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Radar Data Shed Light on Microphysical Processes in Rain
Published: September 07, 2017
Posted: January 26, 2018

ARM dual frequency radar data can be used to study details of microphysical rain processes that are important for improving rainfall amount predictions.

The Science
Understanding the microphysical and dynamical processes that control the vertical evolution of the distribution of rain droplet sizes is important for properly predicting rainfall amount at the ground from radar observations. It is also critical for understanding and improved predictive modeling of the interactions between rain microphysical properties and the life-times of precipitating cloud systems. In this work, scientists use radar observations from the ARM Southern Great Plains (SGP) site and a novel dual-frequency radar technique to study an observed case in which evaporation represents the dominant rain microphysical processes affecting the evolution of the rain droplet size distribution.

The Impact
The study illustrates that dual-frequency radar retrieved profiles of drop size distribution feature signatures of the evaporation process. Under light rain conditions dominated by evaporation, the shrinking of the diameter of raindrops can be used as a signature of the ambient humidity. The case study presented in this work demonstrates the utility of the dual-frequency radar technique and indicates that it can be used in more general cases to study additional rain microphysical processes such as droplet collision and breakup. Future work combining the radar retrieval with ancillary measurements of relative humidity may enable the ability to disentangle the effect of evaporation from other microphysical processes.

This work documents a rain case dominated by evaporation that occurred at the Atmospheric Radiation Measurement site in Oklahoma on the 15th Sept 2011. A recently developed algorithm, applied to radar Doppler spectra measured at Ka and W band, provides the vertical evolution of binned drop size distributions (DSD) and of the vertical wind. Such retrieved quantities are used in connection with relative humidity (RH) profiles to derive evaporation rates and atmospheric cooling rates. In addition, in regions of stationarity and of light rain — when other microphysical processes are negligible, the presented case study suggests the possibility of retrieving RH profiles from the vertical evolution of the drop size distributions. The key is to characterize the gradient of the rain mass flux between successive levels. Such signal is particularly weak and can be enhanced thanks to a substantial averaging of the retrieved DSD over approximately 5 min and 250 m (eight range gates). The derived profile agrees with the retrieval from coincident Raman lidar observations within a 10% RH difference. These results suggest that other rain microphysical processes could be studied by combining the radar-based DSD retrieval with ancillary RH observations.

Contacts (BER PM)
Sally McFarlane
ARM Program Manager

(PI Contact)
F. Tridon
Earth Observation Sciences
Department of Physics and Astronomy
University of Leicester
Leicester, United-Kingdom

The work done by A. Battaglia and F. Tridon was funded by the project ”Calibration and validation studies over the North Atlantic and UK for the Global Precipitation Mission” which was funded by the UK NERC (NE/L007169/1). This research used the SPECTRE and ALICE High Performance Computing Facilities at the University of Leicester. Data were obtained from the U.S. DOE ARM Climate Research Facility

Tridon, F., A. Battaglia, and D. Watters. “Evaporation in action sensed by multi-wavelength Doppler radars.” J. Geophys. Res. Atmos., 122(17) (2017). [DOI:10.1002/2016JD025998]
(Reference link)

Topic Areas:

  • Facility: DOE ARM User Facility

Division: SC-33.1 Earth and Environmental Sciences Division, BER


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