Observations of Transient Linear Organization and Nonlinear Scale Interactions in Lake-Effect Clouds. Part I: Transient Linear Organization

Abstract

Abstract The cold-air outbreak of 13–14 January 1998 during the Lake-Induced Convection Experiment was characterized by large positive buoyancy flux and moderate wind shear. Although theory predicts only cellular organization in these conditions, transient linear organization was observed. Time series of vertical velocity obtained with the Pennsylvania State University 94-GHz vertically pointing cloud radar, which is sensitive to cloud droplets and ice crystals, were used to document the changes in organization that occurred during this wintertime lake-effect event. The cloud radar was deployed on the downwind shore of southern Lake Michigan and measured high-temporal-resolution vertical velocity data at several in-cloud heights. The duration of the event was 18 h, encompassing three cycles of linear organization switching to cellular organization. In Part I of this two-part series the authors document the transient nature of the linearly organized convection and evaluate the role of atmospheric conditions in the mode switching between linear and cellular organization. Within the limits of the available measurements, no correlation was found with mean or low-level shear, surface fluxes, or stability parameters. The mode switching in this case does not appear to be controlled by the atmospheric indicators typically associated with linearly organized convection, suggesting that other factors must have played an important role.