21st Conf. on Severe Local Storms

9.5

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 planetary boundary layer.  The results suggest that, besides producing along-line undulations, areas of enhanced ascent occur where HCR updrafts intersect with the boundaries.Further, convective clouds and/or storms form in the region of enhanced ascent.

 

With the increase in computational resources over the past decade, researchers using numerical models are beginning to investigate the interaction between convective rolls and atmospheric boundaries.  Recent studies have demonstrated that high-resolution numerical 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.

 

Numerical studies of drylines interacting with boundary layer HCRs have been recently reported in the literature. These simulations use a horizontal resolution that is marginally capable of resolving HCRS (ie., 1 km), but produce many of the observed phenomena (e.g., a dryline, HCRs, deep moist convection).  Interestingly, these simulations produce HCRs with aspect ratios much larger than explained by classic linear theory.  One possible explanation for the large aspect ratio is the nonlinear interaction between individual HCRs and/or gravity waves in the free troposphere.  Another possible cause of the large aspect ratios is the limiting of convective roll scales that can occur in nested simulations where the inner grid is too small.  It is now possible conduct single-grid high-resolution simulations of the dryline environment in order to investigate daytime morphology of HCRs near a developing dryline, the HCR - dryline interactions and subsequent convective cloud formation.

 

This presentation will discuss results from a set of high-resolution, single-domain simulations of the dryline environment using the COMMAS mesoscale model.  The simulations exhibit HCR development 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.  HCR aspect ratios range from 3 to 5.  The dryline, which develops in the afternoon, appears as a north-south oriented convergence band along which a strong moisture gradient exists.  Near the intersection points of HCRs and the dryline there are regions of enhanced ascending motion and enhanced low-level moisture.  Further, the interaction between the HCRs and the dryline appear responsible for creating a the east-west undulations along-line along the dryline.  These undulations are transported northward along the dryline by the prevailing flow.  In addition, deep convective clouds develop near the intersections of the dryline and HCRs (see Fig. 1.).  This intersection appears to both enhanced low-level moisture and provide strong, deep ascending motion to lift low-level air to its level of free convection.

Fig. 1.  Contours of vertical motion at roughly 500 m AGL and surface level mixing ratio (shaded).  Contour intervals are every 0.5 m s-1 for the vertical velocity (negative values dashed) and every 1 g kg-1 for the mixing ratio (see colorbar).  One location of deep convective clouds development is labeled ‘A’.

extended abstract  Extended Abstract (1.2M)

Session 9, Convective Initation
Wednesday, 14 August 2002, 10:30 AM-12:00 PM

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