Doppler sounder observations of trade winds and sea breezes along the African west coast near 34 � S, 19 � E

Abstract

Summer weather conditions along the west coast of Africa near 34" S, 18"E are investigated using doppler acoustic sounder profiles. Case studies were selected from a two-year record to form composite analyses over the diurnal cycle. The SE trade wind exhibited a low level jet at the level of the temperature inversion due to a sharp reversal in the thermal wind vector aloft. Mean wind speeds reached 14 m s-i just before midnight as the surface and upper inversions strengthened. Seabreezes were categorised by the supporting gradient wind and found to have mean depths of 400 m, speeds of over 6 m s-r at the 200 m level, and advance/retreat times of 09 hr and 16-20 hr. During seabreezes and weak on-shore gradient flow conditions, the thermal internal boundary layer (TIBL) was monitored with sounder transects in the first 12 km of the coastal zone. The growth height was observed to be 1: 20 in the first 5 km and 1: 50 farther inland. The sounder climatology, together with surface network and aerial survey results, illustrate the four-dimensional characteristics of trade winds and seabreezes near Cape Town.

Measuring the sea breeze from QuikSCAT Scatterometry

“…Figure 2b also suggests that the trade wind cycle is amplified near these mountains; in both locations mean wind differences exceeding 1 m s −1 extend 750 km or more off shore. This is consistent with Dai and Deser [1999], who reported strong diurnal cycles over elevated topography, and with Jury and Spencer‐Smith [1988], who noted that trade winds tend to accelerate at night downstream of topography.…”

Features of the Caribbean low level jet

“…Low level jets (LLJs) have been identified over various regions including North America (Bonner, 1968;Douglas, 1993), South America (Virji, 1981(Virji, , 1982, Africa (Findlater, 1969;Ardanuy, 1979;Kelbe, 1988;Jury and Spencer-Smith, 1988;Jury and Tosen, 1989), Australia (Wilson, 1975;Brook, 1985;Keenan et al, 1989), Asia (Findlater, 1969;Tao and Chen, 1987), and Antartica (Schwerdtfeger, 1975;Chiba and Kobayashi, 1986). Various criteria have been proposed for characterizing them.…”

Winter air pollution episodes over Cape Town

“…Solar insolation values under cloudless skies approach 1000 W m-* (Guastella, 1988) and the summer boundary layer can be characterised as convective. The stabilising influence of the cold Benguela Current bordering the west coast is, at times, advected over the Cape Town area by seabreezes in the autumn (Jury and Spencer-Smith, 1988;Comrie, 1988). In winter, spells of NW wind and rain cleanse the atmospheric boundary layer of pollution.…”

“…(Locations of the air pollution monitor, sounder and radiosonde are given in Figure 1). Further details on the sounder can be found in Keen et al (1987) and Jury and Spencer-Smith (1988). Synoptic analyses for the surface, 850, 700 and 500 hPa levels were obtained from the S A Weather Bureau for the episodes.…”