The 23rd Conference on Hurricanes and Tropical Meteorology

J1.5
GATE- DISTURBED BOUNDARY LAYER STRUCTURE AND PROCESS (INVITED)

David R. Fitzjarrald, SUNY, Albany, NY; and J. A. Businger

The GATE provided the densest array of surface and boun- dary layer observations over the open tropical ocean to date, a data set that is still valuable. The experiment deployed unique arrays of tethered balloons, slow-rise rawinsondes, acoustic sounders (a relatively new instrument at the time), conventional surface layer profile instrumentation as well as hot-film tur- bulence sensors located on ship's boom and on buoys combined with aircraft observations of the boundary layer. Shallow, disturbed mixed layers had been observed during the FSU and BOMEX Caribbean experiments to persist for many hours in the wake of intense con- vective events. In the GATE, the resources were available to fill in this picture, even allowing for the formidible difficul- ties of operating instruments of the day during and just after these disturbances. Combining the ship surface and boundary layer flux measurements with selected aircraft observations led to the following picture: Latent heat fluxes during the wake period registered a 15% increase over the undisturbed trade wind region values. Sensible heat flux during the wake period showed a remarkable 800% increase. At individual ship stations, cou- pling between the ocean surface and the free atmosphere was suppressed for up to 18 hours, similar to the scaling estimate h/CHU. The areal extent of the wake region estimated by compo- siting radar, satellite, surface, and aircraft data for the severe cases exceeded the 2000 km2 area of the C-scale array. That the shallow mixed layers persisted this long in the face of the relatively large surface buoyancy fluxes indicated the pres- ence of strong mesoscale subsident environment. There were suf- ficient GATE boundary layer soundings to allow estimation of the between-cloud subsidence using composite structure alone, and these estimates compared favorably with case study estimates ob- tained using the conventional line integral approach from sound- ings. The relevance of these findings to the cumulus parameteri- zation problem was not widely appreciated until revived by Ray- mond in 1995 following TOGA/COARE. Computers have progressed since 1974 that mesoscale models (e.g. Grabowski et al., 1996, 1998) can now reproduce many of the features of the GATE convec- tive cloud field, taking as input the large scale apparent heat sources, moisture sinks, and estimates of surface fluxes. Howev- er, even this result relies on periodic boundary conditions that do not yet allow for the 18 h wake period to be simulated. While one can now easily access gridded data sets based on the excellent GATE upper air data via the Web, the detailed boundary layer ob- servations of GATE, still among the most extensive such data available over the tropical ocean, are not as easily obtained or used. Clearly, research is still needed to determine the role of the wake decoupling on the spacing of deep convective activity in the oceanic tropics.

The 23rd Conference on Hurricanes and Tropical Meteorology