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V. N. Kudryavtsev and V. K. Makin

Abstract

A model that describes the impact of swell on the marine atmospheric boundary layer is proposed. The model is based on the two-layer approximation of the boundary layer: the near-surface inner region and the outer region above. The swell-induced momentum and energy fluxes are confined within the inner region. Swell loses energy to the atmosphere and enhances the turbulent kinetic energy in the inner region. The transfer of momentum results in acceleration or deceleration of the airflow near the surface. Following-wind swell accelerates the flow, which for a very low wind results in a swell-driven wind. Opposite-wind swell decelerates the airflow, which for a steep swell could cause the reverse airflow. The sea drag in the case of opposite-wind swell is considerably enhanced as compared with the following-wind swell case. Cross-wind swell causes the rotation of the wind velocity vector with height, which leads to the deviation of the turbulent stress vector from the wind velocity vector. The impact of swell becomes more pronounced when the wind speed decreases or when the swell phase velocity increases. In fact, it is the wave age of the swell that characterizes the swell impact because the dimensionless wave-induced fluxes of energy and momentum are proportional to the wave age parameter. Both fluxes are also proportional to the swell slope so that the swell impact is stronger for a steeper swell. The model reproduces qualitatively and quantitatively the main experimental findings for the ocean swell: the impact of swell on the sea drag is very pronounced for opposite-wind swell, is less pronounced for cross-wind swell, and is only at low wind speeds.

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Vladimir N. Kudryavtsev and Alexander V. Soloviev

Abstract

Measurements made in the Equatorial Atlantic during the 35th cruise of the R/V Akademic Vernadsky using a free-rising profiler and drifters revealed a near-surface slippery layer of the ocean arising due to daytime solar heating. The solar heating warms and stabilizes the surface layer of the ocean. This suppresses turbulent exchange and limits the penetration depth of the wind-induced turbulent mixing. The heated near-surface layer is then slipping over the underlying water practically without friction. At daytime warming of 1°C the resistance coefficient in the upper 5-m ocean, C u = (U *U s)2 became smaller by a factor of 25–30 as compared with the case of neutral stratification. The effect of slipping results in forming a daytime near-surface current. At low wind speed the velocity of this current was observed to achieve 19 cm s−1. A simple one-dimensional integral model reproduces the main diurnal variation of the temperature and the current velocity in the near-surface layer of the ocean.

For daytime the experimental data suggest the existence of a self-regulating state of the diurnal thermocline, which predicts linear temperature and velocity profiles and an equilibrium value of the bulk Richardson number. This provides simple relations coupling the temperature and velocity differences and the thickness of thermocline. An estimation of the upper velocity limit of the daytime near-surface current is equal to 29 cm s−1.

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N. Reul, B. Chapron, E. Zabolotskikh, C. Donlon, A. Mouche, J. Tenerelli, F. Collard, J. F. Piolle, A. Fore, S. Yueh, J. Cotton, P. Francis, Y. Quilfen, and V. Kudryavtsev

Abstract

Wind radii estimates in tropical cyclones (TCs) are crucial to helping determine the TC wind structure for the production of effective warnings and to constrain initial conditions for a number of applications. In that context, we report on the capabilities of a new generation of satellite microwave radiometers operating at L-band frequency (∼1.4 GHz) and dual C band (∼6.9 and 7.3 GHz). These radiometers provide wide-swath (>1,000 km) coverage at a spatial resolution of ∼40 km and revisit of ∼3 days. The L-band measurements are almost unaffected by rain and atmospheric effects, while dual C-band data offer an efficient way to significantly minimize these impacts. During storm conditions, increasing foam coverage and thickness at the ocean surface sufficiently modify the surface emissivity at these frequencies and, in turn, the brightness temperature (Tb) measurements. Based on aircraft measurements, new geophysical model functions have been derived to infer reliable ocean surface wind speeds from measured Tb variations. Data from these sensors collected over 2010–15 are shown to provide reliable estimates of the gale-force (34 kt), damaging (50 kt), and destructive winds (64 kt) within the best track wind radii uncertainty. Combined, and further associated with other available observations, these measurements can now provide regular quantitative and complementary surface wind information of interest for operational TC forecasting operations.

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