\documentclass[osd, online, hvmath]{copernicus}

\begin{document}\hack{\sloppy}

\linenumbers

\title{Modelling origin and transport fate of waste materials on the southeastern Adriatic coast (Croatia)}

\Author[1]{M.}{Tudor}
\Author[2]{I.}{Janekovi\'{c}}

\affil[1]{Croatian Meteorological and Hydrological Service, Gri\v{c} 3, Zagreb, Croatia}
\affil[2]{Rudjer Bo\v{s}kovi\'{c} Institute, Bijeni\v{c}ka 54, Zagreb, Croatia}

\runningtitle{Modelling waste transport}

\runningauthor{M.~Tudor and I.~Janekovi\'{c}}

\correspondence{M.~Tudor (tudor@cirus.dhz.hr)}


\received{13~November~2014}
\accepted{30~November~2014}
\published{}


\firstpage{1}

\maketitle



\begin{abstract}
  The southeastern parts of the Adriatic Sea coastline were severely
  polluted by large amounts of accumulated waste material in the
  second half of November~2010.  The waste, reported by major news
  agencies, accumulated dominantly during 21 November~2010 by
  favourable wind -- ocean current transport system (the East Adriatic
  current is northwest so advects floating objects from southeast).
  In the study we
  analysed meteorological and oceanographic conditions that lead to
  the waste deposition using available in situ measurements, remote
  sensing data as well as numerical models of the ocean and the
  atmosphere.  The measured data reveal that an intensive rainfall
  event from 7 till 10 November~2010, over the parts of Montenegro and
  Albania, was followed by a~substantial increase of the river water
  levels indicating a possibility of flash floods that could have splashed the waste
  material into a~river and after to the Adriatic Sea (or to the sea 
  directly). The currents that can bring this waste to Croatia could 
  have been intensified by the strong wind from southeast. In order to
  test these two hypotheses we set a~number of numerical drifter experiments
  with trajectories initiated over southeast Adriatic during the
  intensive rainfall events following their faith in space and time.
  The numerical drifter trajectory experiments that resulted with
  drifters that reached the right position (southeastern Adriatic coast) and
  time (exactly by the time the waste was observed) were initiated on
  00:00 and 12:00\,UTC of 10 November~2010 during the mentioned high
  precipitation event.
\end{abstract}

\introduction On the 21 November~2010, a~dramatic waste accumulation
has been widely reported by public media (web news agencies, television, 
radio, daily papers) at the southeastern coast of Croatia,
particularly area of Pelje\v{s}ac Peninsula; islands Mljet,
Kor\v{c}ula and Lastovo as well as in numerous inlets and beaches 
northwest of Dubrovnik (see map of the area in Fig.~\ref{terrain}).
The heaps of waste were
composed mostly of plastic packages, glass bottles, clothes and other
typical floating municipal garbage while labels suggested that some
part of the waste arrived from Albania. It is a~country on the
southeastern Adriatic coast 100\,\unit{km} southeast of the area
where waste accumulated. 
The labels are no proof that the waste originated from Albania.
One can easily imagine a heap of waste accumulated on a sea shore 
anywhere on Earth with labels "Made in China" since it is well known 
for its massive production and intensive export.
This study describes a possible chain of events that lead to waste 
accumulation on beaches in southeast Croatia.
It is not unusual that a~few pieces of waste
 reach Croatian coast in a~late autumn, however the event
was several orders of magnitude larger than any other in previous
years (according to the local officials, there were no reports in the media).

The meteorological conditions during October and November in southern 
Adriatic included several episodes of intensive precipitation that 
initiated flash floods in Montenegro and Croatia (there were no reports 
available for Albania).
A flash flood event could have washed the waste to the sea 
(or first to a lake or a river that would eventually take it to the sea).
There was no rainfall data from Albania available through standard 
international data exchange so remote sensing data and NWP model data 
were used to estimate which intensive precipitation eventsi (if any) 
could have initiated a flash flood there.
The high precipitation events that could have initiated flash flood in the area are identified by combination of in-situ and remote sensing data.

