Abstract
AbstractThis study investigates the ability of a W‐band radar to measure turbulence structure as a function of height throughout the drizzling marine boundary layer from the fine‐structure of the Doppler velocity field obtained during Dynamics and Chemistry of Marine Stratocumulus Experiment using the NCAR C‐130 aircraft. In situ air velocity measurements are used as a basis for the comparison with radar measurements of turbulence energy and dissipation, and integral length‐scale, after correcting for the effects of velocity averaging within the pulse resolution volume and random uncorrelated noise. One essential step for this study is to estimate the contribution to the fluctuations in Doppler velocity due to the terminal velocity of hydrometeors. To do this, we use microphysics probe measurements of the spatial distribution of the drop counts in each size bin. We find a small velocity standard deviation (0.05 to 0.1 m s−1) contributed by the radar‐measured drizzle fall velocity and a substantial negative correlation between air vertical velocity and this fall velocity in the upper part of the cloud. This correlation has an impact on the turbulent energy and dissipation deduced from the Doppler velocity. However, it does not significantly affect the integral scales, which are in good agreement with the in situ measurements. Thus, the radar enables us to obtain the profile of this key variable through the entire cloud‐capped boundary layer. We obtain estimates of the dissipation in the lower 2/3 of the boundary layer that are in excellent agreement with the in situ measurements and consistent with the predominant production terms in the turbulent kinetic energy budget. Copyright © 2005 Royal Meteorological Society.
Highlights
Turbulence is the means by which properties in the marine boundary layer become vertically well-mixed
(1968) was the first to give a detailed account of the structure of the marine stratocumulus-topped boundary layer (STBL) and the role that turbulence plays in determining its structure and persistence
In order to utilize Doppler velocity measurements to study the characteristics of turbulence in stratocumulus, we use the spatial distribution of drops measured by PMS probes to estimate the effect of the drop fall velocity on the Doppler velocity variance
Summary
Turbulence is the means by which properties in the marine boundary layer become vertically well-mixed. (1968) was the first to give a detailed account of the structure of the marine stratocumulus-topped boundary layer (STBL) and the role that turbulence plays in determining its structure and persistence. The STBL is driven mainly by cloud-top radiative cooling that generates turbulent mixing within the underlying layer. There have been many subsequent studies that describe the main processes in the STBL, the role of turbulence, and point out the climatic importance of this regime When the boundary layer is filled with scatterers, such as cloud and drizzle, Doppler radar measurements can provide more complete two or three-dimensional observations. We evaluate these factors and demonstrate the use of an airborne Doppler radar for measuring turbulence variables
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