Radar Observations of Tropical Convection in GATE, TOGA-COARE, and DYNAMO

Over the last half century, precipitating convective clouds over the tropical oceans have been investigated by large international field programs, one in each of the major ocean basins, and atmospheric radars were important component of each program. In 1974, GATE examined convection over the summertime eastern tropical North Atlantic with array of four shipborne radars making quantitative radar reflectivity measurements. Over the Boreal winter of 1992-93, TOGA-COARE documented convection over the vast western Pacific warm pool with radars aboard both ships and aircraft. In the northern winter of 2011-12 convection in DYNAMO was observed with island and airborne radars at the origin location of the Madden-Julian Oscillation (MJO) in the near equatorial Indian Ocean.

When GATE was being planned, the MJO was in the early stages of its discovery by Madden and Julian’s in 1971-72. As a result, GATE was planned and carried out without the MJO in mind. Instead, GATE emphasized summertime ITCZ convection modulated by the African monsoon and easterly waves coming off the continent of Africa. The later experiments, however, were focused on the MJO. TOGA-COARE observed convection in two mature MJO events. DYNAMO observed convection associated with the initiation stages of two MJOs. Because GATE, TOGA-COARE, and DYNAMO occurred at roughly twenty-year intervals, atmospheric radar technology advanced significantly from reflectivity only in GATE, to reflectivity plus Doppler air motions in TOGA-COARE, to reflectivity plus Doppler winds plus dual-polarimetric determined precipitation microphysics in DYNAMO. The author participated as a radar scientist in all three of these iconic experiments and so was able to see first-hand how radar measurements contributed to the results of all three programs and thus to understanding of near equatorial convection in the tropics.

The GATE shipborne radars showed quantitively the relative importance of convective and stratiform precipitation and that stratiform precipitation was maximized in mesoscale convective systems (MCSs) that occurred during the passage of troughs of synoptic-scale waves.

TOGA-COARE radars employed Doppler radar technology on both ships and aircraft showed that MCSs with large stratiform regions dominated during multi-week active phases of MJO’s but were concentrated in westerly propagating waves of 2-day frequency. The Doppler signals combined with precipitation detection quantified the overturning mesoscale circulations in MCSs and associated wind divergence (and hence heating) profiles of convective and stratiform regions.

In DYNAMO, The NCAR S-PolKa dual-polarimetric sensing of precipitation included particle microphysics along with air motions and precipitation detection. The DYNAMO radars showed that during each of the multi-week MJO active phases, the maximum rainfall occurred in short bursts, each lasting only a few days, coinciding with the passage of westerly propagating synoptic-scale waves of 2-to-5-day frequency. The dual-polarimetry showed that each rainy period began with deepening convective towers containing precipitation particles consisting predominantly of dry aggregates. The periods of peak rainfall were characterized by MCSs featuring both convective towers and stratiform regions characterized by layers just above the melting layer containing mostly wet ice-particle aggregates. These radar measurements provide constraints for models simulating the active periods of developing MJOs in DYNAMO. To be accurate, such models must be consistent with not only the rainfall measured by the radars but also with the radar-observed vertical echo structure, air motions, and microphysical characteristics of the convective and stratiform precipitation during convectively active periods of MJOs.