Changeset 186 for altifloat/doc/ocean_modelling/Draft1.tex

Timestamp:

10/23/15 12:34:34 (9 years ago)

Author:

leila_ocean

Message:

Intro Oct23

File:

• altifloat/doc/ocean_modelling/Draft1.tex (modified) (3 diffs)

altifloat/doc/ocean_modelling/Draft1.tex

@@ -140 +140 @@
 Recurrent marine pollution like the ones observed near the heavily populated coastal regions of the Eastern Levantine Mediterranean basin is a major threat to the marine environment and halieutic resources. Polluting agents, being transported through currents to either deep waters or to another part of the coast, have not only an immediate local effect, but also a long term, large scale one.
 It is clear that a good knowledge of the underlying surface velocity field is necessary to understand the dynamics of this transport process. 
- Geostrophic velocities (measured by satellite altimeter) have been widely used to predict the large mesoscale features of the ocean resolving typically lengths on the order of $100$ km [Refs]. There are however limitations to their usage. They are inaccurate in resolving short temporal and spatial scales of some physical processes, like eddies, which results in blurring these structures. Further errors and inaccuracies occur near the coastal areas (within 20-50 km from land); 
-in fact satellite information there is degraded due to various factors such as ``land contamination, inaccurate tidal and geophysical
+ Geostrophic velocities have been widely used to predict the large mesoscale features of the ocean resolving typically lengths on the order of $100$ km [Refs]. There are however limitations to their usage. They are inaccurate in resolving short temporal and spatial scales of some physical processes, like eddies, which results in blurring these structures. Further errors and inaccuracies occur near the coastal areas (within 20-50 km from land),
+where satellite information is degraded; this is due to various factors such as ``land contamination, inaccurate tidal and geophysical
 corrections and incorrect removal
 of high frequency atmospheric effects at the sea surface. " [Caballero]. 
@@ -148 +148 @@
 To improve geostrophic velocities, especially near the coast, several types of data can be combined: specifically in situ observations, [Bouffard et al., 2010; Ruiz et al., 2009] provided by drifters, are useful. Drifters follow the currents and when numerous, they allow for an extensive spatial coverage of the region of interest. They are relatively not very expensive, easily deployable and provide accurate information on their position and other
 environmental parameters [Lumpkin and Pazos, 2007]. 
-Figure~\ref{fig:cnrs} shows the real-time positions of three drifters launched south of Beirut on August 28 2013, in the context of the ALTIFLOAT* project. We observe from that figure that, unlike the corresponding positions simulated by the geostrophic field (provided here by AVISO), the drifters stay within 10-20 km from the coast. The background field shown in that figure is the geostrophic field, averaged over a period of 6 days. The drifters' data render a more precise image of the surface velocity than the altimetric one, because it includes geostrophic and non-geostrophic components; however, this only possible along the path following their trajectory. These types of data are therefore complementary and we propose to blend them in an optimal way in order to estimate the surface velocity field in the 
-Eastern Levantine basin, with a focus on a region not so-well studied in the literature, close to the Lebanese coast. 
+Figure~\ref{fig:cnrs} shows the real-time positions of three drifters launched south of Beirut on August 28 2013, in the context of the ALTIFLOAT* project. We observe that unlike the corresponding positions simulated by the geostrophic field (provided by AVISO), the drifters stay within 10-20 km from the coast. The background field shown in that figure is the geostrophic field, averaged over a period of 6 days. The drifters' data render a more precise image of the surface velocity than the altimetric one, because it includes geostrophic and non-geostrophic components; however, this only possible along the path following their trajectory. These types of data are therefore complementary. In this work, we propose a new algorithm that blends these types of data in an optimal way, in order to estimate the surface velocity field in the 
+Eastern Levantine basin, taking into account the wind effect. We hope to shed light on a region not so-well studied in the literature before, that is between Cyprus and the Syrio-Lebanese coast. 
 %" not completely reliable when they However, they can be sparse and heterogeneous in space and time, rendering time averages over a mesoscale global grid fraught with possible sampling bias."
 
