Turbulence measurements in marine stratocumulus with airborne Doppler radar
Résumé
This 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) 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.
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