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Examining Horizontal Variability in Ground-Based Microwave Retrievals ofLiquid Water Path

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  • 1 Atmospheric Science Division, NASA/Langley Research Center, Hampton, Virginia
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Abstract

Estimating horizontal length scales associated with variability in cloud liquid water path (LWP) is important in improving both satellite retrievals of cloud properties and in improving cloud models. A microwave radiometer (MWR) at the Atmospheric Radiation Measurement Southern Great Plains (SGP) site provides measurements of the horizontal variability in cloud LWP. Estimating the length scales in time series is typically done using Fourier analysis, but Fourier methods can be quite badly biased by the presence of intermittency in the data. Cloud LWP can contain very localized structures due to convective plumes, so there is some question as to whether decomposing the measurements into a series of Fourier modes will provide physically meaningful results. To explore this problem, LWP measurements are simultaneously decomposed into a series of Fourier modes along with a series of discrete pulses. The pulses in the fitted basis set tend to pick up intermittency, so that the resulting magnitudes of Fourier modes have been filtered of discrete structures. This method, once demonstrated to be effective on proxy data, is applied to analyze the MWR LWP measurements. The results indicate that applying Fourier analysis to gauge scales of variability in LWP over the SGP site can yield misleading results.

Corresponding author address: Dr. Geoffrey D. Considine, NASA/Langley Research Center, Mail Stop 401-B, Hampton, VA 23681-0001.

Email: considin@asdsun.larc.nasa.gov

Abstract

Estimating horizontal length scales associated with variability in cloud liquid water path (LWP) is important in improving both satellite retrievals of cloud properties and in improving cloud models. A microwave radiometer (MWR) at the Atmospheric Radiation Measurement Southern Great Plains (SGP) site provides measurements of the horizontal variability in cloud LWP. Estimating the length scales in time series is typically done using Fourier analysis, but Fourier methods can be quite badly biased by the presence of intermittency in the data. Cloud LWP can contain very localized structures due to convective plumes, so there is some question as to whether decomposing the measurements into a series of Fourier modes will provide physically meaningful results. To explore this problem, LWP measurements are simultaneously decomposed into a series of Fourier modes along with a series of discrete pulses. The pulses in the fitted basis set tend to pick up intermittency, so that the resulting magnitudes of Fourier modes have been filtered of discrete structures. This method, once demonstrated to be effective on proxy data, is applied to analyze the MWR LWP measurements. The results indicate that applying Fourier analysis to gauge scales of variability in LWP over the SGP site can yield misleading results.

Corresponding author address: Dr. Geoffrey D. Considine, NASA/Langley Research Center, Mail Stop 401-B, Hampton, VA 23681-0001.

Email: considin@asdsun.larc.nasa.gov

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