Continental stratocumulus clouds are frequently observed on the cold side of midlatitude frontal systems. Since they can affect the local surface temperature and energy and water budget, as well as the local climate, their impacts need to be accurately represented in weather and climate models. Large-scale numerical models often have difficulty correctly representing the lifetime and impacts of these cloud types, because the small-scale turbulence structures important to cloud maintenance and cloud properties are smaller than the grid spacing of the models. Department of Energy (DOE) scientists developed a new method to derive a turbulence parameter known as the eddy dissipation rate from DOE’s Atmospheric Radiation Measurement (ARM) program’s millimeter wavelength cloud radar Doppler spectrum measurements. Applying this method to radar data from the ARM Southern Great Plains site, the team examined the details of turbulence structures associated with observed stratocumulus clouds.
The team found that forcing processes that maintained turbulence in the cloud varied throughout its lifetime, driven by both surface heating and cloud-top cooling during the day and cloud-top cooling at night. Small-scale turbulence contributed 40% of the total velocity variance at cloud base, but 70% at cloud top, suggesting that small-scale turbulence plays a critical role near the cloud top where entrainment and cloud-top radiative cooling act. This study illustrates the utility of using the Doppler spectrum width from the millimeter wavelength cloud radar to investigate processes driving the turbulence structure of stratocumulus clouds. Turbulence parameters inferred from these observations can be used to evaluate subgrid parameterizations used in numerical models operating on a variety of scales and can aid in the development of parameterizations of dissipation rates for numerical models. Further, the eddy dissipation rate estimates from the radar spectrum have a strong potential for advancing the understanding of important processes such as cloud-top entrainment and the development of drizzle.
References: Fang, M., B. A. Albrecht, V. P. Ghate, and P. Kollias. 2013. "Turbulence in Continental Stratocumulus, Part I: External Forcings and Turbulence Structures," Boundary-Layer Meteorology 149(454), DOI:10.1007/s10546-013-9873-3. (Reference link)
Fang, M., B. A. Albrecht, V. P. Ghate, and P. Kollias. 2013. "Turbulence in Continental Stratocumulus, Part II: Eddy Dissipation Rates and Large-Eddy Coherent Structures," Boundary-Layer Meteorology 149(454), DOI:10.1007/s10546-013-9872-4. (Reference link)
Contact: Wanda Ferrell, SC-23.1, (301) 903-0043, Sally McFarlane, SC-23.1, (301) 903-0943, Ashley Williamson, SC-23.1, (301) 903-3120
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