WMOD00445

• Title: Implications of Time Dependent Convective Z-R Relations for Radar Precipitation Analysis
• Weather Modification - Doc Type: Report

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Figure IA

in figure I having calculated rain rates between 20
and 40 mm/hr. There are about 100 such S!lectrai
having a mean rainrate of 27.S mm/hr. As in ea.r-
lierfigures, dashed lines are "early" o::servations
and chain-dashed lines are "late" values =1 stand-
ard deviation. The excess of large drops. in early
precipitation stages is readily apoarent. Even
late in the rainfall the large drop sizes never
decay to the M-P values (solid line). It may be
that the M-P spectrum ,~hich represents an equilib-
rium condition for stratiform rainfall should be
thought of as a limiting bound which is never
reached in the time-dependent non-eouilil:rium con-
ditions in dissipating convective rainshafts.
Square symbols are "early" model predictions and
crosses are "late" model output values.

The discovery of temporal differences in the rain-
drop data prompted us to search for soatial dif..
ferences as well. Figures IlIA and IIIB show the
variations of N (0) and Lambda on four raincells..
Several passes were made in each cell. Dashed
lines are "early" and chain-dashed lines are "bte"
in the precipitation history of the cell as detE:r-
mined by ground-based radar. The letter U indi.,
cates the upwind side of the cloud and the lettE,r 0
indica.tes the downwind side. It can be seen tha,t
systematic spatial variations within a cloud pass
are even more evident than temporal variations on
a pass-by-pass basis. In fact, the temporal vari-
ations shown in figures I and II would be larger
and in more agreement with the 1-0 model results
if it were not for the spatial cycle superimposed
on the complete pass-by-pass observational data.
The rather low values for each parameter at "U"
are indicative of a relatively "few" lar2e drops
falling through cloudbase. The spectra from this
end of the parametric cycle are sampled on the
upwind-upshear S\~ flank of the rain cells. It i,s
important to note that "older" clouds do not
exhibit such extreme low values of N(O) and Lambda
at the "U" end of the parametric cycle. The
remaining portion of the spatial cycle is the hil~h
rainfall region where a transition from relatively
low to higher values of both parameters, takes plnce.
From the upwind side of the clouds the spectra

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Figure IB

rapidly increase in N(O) and Lambda values, indi-
cating steeper exponential spectra and much larger
concentrations of small drops. This pattern con-
tinues through the core of the high rainfall rate
region.

Z-R relations resulting from the nonstandard drop
spectra shown in earlier figures are given in fig-
ure IV, which shows the distribution of the param-
eters A and B in Eq.(2).

Z-ARB

(2)

There are 17 pass-average Z-R relationships shown
on figure IV, the squares representing "early" data
passes and the crosses "late" estimates of A and B.
The separation of these two data sets again implies
that there are fundamental differences in the Z-R

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Figure II