Turbulent Processes that Influence Boundary-Layer Cloud Structure

The albedo of boundary-layer clouds depends on more than mean liquid water path. It also depends critically on turbulent cloud processes that influence both the horizontal variability of clouds and the cloud properties at two different altitudes.



Efforts to improve understanding and parameterizations of boundary-layer clouds are hampered in part by difficulties in making close comparisons between the parameterizations and observations. In particular, the potential of upward-staring cloud radars maintained at Atmospheric Radiation Measurement (ARM) sites has been hampered by four impediments. First, the spatial scales of the observations and the models are different: upward-staring radars produce point profiles, whereas historically, parameterizations have produced averages over a horizontally extended grid columns. Second, radar retrievals of cloud properties are uncertain. Third, it is difficult to isolate parameterization errors in clouds from those in microphysics. Fourth, the horizontal advection of water vapor and hydrometeors is significant but is difficult to observe and prescribe in models. All these impediments have led parameterization developers to disregard sizable discrepancies between simulations and observations.



In this project, we will make our cloud parameterization’s output more directly comparable with upward-staring radar observations. To avoid retrieval uncertainties, we will use forward radar models. To indirectly assess uncertainties in microphysics and forcing, we will supplement ARM observations with large-eddy simulations.



Our objectives are to 1) improve understanding of how boundary-layer cloud structure is influenced by underlying turbulent cloud processes; 2) enable closer comparison between upward-staring ARM observations and cloud parameterizations; and 3) deliver an improved cloud parameterization.