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)


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.


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).


A large sample of TOGA COARE data is available from both the NCAR Electra and NOAA P3 aircraft (Brown et al 1983).


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:

  1. Processing of the two-dimensional Grey probe data from the NOAA WP-3D aircraft.
  2. Variations in the droplet-size distributions will be computed as a function of altitude, rain rate, and position.
  3. Calibrations and intercomparisons will be completed for all of the radar reflectivity data.
  4. HRD is also participating in the intercomparison of several airborne Doppler wind retrieval processing techniques.


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.

Links to other TOGA COARE WWW pages:

TOGA COARE Information Services Website TOGA COARE Mission Summaries
Directory of COARE DATA

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Updated Tue, May 5, 1998