ARM Radar Measurements Reveal Secrets of Precipitation Droplets

Summary

Precipitation plays a crucial role in the availability of water for people, agriculture, and ecosystems. Quantitative predictions of precipitation amount, frequency, and location still remain one of the ground challenges in the hydrological and atmospheric sciences due to the complex interactions between large-scale atmospheric circulations, local-scale cloud circulations, and cloud microphysical processes that affect the properties of precipitation. Detailed observations of the temporal and spatial variability of rain drop size distributions, in concert with other atmospheric and environmental measurements, can provide important information about what causes changes in precipitation properties between different storms—an important step toward a physically consistent description of precipitation physics that can be included in numerical models.

Scientists using two state-of-the-art radar systems at the Atmospheric Radiation Measurement (ARM) Southern Great Plains (SGP) site have developed a new method for simultaneously obtaining the details of the rain drop size distribution (DSD) and parameters of the local air state related to turbulence and wind shear. The method uses radar observations at two different wavelengths and takes advantage of specific features observed in radar Doppler spectra that are caused by the wavelength dependence of scattering and absorption properties. A fundamental advantage of the new method is that DSD properties are retrieved via the differential attenuation technique, which looks at the differences in signal attenuation between the two wavelengths and is not affected by radar calibration or by standing water on the radome or antenna.

Using the current two radar wavelengths, the approach is applicable to rain rates between roughly 1 and 30 mm/hr. The methodology can be extended to other radar wavelength combinations, which could lead to a seamless retrieval of precipitation properties from light drizzle to heavy rainfall. The proposed methodology shows great potential in linking microphysics to dynamics in rainfall studies, and can be used to study rain microphysical processes such as coalescence and rain drop breakup, ultimately leading to improved parameterizations of rain processes in cloud models.

References

Tridon, F., and A. Battaglia. 2015. “Dual-Frequency Radar Doppler Spectral Retrieval of Rain Drop Size Distributions and Entangled Dynamics Variables,” Journal of Geophysical Research Atmospheres 120(11), 5585-601. DOI: 10.1002/2014JD023023.