to J. Atmos. Sci.), 2012


Garrett, T. J.  C. T. Schmidt*, S. Kihlgren* and Céline Cornet, 2010: Mammatus clouds as a response to cloud base radiative heating J. Atmos. Sci. 67, 3891–3903