Numerical simulation of the interaction between the dryline and horizontal convective rolls

Steven E. Peckham,* Robert B. Wilhelmson, Louis J. Wicker and Conrad L. Ziegler

1. INTRODUCTION

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 (Wilson et al. 1992, Atkins et al. 1995, Atkins et al. 1998). These studies have focused on the relationship between these along-line variations to the presence of horizontal convective rolls (HCRs) within the boundary layer (Etling and Brown 1993, Brown 1980). The results suggest that, besides producing along-line undulations, enhanced ascent occurs 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. For example, Dailey and Fovell (1999) 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, their 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 been reported by Ziegler et al. 1997 and Peckham 1999. However, the horizontal resolutions in these model simulations (1 km and 2 km respectively) was not 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 results (Peckham and Wicker 2000, Peckham 1999), 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.

*Corresponding author address: Steven E. Peckham, Department of Atmospheric Sciences, University of Illinois at Urbana-Champaign, Urbana, Illinois 61801;

e-mail: peckham@uiuc.edu.

This is a product of the dryline tilting the HCRs, converting their vertical circulation into horizontal motions, and locally enhancing the zonal flow (e.g., Dailey and Fovell 1999). Further, the HCR circulation enhances the boundary layer moisture increasing (decreasing) CAPE (CIN) and the likelihood of convective cloud formation near the HCR-dryline intersections.

2. METHODOLOGY

The COllaborative Model for Multiscale Atmospheric Simulation (COMMAS) is used in this study (Peckham 1999, Wicker and Wilhelmson 1995). COMMAS includes a generalized terrain-following coordinate transformation (Gal-Chen and Sommerville 1975), parameterizations for surface radiation for a cloudy atmosphere (Benjamin 1986) and land surface processes (Deardorff 1972, 1978). A 1.5-order sub-grid scale turbulence parameterization (Deardorff 1980) is used with the vertical mixing length within the unstable boundary layer following Sun and Chang (1986).

A series of three simulations are performed on a grid having 500 m horizontal resolution and centered at 100 degrees W longitude and 34.5 degrees north latitude. The horizontal domain size is 600 km x 50 km with periodic boundary conditions in the north-south direction. The vertical domain extends to 15 km with a vertical mesh interval smoothly stretching from 50 m at the lowest grid point to approximately 500 m at the domain top.

Rather than attempt a simulation with detailed terrain information, the terrain elevation is confined to change in the x-direction and is expressed analytically (Peckham 1999). The use of an analytical expression permits the close approximation of the actual terrain without introducing the high frequency variations associated with realistic topography.

The initial thermodynamic profile (Fig 1), derived from observational data obtained during the Cooperative Oklahoma Profiler Studies — 1991 (COPS-91) field program, varies in the east-west direction (Hane et al.1993, Peckham 1999). For the experiments the zonal vertical wind shear is .004 s-1 below 5 km MSL and zero above 5 km MSL. Three experiments examine the sensitivity of HCR-dryline interaction to the mean flow of the u-component of momentum. This is conducted by shifting the .004 s-1 vertical wind shear profile such that winds at 1 km MSL are either 0 m s-1, 5 m s-1, or 10 m s-1 (Fig. 2). Since mechanical lift by the topography is dependent upon the ground-relative winds, the changes to the reference zonal flow impose a weak large-scale ascent, or descent within the dryline environment. The initial v wind component is assumed to be in geostrophic balance and is obtained from the cross-domain horizontal pressure gradient.

3. EXPERIMENTAL RESULTS

Preliminary simulation results indicate that HCRs develop within the convective boundary layer across the entire domain. The rolls are oriented in the direction of the mean PBL wind and 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 as hypothesized by Atkins et al. (1998). 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.

4. DEVELOPMENTS

Additional developments will be made available at:

http://redrock.ncsa.uiuc.edu/~peckham/dryline.html.

5. REFERENCES

References are available upon request.

6. ACKNOWLEDGEMENTS

Fig. 2. Initial vertical wind profile for the u component of horizontal velocity for the three simulations. The long dash represents the weak reference flow case, short dash the moderate reference flow case, and the solid line the strong reference flow case.