TOGA COARE
- Principal Investigator:
- Frank Marks>
- Other Scientists:
- Robert Black
- John Gamache
- Chris Samsury (now at The Weather Channel)
- Paul Willis (CIMAS)
- Dave Atlas (now at Atlas Concepts)
- Dean Churchill (formerly at UMiami/RSMAS)
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) is 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 data is available from both the NCAR Electra and NOAA P3
aircraft (Brown et al 1983).
Accomplishments
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.
References
- 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.
- TOGA COARE
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Updated Tue, May 5, 1998
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