Vertical Air Motion Profiles Available Through New Value-Added Product

Published: 11 March 2013

In the top figure, evidence of stratiform precipitation is observed in the WACR reflectivity profiles. Vertical air motion is retrieved from the WACR velocity spectra for hours 900 to 1200 as depicted on the lower left. An overview of the VAP algorithm is displayed on the lower right. Click to enlarge.
The Vertical Air Motion during Large-Scale Stratiform Rain (VERVELSR) value-added product (VAP) uses the unique properties of a 95-GHz radar Doppler velocity spectra to produce vertical profiles of air motion during low-to-moderate (1–20 mm/hr) rainfall events. It is designed to run at ARM sites that include a W-band ARM cloud radar (WACR) radar with spectra data processing.

The VERVELSR VAP operates by exploiting a resonance effect that occurs in non-Rayleigh (Mie) backscattering, under conditions of moderate stratiform rain. Air motion is retrieved through examination of the velocity spectra, such that falling drops are represented in the spectra according to their fall velocities due to size sorting by air density. The air velocity is measured as the velocity of very small droplets known to follow the air motion. The first minimum in the principal peak is where the resonance effect takes place and is mapped to represent droplets in the 1.65 mm size range. These droplets are known to have a terminal fall velocity of about 5.8 ms-1, depending on the air density, and serve as a relative reference point.

In VERVELSR, air density is calculated using data from the MERGESONDE VAP. The difference between the known terminal velocities (adjusted for air density) and the observed velocity in the Doppler spectra yields an estimate of the mean vertical air velocity. The retrieval is applied only to periods of liquid phase precipitation. In addition, in an effort to provide a robust algorithm, certain criteria have been applied to mitigate potential contaminants related to ice/melting layer, turbulence, or heavy rain.

VERVELSR is currently in evaluation mode, and any comments on the data set are welcome. Evaluation data are available for a few days from the Southern Great Plains (SGP) site and the ARM Mobile Facility Niamey (NIM) deployment. The VAP will run autonomously when the evaluation is completed. More information is available at the VAP web page and in the README file that accompanies the data files in the Data Archive’s evaluation area.

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