The lower atmosphere over a dry season savanna sitePart I: the observations

The Koorin experiment data of 1974 have been re-examined in detail. Synopticscale conditions, although they tend to remain rather steady, show at least two major departures from the norm during the 29-day period of the experiment. Examination of the surface fluxes indicates daytime maxima occurring, more or less as expected, but the heat flux was by no means excessive, compared with that measured in higher latitudes, and the stress showed a strong maximum in the forenoon. Detailed examination of the wind below about 2000 m reveals a strong element of inertial oscillation, which is relatively undamped at 1600 m. The lower levels, below 500-700 m, are dominated by nocturnal sea-breezes which usually arrive before 0100 h and blow for the rest of the night. Only one of 29 nights lacked a sea-breeze, the exception being a day of strong east-southeast wind with marked cold advection. Two kinds of sea-breeze are identified, evidently traceable to the curved coast line, depending on the pre-existing low-level wind before the sea-breeze. If it is from north of east, the sea-breeze is a simpler event than when it is from south of east. In the latter case the low-level jet develops strongly before the sea-breeze arrives, and appears to be lifted to allow the sea-breeze entry beneath it. In this case, the raised jet rather than the sea-breeze appears to dominate the wind field in the lowest kilometre of atmosphere. The inertial oscillation is quite inadequate to explain the pre sea-breeze jet, acting much too slowly and through too great a depth. This leaves only the nocturnal cooling plus orography to explain the often very rapid low-level jet development after sunset when the low-level wind is from south of east. A relatively small difference is observed in the temperature sequences over Daly Waters between the two kinds of sea-breeze, but the data do reveal a tendency for the sea-breeze of the second kind to be weaker, in terms of temperature effects, and to be accompanied by more cold advection above the sea-breeze than those of the first kind. There is also evidence that the second kind of sea-breeze arrives later, after a greater lifting by convection of the capping inversion.