Project Description

The Global Atmospheric Research Program's (GARP) Atlantic Tropical Experiment took place in the summer of 1974. Because of its enormous size (72 nations were involved) and scope, the GATE experiment was divided into a Central Program, which ensured that the central objective was met, and five sub-programs, whose primary objectives supported the central objective.

Scientific Objectives

The central objective of GATE was to look at the effects of smaller-scale tropical weather systems, such as cloud clusters, on larger scale motions (scale interaction), and to improve numerical modeling and prediction methods. This was done through providing datasets suitable for numerical-modeling testing and development of parameterization schemes for use in numerical models. GATE data have been used for evaluation of models varying from large-eddy simulations to numerical weather prediction models, and for testing and evaluation of parameterization schemes for the boundary layer and mesoscale convective systems.

 GATE Ship ConfigurationGATE Ship Configuration
Photo courtesy American Meteorological Society
(Click Image for Full Resolution)

The five sub-programs and their primary objectives were:

  1. Synoptic-Scale Subprogram, to describe the synoptic scale phenomena from West Africa to the western Atlantic Ocean, and from the troposphere through the lower stratosphere.
  2. Convection Subprogram, to document mesoscale convective systems and their interaction with their environment and other scales.
  3. Boundary-Layer Subprogram, to document the surface fluxes, atmospheric boundary layer structure and fluxes, and boundary-layer interaction with clouds, for suppressed (fair-weather) to the highly disturbed conditions associated with precipitating convection.
  4. Radiation subprogram, to document the radiative heating profiles and on the net radiation and its components at the surface, for the range of convective conditions, from suppressed to highly disturbed.
  5. The Oceanographic Subprogram, to study ocean-atmosphere interaction at various scales, including the structure of the upper ocean, and the coupling between ocean currents and surface winds.

Observations

GATE Observations extended from West Africa to the western Atlantic Ocean, and from 10 S to 20 N, with concentric nested hexagons of ships in the eastern Atlantic at around 8 degrees north, 23-24 degrees West), namely the ~800-km diameter A-Scale Array, the 350-km B-Scale array, and, for the third phase, a C-Scale triangle on the NE part of the B-scale array. 39 ships were involved. GATE observations consisted of roughly 30-day phases, interrupted by 10-day periods to allow fine-tuning to adjust for scientific needs. Most ships provided upper air and surface observations. Many of the ships within the B-Scale array had additional instrumentation, including conventional (non-Doppler) radars, tethered balloons (one with flux-measurement capability), and one acoustic sounder. The SMS-1 geostationary satellite provided half-hourly visible and infrared images thatwere useful for mission planning and evaluation.

Thirteen aircraft were deployed from Dakar, Senegal; most of these flew coordinated missions over the B-Scale Array during the three phases, with measurement altitudes ranging from 30 m for the NOAA-DC-6 and the NCAR Electra, to up to 6-7 km for the NASA-CV-990. Three of the aircraft – the NOAA-DC-6, the NCAR Electra, and the UK-C130 – had gust-probe capability; and several were equipped for cloud-physics and/or radiation measurements.

Intensive observing periods and primary and secondary aircraft missions were determined by a five-member Mission Selection Team, on the basis of anticipated weather conditions, platform status, and experiment objectives that needed to be met. This practice, combined with more thorough reviews between the phases, ensured that needed measurements were taken.