Numerical modeling studies that examine how a~floating entity reached
a~certain position by means of atmosphere and sea driven currents have
been done before \citep{beg,doos,liu}. 
The subjects range from explanation of how floating
sweet potato reached Polynesia from South America \citep{monte2008},
spread of oil spills such as the one following the Deepwater Horizon
disaster has received more attention (for a collection of articles 
see http://deep waterhorizon.nooa.gov/) as well as the floating
debris that was washed to the sea by tsunami following the Tohoku 9 Mw
earthquake on 11 March~2011 (see
\url{http://www.marinedebris.noaa.gov/}).

In our case we use meteorological and ocean models to explore how
waste items, once washed to the sea, floated to the area affected by
the accumulated waste.  During this study we assume that strong
southeasterly wind intensified the sea current system that was
favourable to bring the floating waste materials.
Previously, we also assumed that the waste was
splashed into the sea by strong flash floods as a~consequence of the
severe torrential rain. These hypotheses are further investigated
using all available meteorological, oceanographic and hydrological
observational data as well advanced meteorological and oceanographic
numerical models.  

The next section describes the geographical characteristics of the area,
meteorological and oceanographic conditions,
measured data and models used in this study. Results of the analysis
of weather patterns using available measured data and results 
of model simulations are presented in Sect.~3 and summarized in
conclusions in Sect.~4.

\section{Region, data, and models}
\subsection{Region}
The Adriatic Sea is a~narrow sea, connected to the Mediterranean only
by the Otranto Strait at the southern part. Bathymetry varies 
considerably over the basin. The northern part is the shallowest,
with mean depth of 35\,\unit{m}. The central and southern Adriatic are
significantly deeper and divided by Palagru\v{z}a~sill (Fig.~\ref{terrain}).  
The central region reaches up to 280\,\unit{m} depth in Jabuka Pit.  
The southern region is the deepest, up to 1200\,\unit{m} in the South 
Adriatic Pit (SAP).  

The Adriatic Sea surface flow is predominately cyclonic
orientation \citep{cushman2001} with distinct current regime of East
Adriatic Current (EAC) flowing northwest along the eastern coast
characterized with salty and warm water from the Ionian Sea.  During
the rain seasons EAC is further intensified with the outflow of the
Albanian rivers creating region of fresh water (ROFI) dynamics
\citep{burrage2008,burrage2009}. In the central region the sea surface
flow typically bifurcates east of the Palagruza Sill
\citep[e.g.][]{wolf1887} enhancing the cyclonic circulation in the
southern Adriatic \citep{artegiani, horton1997}.

On the other side of the Adriatic Sea there is a~Western Adriatic
Current (WAC) holding fresher and colder water along the western
coast. It carries a~signature of Po river outflow, the most important
source of fresh water in the whole Adriatic Sea.

On the land, the area is surrounded by Apennines in the west, Dinaric
Alps and high mountains of Montenegro and Albania along eastern coast
while on the northern coast reaches low and flat Po Valley. Mountains
are much closer to the shore on the eastern side of the Adriatic Sea,
with several peaks higher than 1.5\,\unit{km} located less than
10\,\unit{km} from the coast (Fig.~\ref{terrain}).  Those mountains
have a~strong effect on the air flow and atmospheric dynamics
\citep{mesing1981} and consequently define the sea current response as well.

Mediterranean cyclones often traverse the area
\citep{horvath2009}.  However cyclones often form in the Genoa Bay, at
the northwest \citep{mesing1981} traverse the Tyrhennian Sea and
continue to the east possibly supporting cyclone development and
intensification in the Adriatic Sea at the east and Ionian Sea at the
south \citep{alpert}.  These cyclones usually cross the Adriatic Sea
but in a~certain synoptic conditions can support development of
a~separate Adriatic cyclone \citep{horvath2008} and other mesoscale
weather activity.  In that case two of these cyclones can coexist
forming a~system of twin cyclones in which moist air converges and
generates large quantities of available precipitable water
\citep{lionello2006}.

