R.M. Wakimoto, J.W. Wilson,
D.E. Kingsmill, N.A. Crook, C.K. Mueller and T.M. Weckwerth
on November 13, 1997 at 19:20:55:
The preferred locations for the initiation of deep convection are
dependent upon the horizontal distribution and vertical depth of
water vapor along convergence zones. It is further hypothesized
that local maxima in water vapor and thus greater potential for
thunderstorm development occur at the intersections between convergence
zones, some of which may include horizontal convective rolls.
Recent simulations using numerical models have highlighted the
sensitivity of thunderstorm initiation to changes in the moisture
and temperature profiles of the order of 1 g/kg and 1-3 deg C, respectively
(e.g., Lee et al. 1991, Mueller et al. 1993, Crook 1996). In addition
recent observational studies have shown that variations in water
vapor of at least this magnitude are common on the mesoscale and
that adequate means for measuring these variations are only now
beginning to emerge. Measurements from CLASS soundings provide relatively
accurate profiles at point locations, however, results have shown
that if horizontal convective rolls are present within the convective
boundary layer, temperature and mixing ratio variations on the order
of 0.5 K and 1.5-2.5 g/kg can be expected depending on the location
of the sounding site relative to the rolls. Furthermore it was shown
that the water vapor variations had the greatest effect on the variations
in CAPE and CIN (Weckwerth et al. 1996). Stationary convergence
lines have been shown to locally increase the magnitude and depth
of the water vapor in their immediate vicinity (Wilson et al. 1992).
Other mesoscale features could cause similar deviations (e.g., variations
in soil moisture).
The above suggests that a thunderstorm initiation project that
hopes to substantially improve upon the results from previous experiments
must include provisions for accurate and detailed measurements of
boundary layer horizontal and vertical distributions of water vapor,
as well as their temporal variability.
Crook, N.A., 1996: Sensitivity of moist convection forced by boundary
layer processes to low-level thermodynamic fields. Mon. Wea. Rev.,
Lee, B.D., R.D. Farley and M.R. Hjelmfelt, 1991: A numerical case
study of convection initiation along colliding convergence boundaries
in northeast Colorado. J. Atmos. Sci., 48, 2350-2366.
Mueller, C.K., J.W. Wilson and N.A. Crook, 1993: The utility of
sounding and mesonet data to nowcast thunderstorm initiation. Wea.
and Forecasting, 8, 132-146.
Weckwerth, T.M., J.W. Wilson and R.M. Wakimoto, 1996: Thermodynamic
variability within the convective boundary layer due to horizontal
convective rolls. Mon. Wea. Rev., 124, 769-784.
Wilson, J.W., G.B. Foote, N.A. Crook, J.C. Fankhauser, C.G. Wade,
J.D. Tuttle, C.K. Mueller and S.K. Krueger, 1992: The role of boundary-layer
convergence zones and horizontal rolls in the initiation of thunderstorms:
A case study. Mon. Wea. Rev., 120, 1785- 1815.
We would use WSR-88Ds, DOWs and ELDORA to identify the location(s)
of the convergence zone(s). Two vertical pointing, mobile water
vapor DIAL systems would be stationed along the zone to measure
the profiles of water vapor at different locations along the line.
Mobile CLASS soundings would also be launched at various locations
along the line, including at the DIAL sites, to examine the along-line
variations in CIN and CAPE. A scanning water vapor DIAL system would
be located within the DOWs' dual-Doppler lobe and would measure
the horizontal water vapor field along the convergence zone. It
is also possible that we could use an airborne downward- or sideways-pointing
DIAL system to map out the water vapor field along various flight
tracks. Additionally the DRI mobile radiometer would be used to
drive along the line to make measurements of the vertically integrated
water vapor. Mobile mesonet stations would also provide useful information
on the surface moisture distribution. Dual-Doppler analyses from
the DOWs and/or ELDORA would then be used to map the three-dimensional
wind fields and locations of thunderstorm initiation. An instrumented
aircraft, such as the King Air, would be required to validate the
water vapor remote sensor fields, as well as the along-line vertical