Cloud droplet spectral relative dispersion is critical to parameterizations of cloud radiative properties, warm-rain initiation, and aerosol-cloud interactions in models; however, there is no consistent relationship between relative dispersion and volume-mean radius in literature, which hinders improving relative dispersion parameterization and calls for physical explanation. Here we show, by analyzing aircraft observations of cumulus clouds during Routine AAF [Atmospheric Radiation Measurement (ARM) Aerial Facility] Clouds with Low Optical Water Depths (CLOWD) Optical Radiative Observations, that the correlation between relative dispersion and volume-mean radius changes from positive to negative as volume-mean radius increases. With the new observation, we postulate that the sign of the correlation is determined by whether or not condensation (evaporation) occurs simultaneously with significant new activation (deactivation). The hypothesis is validated by simulations of both an adiabatic cloud parcel model and a parcel model accounting for entrainment-mixing. A new quantity, first bin strength, is introduced to quantify this new observation. Theoretical analysis of truncated gamma and modified gamma size distributions further supports the hypothesis and reconciles the contrasting relationships between relative dispersion and volume-mean radius, including the results in polluted fog observations. The results could shed new light on the so-called “twilight zone” between cloudy and cloud-free air, which in turn affects evaluation of aerosol-cloud interactions and retrieval of aerosol optical depth.
Bibliographical noteFunding Information:
We appreciate the helpful discussion about the instruments with Drs. Darrel Baumgardner and Fan Mei. This research is supported by the National Key Research and Development Program of China (2017YFA0604001), the Natural Science Foundation of Jiangsu Province (BK20160041), the National Natural Science Foundation of China (41822504, 41975181, and 91537108), the Qinglan Project (R2018Q05), and the Six Talent Peak Project in Jiangsu (2015‐JY‐011). Liu is supported by the U.S. Department of Energy Office of Science Biological and Environmental Research as part of the Atmospheric Systems Research (ASR) Program and Solar Energy and Technology Office (SETO). Brookhaven National Laboratory is operated by Battelle for the U.S. Department of Energy under Contract DE‐SC00112704. Yum is supported by the National Research Foundation of Korea (NRF) grant funded by the Korea Government (MSIT) (NRF‐2018R1A2B2006965). Chen is supported by the U.S. Department of Energy Office of Science Biological and Environmental Research as part of the Atmospheric Systems Research (ASR) Program. Pacific Northwest National Laboratory is operated by Battelle for the U.S. Department of Energy under Contract DE‐AC05‐76RLO1830. The RACORO data are available online ( https://www.arm.gov/research/campaigns/aaf2009racoro ). The fog data and cloud numerical simulations are available online ( https://figshare.com/articles/data_submitted_2019JD031868_zip/12085503 ).
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All Science Journal Classification (ASJC) codes
- Atmospheric Science
- Earth and Planetary Sciences (miscellaneous)
- Space and Planetary Science