TOGA COARE
Principal Investigator:
Frank Marks
Collaborating scientist(s):
Robert Black
John Gamache
Chris Samsury
Paul Willis (CIMAS)
Objective:
A major goal of Tropical Ocean Global Atmosphere Program's Coupled Ocean Atmosphere
Response Experiment (TOGA COARE) was to determine the mechanisms that contribute to the
flux of heat, moisture, and momentum from the ocean to the atmosphere over the western Pacific
warm pool. An important aspect of these mechanisms is related to the effect rainfall from tropical
convective systems has on the ocean fluxes.
Rationale:
The empirical relations between the radar reflectivity factor (Z) and rain rate (R) are commonly
used to estimate the rainfall rate, and this is often tacitly assumed to be the surface rainfall rate.
However, except at very short ranges, surface based radars measure Z aloft where the effects of
vertical drafts on the calculated R can be significant. Although seldom reiterated, the assumption
that vertical air motion has a negligible effect on R estimates is inherent in all use of radar to
estimate rainfall rate, or precipitation water fluxes. Accordingly, in the presence of updrafts,
much, if not all, of the precipitation mass may be rising, and falls to the surface elsewhere later.
The drop distributions, and the Z-R relation, may evolve considerably before the precipitation
reaches the surface. Moreover, in updrafts the actual rain rate (water flux) at the measurement
volume is either reduced, or changes sign; and the converse is true in the presence of downdrafts.
The result is that the Z-R relation is altered by the convective scale motions. The extent of this
alteration is the subject of this study.
The key point is that not only do drafts alter the local rain rates and Z-R relations and alter the
area of the rain reaching the surface, but they partition the precipitation mass into rising and
falling components which follow different trajectories and, if they survive, reach the surface at
different places and times. Thus any attempt to use Z measurements aloft to estimate the water
budget near the surface must integrate over a sufficiently large space-time domain. This is the
basis for the Area Time Integral (ATI) approach to estimating surface rainfall of Donneaud et al
(l984) as explained by Atlas et al (l990).
Method:
A large sample of TOGA COARE PMS data from the NCAR Electra aircraft was reduced into
approximately 14,000 6-s (0.7 km spatial scale) drop size distributions. The high resolution (20
Hz) in situ vertical draft measurements were from a radome gust probe system on the NCAR
Electra aircraft (Brown et al 1983). The data subset for this study consists of 6-s means of vertical
velocity matched to the 6-s samples of rain drop distributions measured at altitudes from 0.3-5.0
km altitude on 26 flights during the four months of TOGA COARE. The reflectivity factor Z and
the rainfall rate were calculated from the measured drop size distributions. The rainfall rate was
calculated using fall speeds for standard temperature and pressure (R0) and with the fall speeds
algebraically adjusted by the measured 6-s mean vertical velocity, <w> (Rw).
Accomplishment:
The results showed that Z-R relations in the
presence of significant updrafts are meaningless. The
Z-R regressions from large samples are dominated by a large stratiform precipitation fraction in
mesoscale convective systems, and thus provide a fair estimate of rainfall rate over large areas.
However, in the presence of convective drafts aloft the Z-R relations are greatly altered. It is
impossible to obtain a measure of this flux from a measure of reflectivity factor alone. To fully
assess the fluxes of precipitation water in convection for water budget studies requires both a
measure of the precipitation water (drop size distributions) and a measure of the air, or
hydrometeor, motions. Since a measure of the air motions is not usually available, the effects of
convective motions need to be considered.
The major effort over the next year will be the completion of the manuscript describing the effect
vertical drafts have on the Z-R relations, and subsequently computation of the water budget.
Future efforts will focus on:
- Processing of the two-dimensional Grey probe data from the NOAA WP-3D aircraft.
- Variations in the droplet-size distributions will be computed as a function of altitude, rain rate, and position.
- Calibrations and intercomparisons will be completed for all of the radar reflectivity data.
- HRD is also participating in the intercomparison of several airborne Doppler wind retrieval processing techniques.
Key reference:
Atlas, D., 1966: The balance level in convective storms. J. Atmos. Sci., 23, 635-651.
Atlas, D., D. Rosenfeld, and D.A. Short, 1990: The estimation of convective rainfall by area
integrals: Part I, Theoretical and empirical basis. J. Geophy. Res., 95, 2153-2160
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.
Brown, E.N., C.A. Friehe and D.H. Lenschow, 1983: The use of pressure fluctuations on the nose
of an aircraft for measuring air motion. J. Climate Appl. Meteor., 22, 171-180.
Donneaud, A.A., S.I. Niskov, D.L. Priegnitz, and P.L. Smith, 1984: The area-time integral as an
indicator for convective rain volumes. J. Appl. Meteor., 23, 555-561.
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.
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