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Werner Alpers
,
Andrei Ivanov
, and
Jochen Horstmann

Abstract

Bora events over the Adriatic Sea and Black Sea are investigated by using synthetic aperture radar (SAR) images acquired by the advanced SAR (ASAR) on board the European satellite Envisat. It is shown that the sea surface roughness patterns associated with bora events, which are captured by SAR, yield information on the finescale structure of the bora wind field that cannot be obtained by other spaceborne instruments. In particular, SAR is capable of resolving 1) bora-induced wind jets and wakes that are organized in bands normal to the coastline, 2) atmospheric gravity waves, and 3) boundaries between the bora wind fields and ambient wind fields. Quantitative information on the sea surface wind field is extracted from the Envisat ASAR images by inferring the wind direction from wind-induced streaks visible on SAR images and by using the C-band wind scatterometer model CMOD_IFR2 to convert normalized cross sections into wind speeds. It is argued that spaceborne SAR images acquired over the east coasts of the Adriatic Sea and the Black Sea are ideal means to validate and improve mesoscale atmospheric models simulating bora events.

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Gerd Müller
,
Burghard Brümmer
, and
Werner Alpers

Abstract

Atmospheric roll convection within an Arctic cold-air outbreak was observed over the Greenland Sea during the ARKTIS 1993 experiment on 24 March 1993 by in situ aircraft measurements and synthetic aperture radar (SAR) imagery from the first European Remote Sensing satellite (ERS-1). Inside a boundary layer heated from below, two kinds of rolls were observed, one aligned parallel and the other perpendicular to the mean wind direction. The wind-parallel rolls occupied the entire boundary layer, whereas the wind-perpendicular rolls were confined to a region around the top of the boundary layer, where a strong vertical shear in the downstream wind component was observed.

A three-dimensional numerical model has been applied to simulate the observed convective pattern. It is shown that the model does not reproduce the observed pattern when using a height-constant geostrophic wind profile. However, when adjusting the vertical wind profile to the one measured from the aircraft, the model reproduces buoyancy-driven wind-parallel boundary layer rolls whose aspect ratio, orientation, and circulation velocity agree well with the corresponding characteristics of the observed rolls. The model calculations show further that shear-driven rolls aligned perpendicular to the buoyancy-driven rolls are generated at the top of the boundary layer. Inside the boundary layer, circulations associated with the shear-driven rolls are suppressed by the buoyancy-driven rolls. The near-surface wind field derived from the ERS-1 SAR image agrees well with the one derived from the model.

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Katrin Hessner
,
Angelo Rubino
,
Peter Brandt
, and
Werner Alpers

Abstract

The dynamics of the Rhine outflow plume in the proximity of the river mouth is investigated by using remote sensing data and numerical simulations. The remote sensing data consist of 41 synthetic aperture radar (SAR) images acquired by the First and Second European Remote Sensing satellites ERS-1 and ERS-2 over the outflow region of the river Rhine. Most of them show sea surface signatures of oceanic phenomena, for example, surface current and wind variations, ship wakes, and oil slicks. In particular, in 36 of these images pronounced frontal features are visible as narrow zones of mainly enhanced, sometimes enhanced/reduced radar backscatter that can be associated with the Rhine surface front. Within the area enclosed by the frontal line, large zones characterized by a lower radar backscatter than in the outer area are often visible. The analysis of the ERS SAR images suggests that the form and the location of the frontal features are mainly linked to the semidiurnal tidal phase in the outflow region, although their variability suggests also that they weakly depend on river discharge, residual currents, and neap-spring tidal cycle. In order to test this observational hypothesis, the results obtained from the analysis of the ERS SAR images are compared with the results obtained from the numerical simulation of the hydrodynamics of the Rhine outflow region carried out using a two-layer, frontal model, which is based on the nonlinear, hydrostatic shallow-water equations on an f plane. The model is forced by prescribing tidal and residual currents and river discharge at the open boundaries. Several simulations are performed by varying the values of these forcing parameters. The numerical results corroborate the observational conjecture: It is found that the form and the location of the simulated interface outcropping lines in the proximity of the river mouth are mainly determined by the semidiurnal tidal phase in the outflow region and that river discharge, residual currents, and neap-spring tidal cycle contribute only secondarily to their determination. Inserting the simulated surface velocity field into a simple radar-imaging model that relates the modulation of the backscattered radar power to the surface velocity convergence in radar look direction, narrow, elongated bands of enhanced radar backscatter emerge near the model frontal line while patches of low radar backscatter appear within the simulated Rhine plume area. The consistency of the model results with the results obtained from the analysis of the SAR images enables one to infer a mean spatial and temporal evolution of the Rhine outflow plume over a semidiurnal tidal cycle from the analysis of spaceborne SAR images acquired during different tidal cycles over the Rhine outflow area and suggests the possibility of using numerical modeling, in conjunction with the analysis of spaceborne measurements, for monitoring the oceanic variability in the Rhine outflow area.

