Alexander Soloviev

Ship Surveillance With TerraSAR-X

Ship detection is an important application of global monitoring of environment and security. In order to overcome the limitations by other systems, surveillance with satellite synthetic aperture radar (SAR) is used because of its possibility to provide ship detection at high resolution over wide swaths and in all weather conditions. A new X-band radar onboard the TerraSAR-X (TS-X) satellite gives access to spatial resolution as fine as 1 m. In this paper, first results on the combined use of TS-X ship detection, automatic identification system (AIS), and satellite AIS (SatAIS) is presented. The AIS system is an effective terrestrial method for tracking vessels in real time typically up to 40 km off the coast. SatAIS, as a space-based system, allows almost global coverage for monitoring of ships since not all ships operate their AIS and smaller ships are not equipped with AIS. The system is considered to be of cooperative nature. In this paper, the quality of TS-X images with respect to ship detection is evaluated, and a first assessment of its performance for ship detection is given. The velocity of a moving ship is estimated using complex TS-X data. As test cases, images were acquired over the North Sea, Baltic Sea, Atlantic Ocean, and Pacific Ocean in Stripmap mode with a resolution of 3 m at a coverage of 30 km 100 km. Simultaneous information on ship positions was available from TS-X and terrestrial as well as SatAIS. First results on the simultaneous superposition of SatAIS and high-resolution radar images are presented.

Observation of large diurnal warming events in the near-surface layer of the western equatorial Pacific warm pool

Abstract Because of the relatively calm winds which prevail over the western Pacific warm pool, the diurnal cycle of temperature in the near-surface layer of the ocean is often quite pronounced. During the TOGA Coupled Ocean-Atmosphere Response Experiment (COARE), very high resolution measurements of near-surface thermohaline and turbulence structures were made using bowmounted probes and a free-rising profiler. Experimental data demonstrate a strong dependence of near-surface thermal structure on weather conditions, In calm weather, SST was observed to exceed 33.25°C; this was associated with a diurnal warming of more than 3°C in the top I m of the ocean. A 1-D model of transilient type reproduces the diurnal cycle at low wind speeds and the evening deepening of the diurnal thermocline. Precipitation influenced the diurnal cycle by trapping heat in the near-surface region. During daytime evaporation, surface salinity increased slightly, but deep convection was inhibited by the strong vertical temperature gradient. Contour plots calculated using observations from bow sensors “scanning” the upper meters of the ocean due to ships pitching in some cases revealed strong horizontal variability of the shallow diurnal thermocline with amplitude ∼ 2°C on scales of 0.2–6 km.

Small-Scale Turbulence Measurements in the Thin Surface Layer of the Ocean

Abstract Small-scale turbulence parameters in the layer below a wind-blown surface of the oc ocean can be measured by means of a tethered free-rising profiler. Spectral analysis of the velocity pulsations, measured in the Atlantic Ocean, shows the presence of a frequency band corresponding to the spectrum of local isotopic turbulence in the inertial viscous range. Vertical profiles of dissipation rates of turbulent energy ϵ were calculated according to the vertical component of the velocity turbulent pulsations. Analysis of the vertical profiles of ϵ on the basis of the similarity theory agreed, as a first approximation, with the wall layer analogy proposed by Csanady (1984, Journal of Physical Oceanography , 14 , 402–411). Turbulence in the surface layer was suppressed due to the diurnal warming.

Observation of Wave-Enhanced Turbulence in the Near-Surface Layer of the Ocean During TOGA COARE

Abstract Dissipation rate statistics in the near-surface layer of the ocean were obtained during the month-long COARE Enhanced Monitoring cruise with a microstructure sensor system mounted on the bow of the research vessel. The vibration contamination was cancelled with the Wiener filter. The experimental technique provides an effective separation between surface waves and turbulence, using the difference in spatial scales of the energy-containing surface waves and small-scale turbulence. The data are interpreted in the coordinate system fixed to the ocean surface. Under moderate and high wind-speed conditions, we observed the average dissipation rate of the turbulent kinetic energy in the upper few meters of the ocean to be 3–20 times larger than the logarithmic layer prediction. The Craig and Banner (J. Phys. Oceanogr. 24 (1994) 2546) model of wave-enhanced turbulence with the surface roughness length from the water side z 0 parameterized according to the Terray et al. (J. Phys. Oceanogr. 26 (1996) 792) formula z 0 = cH s provides a reasonable fit to the experimental dissipation profile, where z is the depth (defined here as the distance to the ocean surface), c ≈0.6, and H s is the significant wave height. In the wave-stirred layer, however, the average dissipation profile deviates from the model (supposedly because of extensive removing of the bubble-disturbed areas close to the ocean surface). Though the scatter of individual experimental dissipation rates (10-min averages) is significant, their statistics are consistent with the Kolmogorovs concept of intermittent turbulence and with previous studies of turbulence in the upper ocean mixed layer.

