Basic Convective and Mesoscale Research
Other Agency – Vertical Vortices in the Convective Boundary Layer
Funding Agency: NSF
Objectives
Identify the dynamical mechanisms of vertical vortex formation
in the convective boundary layer and assess the role of these vertical
vortices on boundary layer processes.
Accomplishments
Environments with no ambient winds. Continuing study of the
dynamical formation mechanisms of atmospheric convective boundary layer
vortices is being conducted. These vortices are typically manifest
visibly as dust devils although there is mounting evidence that they
exist with some frequency in the absence of visible flow tracers. For
example, MacPherson and Betts (1997) point out instrument observations
of invisible boundary layer vertical vortices over the boreal forest.
Furthermore, Kanak et al. (2000; hereafter KLS2000) found that a circulation
on 3 cm mobile Doppler that was associated with a dust devil persisted
on the radar for several minutes after the dust devil had moved over
a vegetated surface and was no longer visible to the eye. Most observational
investigators have reported cases when observed mean winds are low
or nearly calm (e.g., < 7-10 ms-1 (Morton 1966)), which seem to
be their preferred environments. Morton even states that wind speeds
greater than 7-10 ms-1 will break up the dust devil. However, there
have been observations to the contrary where dust devils have been
observed in association with larger scale convergence zones, boundaries
or drylines, where mean winds have been as high as 15 m s-1 (e.g.,
Kanak, unpublished, VORTEX95 03 June 1995 south of Dimmitt, Texas;
Pietrycha and Rasmussen 2004; Markowski and Hannon 2005). Prior simulation
work by the principle investigator has represented only cases without
larger scale boundaries which may be more similar to a typical day
in the deserts of Arizona or California (e.g., Sinclair 1969). It is
these latter cases, in which there is no obvious mean source of vorticity,
which lead to a need to address why dust devils may be preferred in
low wind environments. Furthermore, vertical vortex formation in convection
without a mean shear has not often been documented in laboratory or
numerical simulations. Thus, the main focus of the work thus far has
been on the case of environments without mean winds and the formation
mechanism for vortices in such environments is still being sought through
the continuing study. In current work, a slightly shifted emphasis
is to the vertical structure of vertical vortices which have dust devil-scale
diameters near the surface, but then expand in diameter with height.
In other words, the relationship between mososcale [O(10’s m)]
and misoscale [O(100’s-1000’s m] vortices (Fujita 1981),
their formation mechanisms, and evolution, will be explored. Sinclair
(1966) presents the results from observations taken from upper levels
of dust devils that support the contention that misocyclones may be
related to dust devils. He reports that the radius of disturbed wind
fields associated with dust devils can be up to ten times the radius
of the dust devil column. If dust devils and misocyclones are indeed
associated with one another, dust devil-scale vortices may even been
important to non-supercell genesis (Wakimoto and Wilson 1989). Thus,
the dust devil vortex appears to be embedded within a larger-scale
vortex which seems to be related to the larger-scale convective pattern.
New simulations that are intended to revisit the larger-scale [O(km’s)]
forcing mechanisms have just been performed. The broadening of the
vortex circulations with height is also found in the numerical simulations
(see figure below). The formation mechanisms at the larger scale may
give insight into the dynamical formation mechanisms of the smaller,
near surface vortices.
Environments with ambient wind shears. Wu et al. (1992) showed that in numerical simulations of Rayleigh-Benard convection, the turbulent perturbations had high helicity values when a mean wind that turns with height (helical hodograph) is imposed. To examine whether this is true for vertical vortices only (rather than the combination of vertical and horizontally helical perturbation flows) a new study has been designed. Two experiments have been completed using 6 m horizontal grid spacing. These preliminary results imply that the presence of environmental wind appears to inhibit the formation of vertical vortices as compared with the case of a quiescent environment. The first simulation includes the case of a linear mean wind shear, the second a circular hodograph (shear vector turning with height). The vortex detection algorithm will be used to quantify the effects of the various shear profiles on the number of vortices and their physical characteristics. In addition, other simulations are planned to expand the parameter space to include more shear profiles.
This project is ongoing.
Publications
Kanak, K. M., 2006: On the numerical simulation of dust devil-like vortices
in terrestrial and Martian convective boundary layers, Geophys. Res.
Lett., 33, L19S05, doi:10.1029/2006GL026207. Invited paper for a special
section on Dust Devils.
Horizontal velocity vectors from simulation using 40 m horizontal grid spacing. a) z = 10 m height; diameter of vortex approximately 100 m. b) z = 590 m height; diameter of vortex approximately 500 m.