No abstract

Abstract. TOPEX is a nadir-pointing dual-frequency radar altimeter that has been in orbit for more than 6 years. Empirical methods, based on the close correlation between the C band and Ku band backscatter, yield an estimate of the rain rate. The principal rain bands of the Intertropical Convergence Zone (ITCZ) are shown to be bounded by "marine deserts" receiving only one hundredth of the rainfall of the ITCZ. Analysis of the entire data set reveals variations in the frequency of oceanic precipitation over a range of timescales. For most regions the diurnal cycle of precipitation has a minimum likelihood at evening local time, with maximum being early morning for the tropics and northern oceans, but late afternoon for the Southern Ocean. On the seasonal cycle, the Pacific ITCZ reaches its southernmost limit during March-April, and is then nearly always accompanied by a secondary but weaker band an equal distance to the south of the equator. Finally, on the interannual timescale the 1997-1998 E1 Nifio is seen to cause a broadening of the Pacific ITCZ and its translation to the south and east, with increased global levels of rainfall rather than just a geographical redistribution. High-resolution sampling shows that the length scale of rain events varies not only latitudinally, but also longitudinally according to whether they lie in regions of genesis or decay of storms. Simultaneous wind and wave information from TOPEX indicates that the rainfall in midlatitudes occurs preferentially with high winds, but in the tropics is associated with swell. The long-term near-global single-instrument data set from TOPEX is thus complementary to those from other sensors.

Three case studies of the nocturnal jet at dawn are presented. Observations were made over southern England where the terrain is considerably less homogeneous than that over which previous published data concerning the jet have been obtained, for example the Great Plains and Wangara.It is shown that previously suggested layer models of the boundary layer can be usefully quantified togive the diurnal evolution of a layer-average wind. This model, which involves a decoupling of the flow in middle levels from surface constraints by thermal stratification at night, exhibits a nocturnal jet. Two surfacestress parameterizations, which make the stress proportional to the velocity and to the velocity squared, give a similar wind evolution. The observed abrupt transition in boundary layer structure soon after sunrise is studied by considering the growth and transfer characteristics of perturbations to a homogeneous Boussinesq fluid, which is stratified and possesses an idealized jet wind structure. Depending on the value of an average Richardson number, z, two regimes of dynamically unstable eddies are indicated. When +0*35 kz> -003 the preferred eddies are oriented transverse to the shear and when 5 -0.03 they are oriented parallel to the shear. The latter regime, associated with longitudinal convective circulations, is efficient at smoothing the jet momentum distribution to give the characteristic well-mixed daytime wind profile.

No abstract

JASIN was designed to observe the physical processes causing mixing and to quantify aspects of the heat and momentum budgets in mid-latitude oceanic and marine atmospheric boundary layers. The multiplicity of processes to be sampled necessitated a large experiment, and JASIN involved 14 ships and 3 aircraft, with more than 50 teams of investigators from 9 countries. The experiment took place from July to September 1978 in the North Rockall Trough, an area of deep water several hundred kilometres off the west coast of Scotland. Hydrographic surveys revealed eddies 100 km across, and intensive measurements on smaller scales revealed fronts in the confluence zones of the eddy field. The development of the mixing layer and seasonal thermocline, internal and inertial wave fields, and the Ekman response to winds are all complicated by the fronts and eddies. The atmospheric boundary layer on scales of 200 km was non-steady and inhomogeneous. Surface-generated turbulence occupied the subcloud layer and transfers through the cloud layer occurred on scales greater than 50 km or intermittently. Possible driving factors were the variability in surface fluxes near the oceanic mesoscale eddies, radiative processes in the cloud layer and condensation processes. The lifting of warm sector boundary layer air from widely differing sources into the free atmosphere at a front resulted in well mixed moist layers that exhibited near-thermal wind equilibrium.