The intensive atmospheric dynamics in the area also supports strong wind
\citep{horvath2011,bajic2007,brankovic2008} development with the most
severe and gusty wind from northeast named bura \citep[see][for
a~review]{grisog2009}, as well the local wind from southeast referred
as jugo \citep{jurcec1996}.  Strong bura or strong jugo can last for
several days inducing strong response in the Adriatic Sea
\citep{kuzmic, dorman}.  Onset, duration and spatial distribution of
wind strength in a~bura or jugo episode is controlled by an
interaction of the synoptic and/or mesoscale forcing with local
topography \citep{ivateks2004,pasaric2007,tudor2010}.  
The bura strength varies
significantly in space and time forming usually several stripes of
strong wind across the Adriatic Sea separated by areas of milder wind
\citep{grubi2004} related to mountain gaps and ridges upstream.  On
the other hand, jugo blows along shore, it is more steady and
relatively warm wind related to a~Genoa cyclone \citep{jurcec1996} or
mesoscale cyclone above northern Adriatic
\citep{brzomah1999,brzovic1999}.  Jugo can be associated with
sirocco. However, the terms jugo and sirocco are not synonyms. The
latter is a~southern wind blowing from Sahara in advance of low
pressure moving eastward across southern Mediterranean Sea. Jugo is SE
wind over Adriatic, while in sirocco episodes, wind that brings the
Sahara desert dust over Adriatic can be from south or southwest.

Climatologically, Southern Adriatic region is characterized with warm
and dry summers and mild and wet winters \citep{zaninovic2008}.  The
area receives abundant precipitation amounts as Crkvice in Montenegro
holds the maximum measured on the European continent
\citep{magas2002}.  Parts of the northern Albania is rich in
precipitation but unevenly distributed in space and time.  In those
regions precipitation can be further intensified by increased aerosol
concentration \citep{koren2012} usually advected to the Adriatic Sea
area from the Sahara desert by the sirocco wind.

It is worth to say that wind forcing, when pronounced, dominate over
all other forcing contributions and dynamically shape the sea surface
currents system found in the Adriatic Sea.  The surface wind jets and
wakes of the bura wind have a~profound effect on the surface currents
\citep{orlic1994,pullen2003}, while jugo wind is well known to
influence WAC flow reversals \citep{orlic2007,poulain2004}.
It is therefore important to drive the ocean model with a high resolution wind field that resolves high resolution wind features that develop due to interaction of large scale dynamics with local mountains surrounding the sea.

\subsection{Data}

In order to test our hypothesis and numerical model results we used
available remote sensing data and in situ measurements.  For the
meteorological part we used SYNOP, climatological and rain-gauge
measurements from Croatia, Montenegro, Italy, Greece and Macedonia.
At the time of the event (November 2010) there were no in situ measured 
data available from Albania through standard meteorological network, 
the data from the airport did not contain measurements of precipitation.
The hydrological analysis was based on the water level measurements on
relevant major rivers in Montenegro and Macedonia used to confirm
intensive precipitation as a~possible cause of the flush flood event.  

Remote sensing
data, used in this study, originate from Meteosat Second Generation
(MSG), specifically from The EUMETSAT Network of Satellite Application
Facilities (NWC SAF)\footnote{Products available on the
  \url{http://www.eumetrain.org/}.}.  Satellite derived precipitation
data are used as provided from the Tropical Rainfall Measuring Mission
\citep[TRMM,][]{huffman2007}, in particular we used the diurnal
accumulated precipitation data from the 3B42 product and 3\,hourly
precipitation intensity data from 3B40RT product.  

Precipitation can be enhanced by the presence of aerosols \citep{koren2012}.
The two sets of
aerosol data presented in this study are the aerosol optical thickness
(AOT) from Moderate Resolution Imaging Spectroradiometer (MODIS,
\citealp{remer2008}) aboard Aqua satellite and Ozone Monitoring
Instrument aboard NASA's Earth Observing System (EOS) Aura satellite
(OMI, \citealp{torres2002}; \citealp{veihel2007})\footnote{The OMI and
  TRMM data are available from Giovanni web server interface
  \citep{acker2007} on \url{http://disc.sci.gsfc.nasa.gov}.}. 
The wind  over the sea surface derived from MetOp ASCAT \citep{bentamy2012,bentamy2012a} was used to evaluate 10 meter wind field from the meteorological model.


