Studying Details of Turbulent Eddies in the Atmosphere
Using ARM radar and lidar observations, scientists gain insight into atmospheric turbulence.
Turbulence is one of the most important physical processes in the atmospheric boundary layer, responsible for vertical transport of water vapor, momentum, mass, and pollutants, which affect boundary layer evolution and cloud formation. Turbulent motions also drive entrainment mixing in clouds and influence cloud microphysics and precipitation formation.
The turbulent eddy dissipation rate, or rate at which energy cascades from large to small eddies, is an important parameter in the numerical modeling of clouds. Since climate models cannot resolve all turbulent eddies, they must parameterize their effects, and the eddy dissipation rate is used in the parameterization of the average eddy size or mixing length in the turbulence equations. An improved understanding of in-cloud turbulence can lead to a better understanding of cloud lifecycle and cloud microphysical processes and also to improved representation of clouds in numerical weather prediction and global climate models.
Coincident profiling observations from Doppler lidars and radars operated by the Department of Energy’s Atmospheric Radiation Measurement (ARM) Climate Research Facility are used to estimate the turbulence energy dissipation rate (ε) using three different data sources: (1) Doppler radar velocity (DRV), (2) Doppler lidar velocity (DLV), and (3) Doppler radar spectrum width (DRW) measurements. The agreement between the derived ε estimates is examined at the cloud base height of stratiform warm clouds. Collocated ε estimates based on power spectra analysis of DRV and DLV measurements show good agreement during both drizzling and non-drizzling conditions. This finding suggests that unified (below and above cloud base) time-height estimates of ε in cloud-topped boundary layer conditions can be produced, regardless of the size of the particles present in the radar volume. This finding also suggests that eddy dissipation rate can be estimated throughout the cloud layer without the constraint that clouds need to be nonprecipitating. An important implication of the conditional agreement of the two techniques at the cloud base height is that the influence of the droplet size distribution on the width of the Doppler radar spectrum can be derived. This derived term can be used to constrain retrievals of drizzle properties and provides a means to validate microphysics parameterizations in numerical models.
This research was supported in part under Contract DE-SC00112704 by the Atmospheric System Research program, Office of Biological and Environmental Research, U.S. Department of Energy. Data used were obtained from the ARM data archive at http://www.archive.arm.gov.
Borque, P., E. Luke, and P. Kollias. 2016. “On the Unified Estimation of Turbulence Eddy Dissipation Rate Using Doppler Cloud Radars and Lidars,” Journal of Geophysical Research Atmospheres 121(10), 5972-89. DOI : 10.1002/2015JD024543.