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Werner Alpers
,
Andrei Yu. Ivanov
, and
Knut-Frode Dagestad

Abstract

Foehn wind blowing through the Kolkhida (Kolkheti) Lowland in the southwestern Caucasus (western Georgia) was observed on an Envisat synthetic aperture radar (SAR) image as it encountered an atmospheric cyclonic eddy over the Black Sea on 13 September 2010. This SAR image reveals unprecedented finescale features of the near-surface wind fields that cannot be resolved by other sensors. It shows, among others, the deflection of the foehn wind by the atmospheric eddy. Quantitative information on the near-surface wind field over the sea is extracted from the SAR image.

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Peter Brandt
,
Angelo Rubino
,
Werner Alpers
, and
Jan O. Backhaus

Abstract

A new numerical two-layer model is presented, which describes the generation of internal tidal bores and their disintegration into internal solitary waves in the Strait of Messina. This model is used to explain observations made by the synthetic aperture radar (SAR) from the European Remote Sensing satellites ERS 1 and ERS 2. The analysis of available ERS 1/2 SAR data of the Strait of Messina and adjacent sea areas show that 1) northward as well as southward propagating internal waves are generated in the Strait of Messina, 2) southward propagating internal waves are observed more frequently than northward propagating internal waves, 3) sea surface manifestations of southward as well as northward propagating internal waves are stronger during periods where a strong seasonal thermocline is known to be present, 4) southward propagating internal bores are released from the sill between 1 and 5 hours after maximum northward tidal flow and northward propagating internal bores are released between 2 and 6 hours after maximum southward tidal flow, and 5) the spatial separation between the first two internal solitary waves of southward propagating wave trains is smaller in the period from July to September than in the period from October to June.

The numerical two-layer model is a composite of two models consisting of 1) a hydrostatic “generation model,” which describes the dynamics of the water masses in the region close to the strait’s sill, where internal bores are generated, and 2) a weakly nonhydrostatic “propagation model,” which describes the dynamics of the water masses outside of the sill region where internal bores may disintegrate into internal solitary waves. Due to a technique for movable lateral boundaries, the generation model is capable of simulating the dynamics of a lower layer that may intersect the bottom topography. The proposed generation–propagation model depends on one space variable only, but it retains several features of a fully three-dimensional model by including a realistic channel depth and a realistic channel width. It is driven by semidiurnal tidal oscillations of the sea level at the two open boundaries of the model domain.

Numerical simulations elucidate several observed characteristics of the internal wave field in the Strait of Messina, such as north–south asymmetry, times of release of the internal bores from the strait’s sill, propagation speeds, and spatial separations between the first two solitary waves of internal wave trains.

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Werner Alpers
,
Jen-Ping Chen
,
Chia-Jung Pi
, and
I-I. Lin