\subsection{Models}
\subsubsection{Atmospheric model -- ALADIN}
The numerical weather prediction (NWP) model data used in this study originate from the operational
8\,\unit{km} resolution forecast runs using ALADIN limited area model
(Aire Limit\'{e}e Adaptation Dynamique d\'{e}veloppement
InterNational, \citealp{aladin}) with a~specific local 3-D-var data
assimilation \citep{stanesic2011}.
In autumn 2010, operational forecast run twice per day up to
72\,h in advance starting from 00:00 and 12:00\,UTC analyses.  The
model forecast in 8\,\unit{km} resolution used initial and boundary
conditions from global model ARPEGE \citep[Action de Recherche Petite
Echelle Grande Echelle,][]{cassou2001} run operationally in Meteo
France.  The operational high-resolution dynamical adaptation
\citep{ivateks2004} provides forecast of 10\,\unit{m} wind adapted to
local and upstream topography \citep{horvath2011} in 2 km resolution.  
Unfortunately,
this method provides only wind field at high resolution, but not the
other meteorological variables needed to force the ocean model.  
 The meteorological model 10\,\unit{m} wind field is obtained by vertical
interpolation from the lowest model level \citep[17\,\unit{m} above
sea, see][for more details]{geleyn1988}. 

In order to simulate the mesoscale characteristics and development of the
low pressure field, a~2\,\unit{km} resolution forecast using the
non-hydrostatic set-up of the ALADIN model and the full
parametrization set, including radiation, microphysics and convection
schemes \citep{tudor2010} was used to model the state of the
atmosphere.  The high-resolution forecast uses scale selective digital
filter initialization \citet{termonia2008} and no data assimilation to
initialize the model fields. It is coupled to the ALADIN 8\,\unit{km}
resolution with 3\,h interval.  This might be insufficient to prevent
the fastest of the meteorological features to enter the domain
unnoticed by the lateral boundary coupling procedure
\citep{tudor2010b} and possibly miss or undersample a~storm rapidly
entering the domain through the lateral boundaries.
Since the southern Adriatic is not very far from the southern lateral boundary of the high resolution domain, model could have underestimated a storm arriving from south through Otranto strait, however this would be a short duration event related to a flash flood but too short to affect the sea currents substantially.

\subsubsection{Ocean model -- ROMS}

The quality of simulated currents on the ocean surface depends on the wind field.
Wind field over Adriatic is variable in both space and time, depends on surrounding topography and events with strong and severe wind are better forecast in high resolution NWP models \citep{ivateks2004,brankovic2008,tudor2010}.
The ocean dynamics as a~response to the atmospheric forcing was
computed using ROMS REgional Ocean modelling System (ROMS, 
\cite{schep2005}) numerical model. 
ROMS model belongs to free surface, Boussinesq and hydrostatic
approximation models that solves primitive equations using curvilinear
finite difference grids. Model was forced with ALADIN meteorological
model data (10\,\unit{m} wind, 2\,\unit{m} temp and relative humidity,
sea level pressure, rainfall rate, short wave radiation and cloud
fraction), climatological values for the Adriatic river run-offs and
open boundary values with daily temperature, salinity, currents and
sea level information from AREG (INGV) Mediterranean model. The
advection scheme for tracers (temperature and salinity) is based on
multidimensional positive definite advection transport algorithm --
MPDATA \citep{smolarkiewitz} while for momentum on 3rd order upwind
scheme. More details of model implementation for the Adriatic Sea are
described in \cite{janekovic}.
ROMS model time step was 120s while output frequency of needed current 
fields were stored every hour.
Those hourly fields were used for computations of drifter trajectories.

The sea surface currents, responsible for waste transport, are
computed using 2\,\unit{km} resolution ROMS ocean model and were used
for virtual drifter trajectory simulations. Drifters are set to the
surface layer, without vertical dynamics, ensuring representation of
floating waste material.
For computing numerical drifter trajectories we used Roms
OFfline Floats (ROFF) package \citep{carr}, available for ROMS
comunity (http://web.atmos.ucla.edu/~capet/Myresearch/my\_research\_floats.html).

