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THE OBSERVED VARIATION OF COHERENT UPDRAFT OBJECTS WITH HEIGHT IN THE CONVECTIVE BOUNDARY LAYER
AuthorKeene, Courtney Ann
AdvisorLareau, Neil P
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Buoyant plumes and thermals rising through the convective boundary layer (CBL) modulate the flux of heat, moisture, momentum, and aerosols from the surface to aloft. These fluxes also determine the lower boundary conditions for cumulus development (e.g., shallow vs. deep cumuli) provided that thermals and plumes rise to their condensation level. In coarse resolution climate and weather models these plume fluxes can be represented using “eddy-diffusivity mass-flux” (EDMF) parameterizations, wherein the “mass flux” is accomplished by a spectrum of buoyant plumes. In this presentation, we analyze the properties of ~120,000 coherent CBL updrafts recorded with a network of five Doppler lidars in Oklahoma, USA on days with clear and cumulus topped boundary layers. We present the composite evolution of these updraft objects as a function of height, which reveal a mixture of plume-like and thermal-like objects. The results indicate the expected quasi-linear broadening of updrafts as they ascend and the increasing skewness of the updraft strength with height. Interestingly, updrafts that are vertically continuous (i.e., plume-like) through the CBL tend to be both stronger and wider than their isolated counterparts. To this end, these data also show a clear strength-to-width relationship in updrafts: wider updrafts are stronger than narrow updrafts. We also examine how the structure and evolution of updrafts varies with forcings in the CBL. Updrafts occurring on days with both a large buoyancy forcing (w*) and low cross wind speeds (U) lead to the strongest updrafts, a relationship that can be succinctly summarized using the ratio of w*/U. These results will help inform refinements to subgrid scale parameterizations for the boundary layer and cloud layer processes in coarse resolution numerical models.