Search

The anomalous phenomena induced by the prevailing swell at low wind speeds prevent a complete understanding of air–sea interaction processes. Many studies have considered this complex problem, but most have focused on near-neutral conditions. In this study, the influence of the swell on the atmospheric boundary under nonneutral conditions was addressed by extending the turbulent closure models of Makin and Kudryavtsev and the Monin–Obukhov similarity theory (MOST; Monin and Yaglom) to the existence of swell and nonneutral conditions. It was shown that wind profiles derived from these models were consistent with each other and both departed from the traditional MOST. At low wind speeds, a supergeostrophic jet appeared on the upper edge of the wave boundary layer, which was also reported in earlier studies. Under nonneutral conditions, the influence of buoyancy was significant. The slope of the wind profile increased under stable conditions and became smoother under unstable conditions. Considering the effects of buoyancy and swell, the wind stress derived from the model agreed quantitatively with the observations.

Duct climatology over the South China Sea (SCS) was statistically analyzed in the present study based on 10-year ECMWF reanalysis data. The results show that the spatial distributions of the ducting events over the SCS exhibit obvious monthly variation. May is a key time point that separates the spatial distribution pattern of duct characteristics over the SCS. Ducting events often occur in the first half year and higher ducting probabilities are the broadest area of the SCS in March and April. There are two higher ducting probability regions including the northeast and west of the SCS since June. According to seasonal variations, the duct probability, the duct strength and thickness reach their maximum in spring, followed by winter, autumn, and summer. The atmospheric ducts occur more often in the north than south of the SCS and more ducts especially the surface ducts are present off the mainland. The wind field over the SCS plays a very important role in ducting events and the winter monsoon over the SCS allows ducts to form readily in the middle and north SCS until the onset of the summer monsoon. By contrast, the summer monsoon makes it difficult for ducts to form over the SCS. In the monsoon reversal periods, the higher duct occurrence area decreases and less ducts form in most of the SCS.

Based on the global position system (GPS) radiosonde data near the sea surface, the surface duct characteristics over the South China Sea (SCS) were statistically analyzed. The annual surface duct occurrence over the SCS was about 64%. Of the observed surface ducts, duct heights mainly distributed between 18 and 42 m, with M slopes in the range of–0.3 to–0.2 M units/m. Those ducts accounted for about 80% of the ducting cases. For the total profiles, the duct occurrences in a day changed slowly and were more than 60% in all times. The surface ducts formed more easily in the daytime than in the nighttime and most of the duct height were at bellow about 32 m. Additionally, The seasonal variation of the SCS ducts appeared to be evident, except that the mean duct thickness was almost constant, about 33 m for all seasons. The highest occurrence was about 71% in the autumn, followed by in the summer, spring and winter. In spring, their top-height existed more often at a height of more than 48 m. Their mean duct strength became stronger trend from spring to winter, with the M-slope in the range between–0.26 and–0.18 M units/m. Those results agreed well with other studies, provided considering the data resolution. The statistical analysis was reliable and gave the duct estimation for the SCS. Such duct climatology not only has important implications for communication systems and the reliability of the radar observation, but also can provide useful information to improve the accuracy of the meteorological radar measurements.

The observed characteristics of lower atmospheric ducts over the South China Sea (SCS) were analyzed based on Global Position Systerm (GPS) radiosonde data collected four times daily during autumn open cruises from 2006 to 2012. Duct occurrence, thickness, and strength over the SCS were about 40%, 150-m thick, and 8 M units, respectively, which were larger than during the summer monsoon period. Most ducts occurred at heights