\section{Results and discussion}

\subsection{Atmospheric model results}

To estimate the convective rainfall rate and precipitating clouds we
used derived fields from the NWC SAF products focused on studied area
and time, and rain-gauge measurements and TRMM rainfall data.  The NWC SAF
precipitating clouds \citep[PC,][]{pc} field provides precipitation
probabilities and the convective rainfall rate in mm/hour
\citep[CRR][]{crr} is computed assuming that clouds being both high
and with a~large vertical extent are more likely to induce rain (see
\url{http://www.nwcsaf.org/} for more details).  
CRR gives estimate of intensive rainfall from convective clouds, but PC is useful estimate of rainfall from other types of clouds (eg. nimbostratus).
According to the
available rain-gauge measurements, 6\,hourly PC and CRR fields and
TRMM rainfall data, there were several heavy rainfall events in the
month preceding 21 November~2010 that could have caused flash floods
in the area of southeast Adriatic coast and inland.

We identify those events as four episodes: 23--25 October (E1), 2--3
(E2), 8--10 (E3) and 17--18 November~2010 (E4).

The large scale synoptic conditions responsible for meteorological
setup are described using ERA Interim \citep{dee2011} re-analysis
fields. It turns out that on 24 October~2010, low pressure system
entered western Mediterranean from Atlantic, deepened and formed
a~cyclone, centred over Genoa bay.  The next day the pressure
decreased further and the associated southern wind strengthened from
northern Africa to Adriatic causing intensive rainfall over the
eastern Adriatic coast (E1). The cyclone moved southeast on 26 and
initiated severe bura wind first on northern Adriatic and later spread
over the whole Adriatic Sea by 27 October~2010.  An ensemble of
trajectories initiated over southeast Adriatic on 12:00\,UTC, 25
October~2010 were used to test if this severe rainfall event was the
one that flushed the waste to the sea.
The results of these trajectory computations are
described later in the text (as experiment 2 in Sect.~3.3.1)

Another cyclone from the 1 to the 4 November~2010 (E2) moved from the
Genoa bay southeastward, causing strong jugo wind over the Adriatic
Sea (Fig.~\ref{wind}).  The rainfall was the most intensive over the
northern Italy and central Adriatic region with most of the rainfall
above the Adriatic sea.  Northern Adriatic received more than
100\,\unit{mm} of precipitation within 24\,h, while the rain was weak
in the southeastern region of our interest
(Fig.~\ref{raingauge}). Consequently, E2 case was omitted from further
analysis as was too weak to initiate a~flash flood in southeast
Adriatic. In the following days, meteorological situation was stable
with weak pressure gradient, low wind as well high pressure over
western Mediterranean inducing moderate winds from northwest.

The weather changed again in the period from 7 till 10 November~2010
(E3), dominated by a~large scale cyclone (Fig~\ref{erai850}) that arrived from the
Northern Atlantic causing sirocco wind over Mediterranean
(the colour of the wind vectors in Fig~\ref{erai850} indicates wind speed 
in m/s) that brought warm air and Sahara dust from the northern Africa
(often found in rain gauges after intensive rainfall events), 
aerosols are also shown in Fig~\ref{erai850}. Over Adriatic, the wind was
strong to severe from southwest and south direction (Fig.~\ref{erai850}).  The
wind direction was well forecast by the model, but at Palagru\v{z}a,
Dubrovnik, Prevlaka wind speed was underestimated, while at Mljet and
Biokovo the observed wind speed was correctly modelled.  Pressure
measurements reveal that during this event the Adriatic Sea was
subject to a~deep cyclone that last for several days (Fig.~\ref{wind})
with a~strong pressure gradient over the Adriatic sea.  

