WMOD00311

• Title: Harold D. Orville Symposium - Forty Years of Modeling Clouds and Weather Modification
• Date: 4/26/2003
• Weather Modification - Doc Type: Report

and often produces more intense areas of
precipitation focused over relatively small areas.
The dynamical impact of hail upon the storm is
often significant, because it produces locally
higher hydrometeor contents that result in
negative buoyancies and locally intense
downdrafts. These effects can only be adequately
resolved within a non hydrostatic model of
sufficiently high resolution.

But what happens if hail is not observed? In
fact, the presence of hail is the exception rather
than the rule, because it takes special
environmental conditions to support the formation
of hail in storms, namely high convective available
potential instabilities (CAPE), in order to support
the presence of strong updrafts needed to
transport large, ice particles upward. In fact,
TRMM observations of lightning and passive-
microwave brightness temperatures over the
global tropics are consistent with the absence of
large ice over the oceans (e.g., Toracinta et al.,
2002).

In the microphysical scheme of Rutledge and
Hobbs (1983, 1984; hereafter RH), which has
more similarities to LFO in its formulation than
differences, the large, heavily-rimed ice are
assumed to have the characteristics of "graupel"
based on observations over the Pacific Northwest
(Locatelli and Hobbs, 1974). The graupel (or "soft
hail") in RH has a density that is half that in LFO,
is much smaller in size, and has fall speeds of 2-4
m 5.1. For example, a graupel content of 1 g m.3
would have a mean size of -0.7 mm in RH and
-1.7 mm in LFO, and a fall velocity near sea level
of -2.4 m S.1 in RH and -8.2 m S.1 in LFO. The
fall speeds of graupel in RH also are similar to
what Heymsfield (1978) documented for Colorado
congestus clouds. .

To those unfamiliar with cloud-resolving
models, such differences in the properties of
large, rimed ice may appear to be an esoteric
detail. But whether the large ice has
characteristics of graupel or hail can have a large
impact on the dynamical structure of simulated
convection. The reason is that the simulated
particle trajectories of graupel and hail can be
quite different locally within a convective storm,
impacting the duration and intensity of the
convective updrafts, where downdrafts are
initiated, and their strength and areal extent. In
20 model simulations of squall systems using the
Goddard Cumulus Ensemble (GCE) model (Tao

and Simpson, 1993), we found that the downdrafts
tended to be locally weaker but extend over a
larger area when the large ice was treated as
graupel (McCumber et a/., 1991; Ferrier et a/.,
1995, 1996).

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The cool pools are typically warmer in the
"hail" runs, the updrafts are oriented more in the
vertical, and the cool-pool generated vorticity is
typically smaller (Rotunno et al., 1988). These
storm characteristics are consistent with hail falling
rapidly out of strong updrafts, making the storm-
scale updraft and downdraft circulations smaller in
horizontal scale and more upright in orientation.
The buoyancy (or thermodynamic) contribution to
the upward pressure perturbation force was larger
in the graupel runs (I.e., higher pressures near the
surface, lower pressures aloft), counteracting the
larger negative buoyancies that formed in the
downdrafts, and thus reducing the downward
accelerations and the adiabatic warming
associated with the subsidence. It's as though the
squall convection simulated with graupel spans a
larger area and possesses more of a hydrostatic
behavior within the cold pool (i.e., an approximate
balance between the upward pressure gradient
force and downward buoyancy). This usually
resulted in more rapid upscale growth with the
convective updrafts tilting over the cold pool
("upshear" tilt counter to the low-level shear vector)
and becoming quasi-steady in evolution. Some of
the hail runs would not evolve into a quasi-steady,
upshear-tilted squall system, and when they did it
was with some reluCtance. Those storms that
would not tilt upshear would almost always
eventually dissipate (this could be a positive
result).

There are other improvements that have been
made to the basic SOSMT bulk microphysical
model, which I will only be able to give an
incomplete (and possibly personally' biased)
account. Again, going back to Rutledge and
Hobbs (RH), they added .a formulation for the
conversion of snow to graupel through rapid cloud-
water riming, which was added to the SOSMT
model by Farley et al. (1989). Though this was
probably an improvement, this process tended to
convert all of the snow to graupel almost
instantaneously. Krueger et a/. (1995) proposed
reasonable improvements in treating Bergeron
growth of small ice crystals at the expense of cloud
water to produce snow. These changes led to
thicker, more extensive anvils that agreed better
with satellite observations.

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