Environmental controls on updraft variation with height in the convective boundary layer

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 updrafts recorded with a network of 5 Doppler lidars in Oklahoma, USA on days with clear and cumulus topped boundary layers. Updrafts are identified as coherent regions of positive vertical velocity, and each updraft object is characterized by its geometric and kinematic properties including its width, depth, area, and magnitude.

Updraft composites are constructed for updrafts encountered at different heights in the boundary layer (Z/Z_i), providing an observational approximation for tracking updrafts as they ascend with height. Two groups of composites are examined, first all updrafts at a given height, and second "rooted" updrafts, which are updrafts who's base remains in the lower half of the boundary layer. These composites show that updrafts increase in width and intensity with height, but more so for the rooted updrafts. The increase in width and strength is not linear, which differs from classic plume theory.

Next we examine the environmental factors impacting updraft size, strength, and fraction. Specifically we examine the impact of the mean wind (U), the convective velocity (w_*) and the ratio of the two (w_*/U). These sensitivities show that the updraft speed is stronger for weaker cross winds, but that, surprisingly, there is little sensitivity in the updraft width. Likewise larger w_* yields stronger and somewhat wider updrafts. These two sensitivities are compactly summarized in the w_*/U ratio, which represents the "rise over run" of the updraft in the environmnt. Lastly we examine the number of updrafts per hour and the updraft fraction, finding that w_* is a strong predictor of both.