The precipitation intensity was estimated using the PC and CRR
fields that showed strong convection and rainfall in the afternoon and
evening with periods of weak to moderate rain intensity in the
night and early morning.  The precipitating clouds covered much of the
area, while the convective rainfall rate is far more localized and
very intensive.
It is important to note that PC and CRR fields were available on 6\,h
interval, while heavy rainfall could have occurred outside the
sampling interval and easily could have been missed.  The 24\,hourly
precipitation exceeds 100\,\unit{mm} over parts of southeast Croata,
Bosnia and Herzegovina, Montenegro and northern Albania in TRMM
precipitation estimates (shown as squares in Fig~\ref{rain1})
 and measurements at several rain-gauges in
Montenegro for two consecutive days (rain-gauge measurements are 
shown as circles in Fig.~\ref{rain1}) as well as in the model forecast
(shaded background in Fig~\ref{rain1}). 
Measurements from the rain-gauges showed that
during E3, rainfall was the most intensive on stations in
Montenegro (larger circles and stars on Fig.~\ref{raingauge}) hence on
southeast Adriatic coast and significantly more intensive than in
other episodes in November~2010.  
For example, on 10 November 2010, there was 188.1 mm of rain measured 
at Cetinje and 143 mm measured at Golubovci station, both in Montenegro.
Accumulated precipitation data are shown on maps for 9 and 10 November 2010 
(for all available stations, circles in Fig~\ref{rain1}). 
There were 3 intensive precipitation events before 21 November 2012, 
and measured precipitation exceeded 100 mm/24 hours in Montenegro 
only in the event 7 to 10 November 2012 (Fig~\ref{raingauge}.
TRMM data also show that 24 hourly precipitation exceeded  100\,\unit{mm}
over Albania in the same event.
The forecast of the accumulated 24 hourly rainfall corresponds to the values
measured on rain-gauges, although the model exaggerated slightly the
rainfall on the coastline and underestimated the rainfall on several
locations further inland (Fig.~\ref{rain1}).  Wind measurements
(Fig.~\ref{wind}) show that wind in E3 episode was from south
direction, more energetic and lasted longer than for other strong wind
episodes during November~2010.

After E3, in the period from 11 till 15 November~2010, the weather was
mostly dry with weak to moderate wind and direction typical for the sea
breeze diurnal cycle (Fig~\ref{wind}). 
In the next days a~cyclone formed in the Genoa
bay (15 November~2010) supporting again strong jugo wind over the
whole Adriatic Sea (Fig.~\ref{eraiuvnp}). Moreover, the wind
strengthened (Fig.~\ref{wind}) with prevailing direction from
southeast as measured in Dubrovnik, Mljet and Prevlaka
(Fig.~\ref{wind16}). Precipitation was intensive with peaks above
100\,\unit{mm} within 24\,h on 17 November (E4), but the maxima were
localized on northeast Adriatic. 
The ALADIN model forecast had similar rainfall distribution, as a~result,
E4 was omitted from detailed analysis as a~period favourable with
respect to the flash flood. However, it is important as the wind field 
has driven the currents on the sea surface. During E4 wind was stronger on
the coast (Dubrovnik and Prevlaka) than in the off-shore region
(Mljet), as a~consequence of channelling effect the coastal
mountains (Fig~\ref{wind16}). 
 Model yields stronger wind above open sea (thiner arrows on regular grid 
in Fig~\ref{wind16}) over southern Adriatic than MetOp ASCAT wind data 
(thicker arrows in Fig~\ref{wind16}) for 16 November 2010, but the wind 
strength and direction are correct for 17 November 2010 (Fig~\ref{wind16}).
The global pressure gradient over the Mediterranean and
Central Europe supported the wind regime from south and southeast over
the whole southern Adriatic (Fig~\ref{eraiuvnp}).  
Later, by 19 November~2010, the wind
changed direction to southwest.  A~cyclone moved from Atlantic
southeast, to the western Mediterranean.  The wind changed to strong
and severe jugo wind on the 21 November~2010.

Based on the analysis of available precipitation fields, it turns out 
that E3 episode was the one when intensive rainfall occurred over
coast and inland of southeast Adriatic  and was the most
likely event that could have triggered a~flash flood there.

\subsection{Hydrology}

Annual river run-off distribution for the Albanian rivers usually
varies for an order of magnitude during the year (embedded diagram in
Fig.~\ref{rivers}) with one pronounced peak in November and another in
January.
Bojana river collects the water flowing from Drim river and Skadar lake 
and flows into Adriatic along the border between Albania and Montenegro.

The largest lake in the region, the Skadar lake, is filled by river
Mora\v{c}a~and Crnojevica in Montenegro and drained into Bojana River
(the name is Bojana in Montenegro and Buna in Albania).  Bojana River
also receives Drim river as a~major tributary on the way to the
Adriatic Sea.  Drim (Drim in Montenegro, Drin in Albania) river powers
3 hydroelectric power plants in Albania. Downstream it splits into two
flows, the smaller one reaches the Adriatic sea directly, and the
larger part flows into Bojana River.

