Wednesday, 14 August 2002: 11:45 AM
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
Poster PDF
(1.2 MB)
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’.
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