P4.3
Numerical simulation of the interaction between the dryline and horizontal convective rolls
Steven E. Peckham, University of Illinois, Urbana, IL; and R. B. Wilhelmson, L. J. Wicker, and C. L. Ziegler
Over the past decade, fine-scale observational studies have revealed that atmospheric boundaries (e.g., sea breezes, convergence zones, drylines) can possess a large amount of along-line variation. These studies have focused on the relationship between these along-line variations to the presence of horizontal convective rolls (HCRs) within the boundary layer. The results suggest that, besides producing along-line undulations, enhanced ascent occur where HCR updrafts intersect with the boundaries. Further, convective clouds form in the region of enhanced ascent.
With the increase in computational resources over the past decade, modelers are beginning to investigate the interaction between convective rolls and atmospheric boundaries. Recent studies have demonstrated that high-resolution (500 m) simulations of the sea breeze can reproduce many of the observed phenomena (e.g., sea-breeze boundary, HCRs). Further, these simulations demonstrate how the interaction between HCRs and boundaries plays an important role in the formation and evolution of convective clouds along the sea breeze. Studies of drylines interacting with convective rolls have recently been reported. However, the horizontal resolutions in these model simulations (e.g., 1 km and 2 km respectively) was adequate to resolve the HCRs. Therefore, there is a need to conduct high-resolution simulations of the dryline environment in order to investigate the HCR - dryline interactions and subsequent convective cloud formation.
Expanding upon previous study results, this investigation is examining the formation of HCRs within the dryline environment using significantly higher horizontal resolution (500 m). One goal of this study is to understand the impact of the wind profile on the orientation and evolution of HCRs near the dryline and their subsequent interaction. It is hypothesized that the along-line variability is predominately a result of the HCR/dryline interaction where the HCRs form west of the dryline.
A series of three 12 hour simulations, starting at 1200 UTC, are being performed in which the initial zonal wind profile is systematically varied. An initial east-west thermodynamic profile is derived from observational data obtained during the COPS-91 field program and expanded to three-dimensions using geostrophic constraints. Three grids (two nested) are employed in the simulations with an inner grid horizontal resolution of 500 m.
Preliminary simulation results indicate that HCRs develop within the convective boundary layer across the entire domain. The rolls are oriented across (along) the north-south oriented dryline boundary in the western (eastern) boundary layer with the western HCR circulations being the most intense. The interaction of the western HCRs with the dryline appear responsible for creating a considerable amount of along-line variation. Further, the amount of along-line variation appears to be dependent upon the initial zonal wind profile.
Two mechanisms are identified that might explain the formation of the along-line undulations. The first is the interaction between the HCR circulations and the relatively north to south oriented dryline. Along the descending branch of the HCRs, the strong westerly flow transports the dryline boundary farther eastward. Conversely, along the ascending branch of the HCRs the dryline is transported westward by the stronger easterly winds within the eastern CBL. The along-line differences in wind speed combine to create the east-west undulations. The second is the convergence of moisture under the ascending branches of the HCRs. The alternating regions of increased (decreased) moisture within the western CBL produces the westward (eastward) shift in the dryline.
Poster Session 4, Convective
Wednesday, 9 August 2000, 6:00 PM-9:00 PM
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