ARM Shortwave Spectrometers to Study the Clear-Cloud Transition Zone and Mixing Processes

Principal Investigator(s):
Alexander Marshak, NASA/Goddard Space Flight Center

Alexander Khain, The Hebrew University of Jerusalem, Israel
Weidong Yang, USRA


The transition zone between cloudy and clear air is a region of strong aerosol-cloud interactions where aerosol particles humidify and swell when approaching the cloud, while cloud drops evaporate and shrink when moving away from the cloud. This zone is much larger than was previously thought and thus it complicates estimates of the aerosol indirect effect and of aerosol radiative forcing — excluding aerosols near clouds will dramatically reduce the database and underestimate the forcing, while including them may overestimate the forcing because of unaccounted cloud contamination.

Few ARM instruments measure rapidly enough to resolve the transition zone.  One-sec-resolution data from the ARM shortwave spectrometers provide a unique opportunity to study the transition zone.  The main goal for this proposal is to interpret spectral radiative measurements in terms of aerosol and cloud properties in the transition zone.  In addition, using spectral radiative measurements we propose to study the factors controlling the entrainment and homogeneous vs. inhomogeneous mixing process.

Applying the spectral methods developed by our group to the shortwave spectrometer measurements, we propose to:

  • study the causes of particle changes in the transition zone;
  • distinguish between aerosol particles and weak cloud elements; and
  • test the hypothesis of cloud inhomogeneous mixing in a new way.

Our ultimate goal is to improve the estimates of aerosol radiative forcing and understanding of mixing processes in the transition zone as a function of cloud and aerosol microphysical properties.