WMOD00474

• Title: The Evolution of Raindrop Spectra in Warm-Based Convective Storms as Observed and Numerically Modeled
• Date: 12/12/1978
• Weather Modification - Doc Type: Report

DECEMBER 1978

R. E. CARBONE AND LOREN D. NELSON

2313

large. The authors would not anticipate: that specifics
of the results herein could be successfully applied to
different environmental condiitions or storm types.

8. Conclusions

'. ';I

Radar and aircraft observations together with one-
dimensional, time-dependent model simulations have
revealed the following characteristics of convective
storms in Texas:

1) During July and August early echoes are usually
the result of warm rain processes.
2) On the intensively studied days where storms pro-
duced large quantities of precipitation, early echoes'
resulted from both liquid and ice phase processes.
3) Drop-size distributions at cloud base show sys-
tematic variability in time and space.
4) Temporal evolution is dominated by updraft
sorting (sedimentation) effects where small drops do
not have sufficient terminal velocity to fall through
cloud base. The result is a high number density of
drops > 3 mm diameter and a low number density of
drops < 2 mm diameter during the cloud growth stage.
During this stage number densities are typically one
order of magnitude lower than MP values at the 1 mm
diameter size.
. 5) During the dissipation stage drop spectra assume
MP characteristics in the model. The observations,
while evolving toward the MP form, rarely achieve
comparably high concentrations of small drops or
comparably low concentrations of large drops.
6) Model runs including and excluding the collision
breakup process suggest that collision breakup is not
important at and below cloud base when prior updraft
sorting has drastically reduced the total number con-
centration of drops.
7) Close agreement in observed and modeled
spectral form has been shown with the numerical
results of Srivastava (1971, 1978). These results suggest
that the spontaneous breakup process may be important
for rainfall rates ~ 30 mm h-1 and liquid water content
~ 1 g m-3.
9) The high variability of drop spectra precludes
the reliable use of conventional radar (Z-R) rainfall
estimation techniques in convective precipitation unless
some allowance for spatial and temporal change is
made.

The principal limitations of this investigation are
related to sample volume considerations in the observa-
tions and one-dimensionality of the model. To achieve
temporal stratification of the data, it was necessary to
average data over a finite path length in space. To
discern spatial variations, it was necessary to average
data in time. Updraft sorting, while clearly evident in
the model, may be an artifice of one-dimensionality.
Consideration of alternative explanations to updraft
sorting requires a two-dimensional model, such that

rain is not necessarily forced to fall through an updraft. .
The authors propose that future numerical work should
address the situation where rainfall rate is ~ 30 mm
h-1 and liquid water content is ~ 1 g-3 'to tully evaluate
the importance of various physical processes. In
particular, the question arises as to whether an en-
semble of drops will evolve to a unique equilibrium if
given enough time. The findings of this investigation
cast doubt on the concept of a unique equilibrium in
the presence of strong vertical air motions. Conse-
quently, the authors strongly encourage further
numerical work on the- effects of sedimentation.
Acknowledgments. The authors are indebted to Mr.
Donald Takeuchi for the processing of aircraft data
and to Mssrs. S. Howard, R. L. Reace, Jr., and D.
Suder for processing of radar data. Dr. Eugene Mueller
and Mr. Mark Gardner provided valuable assistance
with respect to digitization of radar data. Professor
R. C. Srivastava, Dr. T. B. Smith and Dr. Bernard
Silverman are gratefully acknowledged for stimulating
discussions and helpful suggestions. Ms. Gail Rust
performed typing and editing of the manuscript
admirably.
This research has been supported in part at
Meteorology Research, Incorporated, under contracts
from the Bureau of Reclamation and the Texas Water
Development Board. The National Center for Atmo-
spheric Research is sponsored by the National Science
Foundation and operated by the University Corporation
for Atmospheric Research.

APPENDIX

Model Changes and Differences from the
Warm Cloud Model of Silverman
and Glass (1973)

. Ice hydrometeors have been added to the model.
All ice is assumed spherical with adensityofO.9g cm-3.
. There are 45 liquid and 45 ice hydrometeor classes
in this model.
. Ice is formed in the model by nucleation from the
vapor and drop freezing.
. Riming is computed via a stochastic collection model
assuming the same collection efficiencies as used for
liquid coalescence.
. All liquid touching ice freezes to ice. No wet growth
of hail is allowed. No ice multiplication process
is allowed.
. Ice phase effects have been incorporated in the
thermodynamic, motion and continuity equations.

REFERENCES

Alusa, Alexander L., 1975: The role of drop breakup in the
development of raindrop-size distributions. J. Recl1. Almas.,
9, 1-10.
Asai, T., and A. Kasahara, 1967: A theoretical study of the
compensating downward motions associated with cumulus
clouds. J. Almos. Sci., 24, 487-596.