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Spectral Density of Cloud Liquid Water Content at High Frequencies

H. GerberGerber Scientific Inc., Reston, Virginia

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J. B. JensenCSIRO Atmospheric Research, Aspendale, Victoria, Australia

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A. B. DavisSpace and Remote Sensing Sciences Group, Los Alamos National Laboratory, Los Alamos, New Mexico

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A. MarshakJoint Center for Earth Systems Technology, University of Maryland Baltimore County, Baltimore, Maryland

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W. J. WiscombeClimate and Radiation Branch, NASA Goddard Space Flight Center, Greenbelt, Maryland

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Abstract

Aircraft measurements of liquid water content (LWC) made at sampling frequencies of 1 and 2 kHz with a particle volume monitor (PVM) probe from horizontal traverses in stratocumulus clouds during the Southern Ocean Cloud Experiment and cumulus clouds during the Small Cumulus Microphysics Study are described. The spectral density of the LWC measurements is calculated and compared to the −5/3 scaling law. The effect of PVM sampling noise is found to be small in most cases. Most measurements follow approximately the −5/3 law until cloud scales decrease below about 5 m in length. Below this length LWC variance can exceed that predicted by the −5/3 law. It is suggested that the enhanced LWC variance at small scales is related to entrainment of environmental air into the clouds, which changes primarily the droplet concentration.

Corresponding author address: H. Gerber, Gerber Scientific Inc., 1643 Bentana Way, Reston, VA 20190.

Email: gerber.gsi@erols.com

Abstract

Aircraft measurements of liquid water content (LWC) made at sampling frequencies of 1 and 2 kHz with a particle volume monitor (PVM) probe from horizontal traverses in stratocumulus clouds during the Southern Ocean Cloud Experiment and cumulus clouds during the Small Cumulus Microphysics Study are described. The spectral density of the LWC measurements is calculated and compared to the −5/3 scaling law. The effect of PVM sampling noise is found to be small in most cases. Most measurements follow approximately the −5/3 law until cloud scales decrease below about 5 m in length. Below this length LWC variance can exceed that predicted by the −5/3 law. It is suggested that the enhanced LWC variance at small scales is related to entrainment of environmental air into the clouds, which changes primarily the droplet concentration.

Corresponding author address: H. Gerber, Gerber Scientific Inc., 1643 Bentana Way, Reston, VA 20190.

Email: gerber.gsi@erols.com

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