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

2302

JOURNAL OF THE ATMOSPHERIC SCIENCES

VOLUME 3S

The Evolution of Raindrop Spectra in Warm-Based Convective Storms
as Observed and Numerically Modeled

R. E. CARBONE

National Center for Atmospheric Research,! Boulder, CO 80307

LOREN D. NELSON

Bureau of Reclamation, Denver, CO 80225
(Manuscript received 14 December 1977, in final form 12 September 1978)

ABSTRACT

Airborne optical spectrometer measurements of raindrop spectra were performed in Texas together
with 10 and 3 cm radar observations. The observations reveal large drop spectra emanating from cloud
base with low concentrations of small drops compared to the Marshall and Palmer (1948) distributions.
Systematic patterns of exponential spectrum parameters (No and A) are revealed to constitute a "pa-
rametric cycle" in time and space. Drop spectrum form is attributed principally to updraft sorting of
smaller drops. Subsequent to updraft sorting, the importance of the collision breakup process with
respect to spontaneous breakup appears to be greatly diminished. Recent theoretical studies by Sri-
vastava (1971, 1978) partially support conclusions drawn with respect to the relative importance of
spontaneous versus collisional breakup.
A one-dimensional, time-dependent numerical model was employed for initial conditions simulating
the observations. The resultant spectra emanating from the model were quantitatively similar to the
measurements and exhibited temporal evolution consistent with the observed parametric cycle.
Implications for estimation of spectral moments such as liquid' water content and radar reflectivity
factor as well as rainfall rate are discussed briefly.

1. Introduction

a. Objectives

Evolution of raindrop size distributions is a subject
of considerable importance to the fundamental under-
standing of precipitation formation. Although warm
rain processes are believed to be of secondary import-
ance with respect to precipitation initiation in mid-
latitude convection, these processes generally are not
negligible during subsequent growth and fallout when
cloud bases are warm. The processes involved are
condensation, evaporation, spontaneous (aerodynamic)
breakup, sedimentation, and binary events including
stochastic coalescence and collisional breakup. The
purpose of this investigation is to combine observa-
tions and numerical modehesults in order to character-
ize the evolving spectra and deduce the relative im-
portance of certain processes for a given set of environ-
mental circumstances. In particular, the authors have
examined temporal and spatial changes in drop spectra
and have compared these observations to previously
and newly presented results from numerical models
which treat some of the known processes.

1 The National Center for Atmospheric Research is sponsored
by the National Science Foundation.

0022-4928/78/2302-2314$06.50
@ 1979 American Meteorological Society

Detailed measurements of drop spectra were obtained
near cloud base in convective storms near Synder,
Texas, as part of the High Plains Cooperative Experi-
ment in 1975. Simultaneous 10 and 3 cm radarobserva-
tions of aircraft-sampled storms provided information
with respect to the precipitation history of each storm.
In addition, longer term radar observations provided a
limited early echo climatology of the Snyder region,
located in west-central Texas. Ancillary observations,
such as rawinsondes and raingages, were also available,
A one-dimensional, time-dependent model was run for
conditions simulating the environment during aircraft
sampling. The details of this model and the observa-
tions are discussed in Section 2.
Considerable discussion in the text is devoted to the
climatological and cloud-mesoscale context of the
measurements. The authors feel compelled to clearly
describe the context, since this knowledge later aids
in the explanation of results which differ considerably
from some previous work.

b. Literature review

Numerous investigators have observed averaged
drop spectra to be exponential in form such as the
distributions given by Marshall and Palmer (1948).
In the case of Marshall-Palmer (hereafter referred to as