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Convective Initiation Near Dryline

Table of Contents

Overview | Questions | Previous Research | Recent Developments | Publications | References |

Principle Investigators

Dr. Steven Peckham
Mr. Glen Romine
Dr. Bob Wilhelmson

Overview

Recent work by Peckham (1999) extends the coarse resolution experiments of Peckham and Wicker (1999) by introducing a nested grid that contains cloud-resolving horizontal resolution over the dryline.

  • The experiments are designed to investigate how interactions between boundary layer circulations and the dryline aid the formation of convective clouds and storms.

    Over the past decade several field programs have produced evidence indicating that convective initiation can develop when horizontal convective rolls intersect preexisting convergence lines or atmospheric boundaries. (Crook et al., 1991; Wilson et al., 1992; Wakimoto and Atkins, 1994; Atkins et al., 1995; Atkins et al., 1998). At these intersection points the overall height and magnitude of the ascending motion are locally enhanced along the boundary increasing the potential for convective cloud formation. Similar results have been produced in a high resolution numerical simulations of the sea breeze (Dailey and Fovell, 1999).

    Questions

  • The interaction between the boundary layer circulations and the dryline control the organization and evolution of clouds (Atkins et al., 1998).
    This hypothesis will be further investigated with a set of simulations using higher horizontal resolution. Several papers (Dailey and Fovell, 1999: and Etling and Brown, 1993) have suggested that at least this fine a resolution is needed to resolve boundary layer convective rolls. Trajectory analysis will then be employed to ascertain the origin of the air feeding the dryline and convective updraft and to investigate dynamical properties of the system.

  • The dryline simulations will be used to study the formation and morphology of numerically simulated boundary layer rolls.
    Over the past decades, many theoretical (Asai, 1970, 1972; Lilly, 1966), observational (LeMone, 1973; Weckwerth et al., 1997) and numerical studies (Sommeria and LeMone, 1978; Sykes et al., 1988; Sykes and Henn, 1989) have provided insight into the formation and behavior of convective rolls. These studies have shown that some combination of surface heat flux and wind shear are necessary for roll development. The wavelength of the boundary layer rolls is usually scaled by the boundary layer depth and the orientation of the roll axis is typically along the mean boundary layer wind and/or shear direction. Numerical simulations by Ziegler et al. (1997) produce convective rolls with aspect ratios of 7 to 23 within the dryline environment. Linear theory suggests that the convective rolls occur with spacing between 2 and 4 times the depth of the boundary layer, or aspect ratios of 2 to 4. They suggest that the large aspect ratios are a product of various scales of motion interacting in the boundary layer (Etling and Brown, 1993). The objective of this study will be to define the environmental parameters that determine the convective roll formation, wavelength, orientation and motions that determine the roll evolution within the dryline environment.

    Several new hypotheses will also be examined during the next several years. The hypotheses are:

  • Topography can influence the distribution of mechanical lifting and focus low-level convergence that increases the likelihood of convective storm formation.

  • The westerly jet width in the prescribed wind profile can influence convective formation.

  • Heterogeneous surface characteristics can influence dryline formation. The objective of this study is to examine the hypothesis that the variations in surface characteristics (i.e., soil moisture gradients, vegetation) strongly influence the morphology of the dryline.

    Previous Research

    Recent Developments


    Idealized dryline

    Simulation parameters:

  • Single grid with dimensions of 500km x 30km x 15km
  • 500 m horizontal resolution
  • 25 m vertical resolution near the surface stretching to 400 m at domain top
  • Reference shear in u is .004 (0) s-1 below (above) 5 km MSL
  • 09 hour simulation
  • Solar radiation for 34.5 N Latitude
  • Periodic lateral boundary conditions in y
  • SC86 vertical mixing length

  • Reference flow @ 1 km MSL = 0 m s-1
    Vertical velocity in xy plane Mixing ratio in xy plane
    4 hours
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    Homogeneous environment

    The following simulations use a homogeneous initial environment. The initial sounding is representative of the environment to the west of the dryline. One purpose of the simulations is to show the behavior of the simulated convective rolls without lateral boundaries impacting their behavior. The homogeneous environment and idealized dryline simulations can then be compared to determine what impact, if any, the open boundary conditions have upon convective roll development and evolution.

    Simulation parameters:

  • Single grid with dimensions of 50km x 50km x 15km
  • 500 m horizontal resolution
  • 25 m vertical resolution near the surface stretching to 400 m at domain top
  • Reference shear in u is .004 (0) s-1 below (above) 5 km MSL
  • 12 hour simulation
  • Solar radiation for -101 W Longitude, 34.5 N Latitude
  • Periodic lateral boundary conditions (both x and y)
  • SC86 vertical mixing length

  • Reference flow @ 1 km MSL = 0 m s-1
    Vertical velocity in xy plane Average potential temperature Average mixing ratio profile
    4 hours
    5 hours
    6 hours
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  • Reference flow @ 1 km MSL = 5 m s-1
    Vertical velocity in xy plane Average potential temperature Average mixing ratio profile
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  • Reference flow @ 1 km MSL = 10 m s-1
    Vertical velocity in xy plane Average potential temperature Average mixing ratio profile
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    Domain-average soundings for the simulation at:

    COMMAS simulation Observed western PBL on 15 May 1991
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    Publications

  • Peckham, S. E., R. B. Wilhelmson, L. J. Wicker, and C. L. Ziegler, 2000: Numerical simulation of the interaction between the dryline and horizontal convective rolls. Proc. 14th Symp. on Boundarylayers and Turbulence, Aspen Amer. Meteor. Soc.
    Preprint

  • Peckham, S. E., R. B. Wilhelmson, L. J. Wicker, and C. L. Ziegler, 2000: Numerical simulation of the interaction between the dryline and horizontal convective rolls. Proc. 20th Conf. on Severe Local Storms, Orlando Amer. Meteor. Soc.
    Preprint

  • Peckham, S. E. and L,. J. Wicker, 2000: The influence of topography and lower-troposphere winds on dryline morphology. Mon. Wea. Rev., 128, 2165-2189.
    Abstract | Article