Where is the updraft? The influence of heterogeneous surface heating on organized vertical motions within and above a sheared, unstable atmospheric boundary layer
Pennsylvania State University
Abstract
Large-eddy simulation (LES) runs are performed to understand the influence of a one-dimensional (1D) surface heating heterogeneity on organized vertical motions within and above the atmospheric boundary layer (ABL). Two knowledge gaps are of interest: i) how updrafts develop in the low free troposphere, and ii) what parameters control updraft location and strength within the ABL? LES runs are performed for a sheared, unstable ABL driven by geostrophic winds of the same magnitude but in various directions relative to a 1D surface-heat-flux heterogeneity. Quasi-steady-state LES results are phase-averaged over time and the horizontal dimension perpendicular to the surface-heat-flux gradient to quantify secondary circulations. Regarding the first knowledge gap, results show that organized vertical motions in the low free troposphere can be modeled as two-dimensional (2D), stationary gravity waves, whose amplitudes depend on ABL updraft strength and instability development within the free troposphere. For the second gap, results show that organized updrafts within the ABL may form above warm surfaces or downwind of warm-to-cool transitions. These different locations are well explained by both the relative contributions of horizontal and vertical velocities to the phase-averaged vorticity fluctuations tied to secondary circulations, and the relative importance of horizontal advection and turbulent transport in the phase-averaged internal energy fluctuation equation. The main balances associated with each updraft location are used to propose empirical models of updraft strength, and it is shown that the presence of sufficiently strong organized vertical motions can potentially change parameters used by atmospheric models that do not resolve ABL turbulence.
About the speaker
Mr. Garett R. T. Warner is a Ph.D. candidate in Meteorology and Atmospheric Science at The Pennsylvania State University, where his research focuses on atmospheric turbulence, boundary-layer processes, and shallow convection. Using LES, his work examines how heterogeneous surface heating through cloud–surface–radiation feedbacks affects shallow convective currents such as horizontal convective rolls. His recent studies have advanced understanding of the dimensionless parameters governing updraft location and strength in the presence of heterogeneous surface heating, with results published in the Journal of the Atmospheric Sciences. Current research focuses on quantifying the spatial characteristics of horizontal convective rolls and assessing how nonsteady surface heating influences the spacing and alignment of their updrafts. Garett has presented his research at national conferences, including meetings of the American Meteorological Society and the American Geophysical Union.