Abstract

Frontal lines having offshore distances typically between 40 and 80 km are often visible on synthetic aperture radar (SAR) images acquired over the east coast of Taiwan by the European Remote Sensing Satellites 1 and 2 (ERS-1 and ERS-2) and Envisat. In a previous paper the authors showed that they are of atmospheric and not of oceanic origin; however, in that paper they did not give a definite answer to the question of which physical mechanism causes them. In this paper the authors present simulations carried out with the fifth-generation Pennsylvania State University–National Center for Atmospheric Research Mesoscale Model, which shows that the frontal lines are associated with a quasi-stationary low-level convergence zone generated by the dynamic interaction of onshore airflow of the synoptic-scale wind with the coastal mountain range of the island of Taiwan. Reversed airflow collides with the onshore-flowing air leading to an uplift of air, which is often accompanied by the formation of bands of increased cloud density and of rainbands. The physical mechanism causing the generation of the frontal lines is similar to the one responsible for the formation of cloud bands off the Island of Hawaii as described by Smolarkiewicz et al. Four SAR images are shown, one acquired by ERS-2 and three by Envisat, showing frontal lines at the east coast of Taiwan caused by this generation mechanism. For these events the recirculation pattern, as well as the frontal (or convective) lines observed, were reproduced quite well with the meteorological model. So, it is argued that the observed frontal lines are not seaward boundaries of (classical) barrier jets or of katabatic wind fields, which have characteristics that are quite different from the flow patterns around the east coast of Taiwan as indicated by the SAR images.

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Werner Alpers
,
Jen-Ping Chen
,
I-I. Lin
, and
Chun-Chi Lien

Abstract

The existence of quasi-stationary alongshore atmospheric fronts typically located 30–70 km off the east coast of Taiwan is demonstrated by analyzing synthetic aperture radar (SAR) images of the sea surface acquired by the European Remote Sensing Satellites ERS-1 and ERS-2. For the data interpretation, cloud images from the Japanese Geostationary Meteorological Satellite GMS-4 and the American Terra satellite, rain-rate maps from ground-based weather radars, sea surface wind data from the scatterometer on board the Quick Scatterometer (QuikSCAT) satellite, and meteorological data from weather maps and radiosonde ascents have also been used. It is shown that these atmospheric fronts are generated by the collisions of the two airflows from opposing directions: one is associated with a weak easterly synoptic-scale wind blowing against the high coastal mountain range at the east coast of Taiwan and the other with a local offshore wind. At the convergence zone where both airflows collide, air is forced to move upward, which often gives rise to the formation of coast-parallel cloud bands. There are two hypotheses about the origin of the offshore wind. The first one is that it is a thermally driven land breeze/katabatic wind, and the second one is that it is wind resulting from recirculated airflow from the synoptic-scale onshore wind. Air blocked by the mountain range at low Froude numbers is recirculated and flows at low levels back offshore. Arguments in favor of and against the two hypotheses are presented. It is argued that both the recirculation of airflow and land breeze/katabatic wind contribute to the formation of the offshore atmospheric front but that land breeze/katabatic wind is probably the main cause.

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Peter Brandt
,
Angelo Rubino
,
Detlef Quadfasel
,
Werner Alpers
,
Jürgen Sellschopp
, and
Heinz-Volker Fiekas

Abstract

On 24 and 25 October 1995, high-resolution oceanographic measurements were carried out in the Strait of Messina by using a towed conductivity-temperature-depth chain and a vessel-mounted acoustic Doppler current profiler. During the period of investigation the surface water of the Tyrrhenian Sea north of the strait sill was heavier than the surface water of the Ionian Sea south of the strait sill. As a consequence, during northward tidal flow surface water of the Ionian Sea spread as a surface jet into the Tyrrhenian Sea, whereas during southward tidal flow heavier surface water of the Tyrrhenian Sea spread, after having sunk to a depth of about 100 m, as a subsurface jet into the Ionian Sea. Both jets had the form of an internal bore, which finally developed into trains of internal solitary waves whose amplitudes were larger north than south of the strait sill. These measurements represent a detailed picture of the tidally induced internal dynamics in the Strait of Messina during the period of investigation, which contributes to elucidate several aspects of the general internal dynamics in the area: 1) Associated with the tidal flow are intense water jets whose equilibrium depth strongly depends on the horizontal density distribution along the Strait of Messina; 2) although climatological data show that a large horizontal density gradient in the near-surface layer along the Strait of Messina exists, its reversal can occur; 3) fluctuations in the larger-scale circulation patterns that determine the inflow of the modified Atlantic water into the Eastern Mediterranean Sea can be responsible for this reversal. As the tidally induced internal waves reflect the variability in the horizontal density distribution along the Strait of Messina, it is suggested that from the analysis of synthetic aperture radar imagery showing sea surface manifestations of internal waves in this area fluctuations of larger-scale circulation patterns in the Mediterranean Sea can be inferred.

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