The river water level measurements on the rivers in Montenegro that
belong to the Adriatic Sea catchment (Fig.~\ref{rivers})
increase substantially for the E3 episode.  The water level surge is
most intense for the rivers that fill the Skadar lake.  The level of
Bojana River raised as well during the same event.  This is followed
by a~rise of 1.5\,\unit{m} in the water level of the Skadar lake.
Bojana River level rose before the level of Skadar Lake, 
this could have happened due to an
increase in contribution from the Drim river tributary. The water
levels of Skadar lake and Bojana River stayed high until the end of
November~2010.

Those measurements suggest that the event (E3) from 8 till 10
November~2010 was capable of flushing the waste material into the
Adriatic sea or any of the rivers in the area that flow into it.

\subsection{Ocean model results}

Ocean model results show (Fig~\ref{roms}) consistent development of strong surface
northwest currents after strong jugo wind episodes and small eddies
close to the eastern Adriatic coast in the periods of weak wind
forcing. As stated before in the text, during the November~2010, we
can find three periods of different wind conditions over the Adriatic
Sea.  The first one from 7 till 11, when strong southeast wind
generated strong northwest current system in southeast Adriatic 
(Fig.~\ref{roms}a) followed
by a~weak wind period when the sea-current transport was weaker
(Fig.~\ref{roms}b) and ocean model formed a pool of colder water 
in southeast Adriatic in the area where Bojana river enters the Adriatic Sea.
Finally, the period with moderate to strong southeast
wind (Fig.~\ref{roms}c) strengthened the northwest current. This event
was most likely responsible for waste transport and deposition.

\subsubsection*{Numerical drifter trajectories}

The waste consisted of floating items, so its movement was computed 
as virtual floating drifters released in southeast Adriatic.
Trajectories of virtual drifters were computed using ROFF package 
and surface currents from 2 km resolution ROMS run.
The computation of the trajectories of the drifters stopped when 
they reached coastline for the first time.
The drifters would then stay at this position on the coastline.
This allowed accumulation of drifters.

A~number of numerical drifter experiments was set in which trajectories 
were initiated in southeast Adriatic on the 12:00\,UTC, 19 (experiment 1) and 25 October~2010
(experiment 2), and then sequentially at 00:00 and 12:00\,UTC on each
day starting from 8 till 12 November~2010 (experiments 3--11).  All
virtual drifters were released within a~polygon covering an area over
southeastern Adriatic.  The initial points of
virtual drifter trajectories are separated by 0.01{\degree}
($\approx 1$\,\unit{km}) along longitude and latitude (total 3071
drifters) and fill a polygon with longitude and latitude coordinates of 
southwest corner (19.1,41.0) and northeast corner (19.4,41.9) that 
cover a portion of southeastern Adriatic Sea in the vicinity of coast 
of Albania and Montenegro (Fig~\ref{trajs}).  
Furthermore we divided the polygon into 9 areas
(A1,\ldots, A9) to better cluster track different regions, hence
possible source origin.  The drifters starting from different areas
are plotted in different colours, as marked on the Fig.~\ref{trajs}.
The longitude and latitude coordinates of southwest and northeast 
points of the polygons are
\begin{itemize}
\item A1: SW (19.1,41.0), NE (19.2,41.3) shown in red,

\item A2: SW (19.2,41.0), NE (19.3,41.3) shown in green,
\item A2: SW (19.2,41.0), NE (19.3,41.3) shown in green,
\item A3: SW (19.3,41.0), NE (19.4,41.3) shown in blue,
\item A4: SW (19.1,41.3), NE (19.2,41.6) shown in magenta,
\item A5: SW (19.2,41.3), NE (19.3,41.6) shown in cyan,
\item A6: SW (19.3,41.3), NE (19.4,41.6) shown in yellow,
\item A7: SW (19.1,41.6), NE (19.2,41.9) shown in dark green,
\item A8: SW (19.2,41.6), NE (19.3,41.9) shown in orange,
\item A9: SW (19.3,41.6), NE (19.4,41.9) shown in gray,
\end{itemize}
A plot of drifter positions was done with 6 hourly interval for each 
experiment (not shown), and the summary trajectories are shown in Fig~\ref{trajs}.

