Ice terminal fall speed and in-cloud vertical air velocity from vertically pointing doppler radar measurements

 

Authors

Christopher R Williams — University of Colorado, Boulder
Alain Protat — Australian Bureau of Meterology

Category

Cloud Properties

Description

Mean vertical profile of the terminal fall speed residual (technique-reference) for the Vt-Z technique (blue), the running-mean technique (red), the Vt -Z-H technique (green), and the DOP-Z-H technique (orange).
Doppler radar measurements at different frequencies (50 MHz and 2835 MHz) are used to characterize the terminal fall speed of hydrometeors and the vertical air motion in tropical ice clouds and evaluate statistical methods for retrieving these two parameters using a single vertically pointing cloud radar. In this work it is found that the natural variability of terminal fall speeds within narrow reflectivity ranges is typically within the acceptable uncertainties for using terminal fall speeds in ice cloud microphysical retrievals. This study also evaluates the performance of previously published statistical methods of separating terminal fall speed and vertical air velocity from vertically pointing Doppler radar measurements using the 50/2835 MHz radar retrievals as a reference. It is found that the variability of the terminal fall speed-radar reflectivity relationship (Vt-Z) is large in ice clouds and cannot be parameterized accurately with a single relationship. While a well-defined linear relationship is found between the two coefficients of a power-law Vt-Z relationship, a more accurate microphysical retrieval is obtained using Doppler velocity measurements to better constrain the Vt-Z relationship for each cloud. A new technique is proposed that incorporates simple averages of Doppler velocity for each (Z, height H) couple in a given cloud. This technique, referred to as DOP-Z-H, was found to outperform the three other methods at most heights, with a mean terminal fall residual less than 10 cm s-1 at all heights. This error magnitude is compatible with the use of such retrieved terminal fall speeds for the retrieval of microphysical properties.