Regime Dependence of Cloud Water Variability

Using ARM data from multiple observational sites around the globe, scientists gain insights into cloud variability.

The Science

Physical processes, such as the interaction of clouds with sunlight or the rate at which clouds produce rain, depend on the amount of cloud water available locally at scales smaller than can be represented by current global model grid resolutions.

The Impact

Using observations from U.S. Department of Energy Atmospheric Radiation Measurement (ARM) program sites across the world, scientists have improved understanding of how the amount of water in clouds varies at small scales. Based on these observations, a new way of describing how cloud water varies with cloud regime has been developed for use in global weather and climate models. The new parameterization will enable models to calculate more accurate process rates by providing a better representation of how the amount of cloud water varies within the cloud at these subgrid scales.


A number of different retrieval products for cloud condensate from ARM observations are assessed for five different geographical regions for multiple years and seasons. The retrieval reliability varies with cloud type, but for cloud categories largely unaffected by precipitation, a comparison across sites and longer time periods is possible. These observations confirm previously documented variability behavior as a function of cloud fraction, but also reveal a systematic regime dependence that is not captured by existing parameterizations. Condensate variability measured as a fractional standard deviation (FSD) in warm boundary-layer clouds is greater in the tropics than in the mid- and high-latitudes for scenes with comparable cloud type and fraction, with the observed FSD varying from 1.2 in the tropics to 0.4 in the Arctic. A parameterization of the cloud liquid condensate FSD based on the grid box mean total water amount and cloud fraction is formulated and shown to better capture the observed range of FSD values across the different geographical sites and seasons.

Principal Investigator(s)

Maike Ahlgrimm
European Centre for Medium-Range Weather Forecasts


This work was supported by the U.S. Department of Energy’s Office of Science, Office of Biological and Environmental Research through the Atmospheric System Research (ASR) activity. Grant Number: DE-SC0005259. Data was obtained from the Atmospheric Radiation Measurement (ARM) Climate Research Facility.


Ahlgrimm, M., and R. M. Forbes. 2016. “Regime Dependence of Cloud Condensate Variability Observed at the Atmospheric Radiation Measurement Sites,” Quarterly Journal of the Royal Meteorological Society 142(697), 1605-17. DOI: 10.1002/qj.2783.