Updraft Effects on Z-R Relations
If we examine the effects of the significant downdrafts and
updrafts separately, the downdrafts tend to shift the data
points to a higher rainfall rate for a given Z. However, the
effect is fairly small since there were virtually no large
organized downdrafts in this sample (<w> < -2.0 m/s).
There were numerous significant updrafts, however, and their
effect on the Z-R relations was to shift the data points so the
resultant regression line is to the left of that for the
whole population. To illustrate the
effects of updrafts on the Z-R relations the sample was limited
to the upper 5% (268 samples) of the 6-s mean <w> >
The measured updrafts reduced the resultant
significantly. In some cases Rw actually
became negative (all of the precipitation water is rising) and
could not be plotted on the log scale. This was particularly
true at low rainfall rates.
Because much of the rain is moving upward in updrafts, in
convection we should consider the flux of precipitation, or
rain water, and not rainfall rate per se. For example, for the
5% of points with <w>>2.0 m/s, the sum of the
precipitation flux (sum R0, sum R
w) changes from 4990.4 to 1240.1 mm/hr
(i.e., only 25% of the rain is falling) upon the application of
updraft velocity, and in a number of samples virtually all of
the precipitation hydrometeors are rising. Moreover, the large
mass of water which is rising will fall out later at places far
removed from the measurement volume, the smaller the drops the
further away. Of course, some of these small drops will not
survive evaporation. The drop distributions will undergo
significant evolution from their initial formation in
convective updrafts and their complex trajectories to the surface.
Another way of looking at the effect updrafts have on the
resultant rainfall rate is by plotting <w> versus the
When Rw/R0=0 the
bulk of the rain is suspended. This occurs at the intercept
<w>=3.9 m/s. The intercept is nearly equal to the mean
and median <Vm> for the entire
sample (3.7 m/s and 4.0 m/s, respectively). Most of the
distributions observed during TOGA COARE had a mean
<Vm> in this range. Distributions
of <Vm> and <w> for a given
distribution can be used to determine whether a substantial
portion of the rain mass is rising.
- Atlas, D., P. Willis, and F. Marks, 1995: Draft effects
upon reflectivity-rain rate relations. Proceedings of the
27th Conference on Radar Meteorology, Vail, CO, AMS.
- Willis, P., R. Black, F. Marks, and D. Baumgardner, 1995:
Airborne rain drop size distributions in TOGA COARE.
Proceedings of the 21st Conference on Hurricanes and Tropical
Meteorology, Miami, FL, AMS, 431-433.
Updated Thur, May 7, 1998