Macro-Physical Properties of Shallow Cumulus from Integrated ARM Observations

Principal Investigator(s):
Jessica Kleiss, Lewis & Clark College

Evgueni Kassianov, Pacific Northwest National Laboratory
Chuck Long, University of Colorado Boulder
Laura Riihimaki, Pacific Northwest National Laboratory
Erin Riley, Lewis & Clark College

Fair-weather cumulus clouds play an important role in many climate-related processes, yet they are challenging to represent correctly in climate models and difficult to observe holistically. Efforts to improve model parameterizations of fair-weather cumulus clouds utilize the cloud horizontal extent, vertical structure, and liquid water content within a model grid cell; however, corresponding observations are commonly zenith pointing with a very narrow field of view (FOV). Therefore, the cloud properties provided by the measurements may not be representative of a larger surrounding area. As a result, difficulties can ensue when model outputs (for a given area) and “narrow-FOV” products (for a given point) are compared.

This project proposes to develop a new “wide-FOV” cloud product required for the comprehensive evaluation of the large eddy simulations (LES) at the ARM SGP ‘megasite’ and will focus on summertime periods where conditions for cumulus growth and evolution are favorable.  The product development will combine advantages of the “wide-FOV” passive sky imaging from the Total Sky Imager and “narrow-FOV” Active Remotely-Sensed Cloud Locations (ARSCL). Conditions with spatially uniform shallow cumulus cloud coverage will be determined by a synthesis of “wide-FOV” passive sky imaging with “narrow-FOV” active remote sensing.  Cloud aspect ratio (CAR; vertical over horizontal cloud size) will be derived from ground-based hemispherical sky images due to the difference in cloud cover over moderate (100 degree) and wide (160 degree) fields of view. Cloud-chord length (CCL) will be derived from “narrow-FOV” active remote sensing and wind speed information at the cloud base height. Cloud base height (CBH) will be derived from “narrow-FOV” active remote sensing and paired ground-based TSI hemispherical observations.

From the above products, additional “wide-FOV” cloud products can be computed. For example, geometrical thickness (cloud depth, CD) as CD = CAR x CCL and cloud-top height (CTH) as CTH = CBH + CD. The classification of shallow cumulus conditions and retrievals of “wide-FOV” macrophysical properties will be evaluated using an existing multi-year climatology of cumulus clouds. Errors and uncertainties will be determined by consideration of instrumental resolution and methodological assumptions. The product will then be retrieved for periods of interest to the Large-Eddy Simulation (LES) ARM Symbiotic Simulation and Observation (LASSO), and also for the Holistic Interactions of Shallow Clouds, Aerosols, and Land-Ecosystems (HI-SCALE) times of interest.

This product will be a valuable contribution to the so-called “data cube” concept of model-observation synergism. The expected new cloud product will provide an opportunity to link changes in these macrophysical cloud properties with changes in the atmospheric and land surface characteristics. This important link will be helpful in characterizing complex atmosphere-cloud-surface interactions and improving cumulus cloud parameterizations.