@@ -156 +156 @@
 From the methodological point of view, combining altimetric and drifters data has been done using statistical approaches, in the presence of extensive data sets. A common approach is to use regression models to combine geostrophic, wind and drifters components, the drifters' velocity component being computed from drifters' positions using a pseudo-Lagrangian approach. With large data sets, this approach produces an unbiased refinement of the geostrophic circulation maps, with better spatial resolution. [Poulain et al. 2012, Mena et al. 2012, Niller 2003, Centurino 2008, Uchida and Imawaki, 2003 ].
 Another approach relies on variational assimilation, a method classically used in weather predictions [Courtier, Talagrand, etc...]. 
-In the context of bending altimetric and drifters' data, the method was (first?) used by [Taillander 2006] and it is based on a simple advection model for the drifters' positions, matched to observations via optimisation. The implementation of the method relies on the time-independent approximation of the velocity correction during a time interval shorter than the typical time scale of the mesoscale field. Improvement of the method consists of 
+In the context of bending altimetric and drifters' data, the method was used by [Taillander 2006] (should cite other people as well? look at Taillandier's intro) and it is based on a simple advection model for the drifters' positions, matched to observations via optimisation. The implementation of the method relies on the time-independent approximation of the velocity correction during a time interval shorter than the typical time scale of the mesoscale field. Improvement of the method consists of 
 considering  inertial oscillations superimposed on the mesoscale field. This work 
 led to the development of the LAVA algorithm [Refs],  initially tested and applied to correct model velocity fields using drifter trajectories [Taillandier et al., 2006b, 2008] and later 
  customised to several other applications such as model assimilation [Chang et al., 2011; Taillandier et al., 2010]. Recently,  [Berta
-et al.]. applied it to estimate surface currents in the Gulf of Mexico, where they added "skill scores that compare 
-the separation between observed and hindcast trajectories to the observed absolute dispersion".
-From the application point of view, blending drifters and altimetric data has been applied to several basins, including the gulf of Mexico [Berta et al.], the black sea [A. A. Kubryakov and S. V. Stanichny], the North Pacific ? [Uchida et al.], and the Mediterranean basin [Poulain et al.]  In [Menna et al.], there was a particular attention to the Eastern Mediterranean, but the region of interest to us (Between the coasts of Lebanon, Syria and Cyprus) is characterised by sparsity of data. (NEMED?)
+et al.] applied it to estimate surface currents in the Gulf of Mexico, where they also added a measure of performance consisting of skill scores, that compare 
+the separation between observed and hindcast trajectories to the observed absolute dispersion.
+From the application point of view, blending drifters and altimetric data has been applied to several basins, including the gulf of Mexico [Berta et al.], the black sea [A. A. Kubryakov and S. V. Stanichny], the North Pacific [Uchida et al.], and the Mediterranean basin [Poulain et al.]  In [Menna et al.], there was a particular attention to the Eastern Mediterranean, but the region of interest to us, which lies between the coasts of Lebanon, Syria and Cyprus, is characterised by sparsity of data. (NEMED?).
 %"The second purpose of this paper is to improve the knowledge on the sub-basin circulation and eddy generation in the eastern LSB. The study is mainly focused on the currents trapped on the topographic slope and on the seasonal and interannual varia- bility of specific sub-basin and mesoscale eddies in the period 2009Ð2010."
 
 
 
-Our contribution focuses on the methodical aspect, and it can be considered as an extension of the variational approach used in Taillander, where we provide a time-continuous correction by: (i) assimilating a whole trajectory of drifters at once and (ii) using a moving time window where observations are correlated. We also take into account the effect of the wind, by adding a wind component to the surface velocity, following Poulain et. al. Finally, a divergence free constraint is imposed on the geostrophic component of the total velocity. We consider two main regions of application and show that the method improves the estimation of an eddy where?, and predicts drifters trajectories along the Lebanese coast. This particular coastal region is of interest and not studied in the literature.  
+Our contribution focuses on the methodical aspect, and it can be considered as an "extension" of the variational approach used in [Taillander, 2006], where we 
+the velocity used in the advection of the drifters is made of a geostrophic component that is divergence-free, and a component due to the effect of the wind. We 
+also provide a time-continuous correction by: (i) assimilating a whole trajectory of drifters at once and (ii) using a moving time window where observations are correlated. We show that our method (i) improves the estimation of an eddy between the Lebanese coast and Cyprus, and (ii) predicts real drifters trajectories along the Lebanese coast. 
+%This particular coastal region of the Eastern Levantine Mediterranean is of interest, not studied in the literature.