Evolution of Cool Skin and Direct Air-Sea Gas Transfer Coefficient During Daytime

Absorption of solar radiation within the thermal molecular sublayer of the ocean can modify the temperature difference across the cool skin as well as the air-sea gas transfer. Our model of renewal type is based on the assumption that the thermal and diffusive molecular sublayers below the ocean surface undergo cyclic growth and destruction, the heat and gas transfer between the successive burst events are performed by molecular diffusion. The model has been upgraded to include heating due to solar radiation. The renewal time is parameterized as a function of the surface Richardson number and the Keulegan number. A Rayleigh number criterion characterizes the convective instability of the cool skin under solar heating. Under low wind speed conditions, the solar heating can damp the convective instability, strongly increasing the renewal time and correspondingly decreasing the interfacial gas exchange. In the ocean, an additional convective instability caused by salinity flux due to evaporation becomes of importance in such cases. The new parameterization is compared with the cool skin data obtained in the western equatorial Pacific during the Tropical Ocean Global Atmosphere Coupled Ocean Atmosphere Response Experiment in February 1993. In combination with a model of the diurnal thermocline it describes main features of the field data both in nighttime and daytime. Under low wind speed conditions (< 5 m s-1) diurnal variations of the sea surface temperature due to the formation of a diurnal thermocline were substantially larger than those across the cool skin. Under wind speeds > 5 m s-1, diurnal variations of the surface temperature due to the variations of the thermal molecular sublayer become more important.

COOL AND FRESHWATER SKIN OF THE OCEAN DURING RAINFALL

Rainfall over the sea modifies the molecular boundary layers of the upper ocean through a variety of different effects. These cover the freshwater flux stabilizing the near-surface layer, additional heat flux established due to rain versus surface temperature differences, modification of physical parameters by temperature and salinity changes, enhancement of the surface roughness, damping of short gravity waves, surface mixing by rain, and transfer of additional momentum from air to sea. They are separately described and included in our surface renewal model to investigate the rains influence on the cool skin of the ocean and the creation of a haline molecular diffusion layer. Simulations with the upgraded model show that the most important effect on the conductive layer is that of reduced renewal periods followed by additional surface cooling due to rain on the order of 0.1 K. At rain rates below 50 mm h-1 rainfall is not able to completely destroy the mean temperature difference across the cool skin. A freshwater skin is created that exhibits a salinity difference exceeding 4‰ under strong rainfall. Comparisons with field data of the cool skin taken during the Coupled Ocean Atmosphere Response Experiment confirm the upgraded renewal model. Surface salinity measurements taken during the same field campaign are consistent with the calculated salinity differences across the freshwater skin. The enhancement of surface roughness by natural rain is less pronounced than described in earlier laboratory studies of rain with large drop sizes only.

Observation of Spatial Variability of Diurnal Thermocline and Rain-Formed Halocline in the Western Pacific Warm Pool

Abstract High-resolution measurements of temperature and salinity were made in the near-surface layer of the ocean during the Tropical Oceans-Global Atmosphere Coupled Ocean-Atmosphere Response Experiment, using probes mounted on the bow of the R/V Moana Wave. Because of surface waves and pitching of the vessel, the bow probes profiled the near-surface layer of the ocean within depths of 0–4.0 m. In the near-surface layer of the ocean in the western Pacific warm pool, strong variability of temperature and salinity produced by diurnal heating and/or rain was often observed. The contoured density field revealed cases of pronounced spatial variability. The shallow diurnal thermocline and rain-formed halocline are subject to perturbations that sometimes look like large amplitude internal waves. Possible sources of the internal waves in the near-surface layer of the ocean are discussed.

Very high-frequency radar mapping of surface currents

An ocean surface current radar (OSCR) in the very high frequency (VHF) mode was deployed in South Florida Ocean Measurement Center (SFOMC) during the summer of 1999. During this period, a 29-d continuous time series of vector surface currents was acquired starting on 9 July 1999 and ending 7 August 1999. Over a 20-min sample interval, the VHF radar mapped coastal ocean currents over a 7.5 km /spl times/ 8 km domain with a horizontal resolution of 250 m at 700 grid points. A total of 2078 snapshots of the two-dimensional current vectors were acquired during this time series and of these samples, only 69 samples (3.3%) were missing from the time series. During this period, complex surface circulation patterns were observed that included coherent, submesoscale vortices with diameters of 2 to 3 km inshore of the Florida Current. Comparisons to subsurface measurements from moored and ship-board acoustic Doppler current profiles revealed regression slopes of close to unity with biases ranging from 4 to 8 cm s/sup -1/ between surface and subsurface measurements at 3 to 4 m beneath the surface. Correlation coefficients were 0.8 or above with phases of - 10 to - 20/spl deg/ suggestive of an anticyclonic veering of current with depth relative to the surface current. The radar-derived surface current field provided spatial context for an observational network using mooring-, ship- and autonomous underwater vehicle-sensor packages that were deployed at the SFOMC.

The Near-Surface Layer of the Ocean

Under light winds or heavy rainfall, upper ocean turbulence is strongly suppressed by stratification and large vertical gradients of any properties can develop in the upper few meters of the ocean. We consider the penetrative solar radiation and the impacts of the distribution of radiant heating on the nearsurface layer dynamics. Stable stratification in the near-surface ocean due to diurnal warming or rainfall can reduce the turbulence friction, which results in the intensification of near-surface currents. Unstable stratification leads to convective overturning, which increases turbulent friction locally. In addition, discrete convective elements—analogs of thermals in the atmosphere—penetrate into the stably stratified layer below and initiate nonlocal transport. Experimental studies at the equator have produced striking examples of local and nonlocal effects on the dynamics of the diurnal mixed layer and thermocline. A discussion of new approaches to modeling the diurnal cycle of sea surface temperature (SST) using computational fluid dynamics (CFD) capabilities is included. Fine thermohaline structure of the near-surface layer of the ocean in polar seas influences the sea ice coverage, which has important climate consequences.

Slippery Near-Surface Layer of the Ocean Arising Due to Daytime Solar Heating

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, Cu = (U*/ΔUs)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. Fo...