Z-R Relations from Airborne Disdrometers
The least squares power law regression for the total sample of
14,254 6-s values gives an overall Z-R relation of Z=192R
01.42. This regression is dominated by
the preponderance of stratiform precipitation. In a typical
flight leg the plan view and vertical cross section of
reflectivity shows the convective cells, the prominent bright
band, and the large extent of stratiform precipitation. The
same features are immediately evident in the in situ data
profiles of <w> and R0.
In order to correct R0 for vertical air
motions we use the relation
= (<Vm> - <w>) /
where <Vm> is the mass weighted
mean fallspeed of the drop size distributions and <w>
is the 6-s mean updraft, positive upwards. We have computed
<Vm> from the relation R
0 = K <Vm>
M, where M is the rain mass in the drop size distribution, and
K is a constant that includes the sample volume and unit
conversion (for a typical sample volume of 1,454 liters,
K=2.5). Corrections have been made for the decreasing air
density aloft. Clearly as <w> approaches <V
m>, Rw approaches
zero implying that the mass of rain is suspended. In fact,
drops having fall speeds less than <w> rise, and
conversely for the larger drops. This is analogous to the
"balance level" for radar reflectivity where the mean Doppler
velocity at vertical incidence is zero (Atlas,
l966), except that <Vm> =
This means that the balance level for Z usually occurs
somewhat higher than that for M where the updrafts are stronger.
The Z-R relations for these two distributions are nearly
identical, due to the preponderance of points with <w>
< ± 0.5 m/s (75%).
- Atlas, D., 1966: The balance level in convective storms. J. Atmos. Sci., 23, 635-651.
- 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