\begin{itemize}
\item In the experiment 1 the drifters were released at
  12:00\,UTC, 19 October~2010 and were first pushed offshore into
  EAC. It turns out that a~considerable number of drifters originated
  from regions A7, A8 and A9 reached Croatian coast and Mljet island
  already on 27 October~2010.  The drifters from regions A4,
  A5 and A6 reached Mljet channel by 3 November, but were pushed back
  southeast in the following days. Those drifters continued further
  to the northwest and finally accumulated on the islands much
  further northwest than observed (Fig.~\ref{trajs}a). There were no
  reports of significant accumulation of waste on the Croatian coast
  that would be a~consequence of this event. In that sense, we can
  reject the hypothesis that this rainfall event was the one that
  caused the flash flood that got the waste material to the sea.

\item For experiment 2 the drifters were released at 12:00\,UTC on 26
  October~2010.  Soon the drifters were advected in the westward
  direction. When drifters entered EAC, they moved more to the
  northwest and were deposed on Mljet Island already on 9
  November~2010. However, several drifters starting from A6 region
  entered Mljet channel on 18 and later deposed on Pelje\v{s}ac on 21
  November (Fig.~\ref{trajs}b). Based on those results we can assume
  that it is possible but unlikely that the rainfall event on 
  26 October~2010 has
  initiated the chain of events that led to severe waste disposal in
  the region.

\item Drifters initiated on 8 November (both 00:00 and 12:00\,UTC --
  experiments 3 and 4) mostly arrived to southeast Adriatic Sea
  coast, the northern Albania and Montenegro as soon as on the 11
  November~2010 (Fig.~\ref{trajs}c and d) as a~consequence
  of sea current system supported by strong southern and SSW wind
  blowing on 8 and 9 November~2010.  Furthermore, strong wind changed
  direction into NW on 12 and 13 November~2010 generating currents
  that transported numerical drifters off the coast, resulting only
  with a~small number of them (initiated from A4 region), to reach
  Mljet island and coastline northwest of Dubrovnik by 18
  November~2010.  The rest of the drifters dominantly stayed in the
  southeast region, while only a~small number of them moved
  northeastward not entering Mljet Channel, but instead floated much
  closer to coast, at the end finally accumulated in the Kolo\v{c}ep
  Channel and Ston bay area on 25 November~2010.

\item The quickest drifters initiated from A7, A4 and A8 regions at 00:00
  and 12:00\,UTC on 9 November~2010 (experiments 5 and 6) reached
  Mljet Island and entered Mljet Channel already on 17 and 18, while
  a~majority of drifters from other areas accumulated in the Ston bay
  after 21 November~2010 (Fig.~\ref{trajs}e and f).

\item A~small number of drifters from A7, A8 and A9 regions, released at
  00:00 and 12:00\,UTC on 10 November~2010 (experiments 7 and 8)
  reached Mljet, Dubrovnik and Kolo\v{c}ep channel by 18, while other
  drifters initiated from the same area accumulate on the Croatian
  shores on 21 November~2010 (Fig.~\ref{trajs}g and h). 
  The drifters initiated further south lagged behind former ones and
  approached the affected area on 22 November~2010. Those dates were
  reported in the media as the onset of severe pollution at the
  Croatian coast.

\item The last three sets of drifters, released at 00:00 and
  12:00\,UTC on 11 and 00:00\,UTC on 12 November~2010 (experiments
  9--11) were first pushed westward off the Albanian coast and stayed
  in the area off shore of Albania and Montenegro for a~few
  days. Later they were transported into southward direction on 14 to
  16 November~2010 (Fig.~\ref{trajs}i). Apparently, EAC was detached,
  at that time, from the shore and it's typical path. As a~result,
  drifters from A1 and A2 regions arrived further northwestward than
  drifters initiated more to the north or closer to the coastline.

\end{itemize}

One should bear in mind that the drifter trajectories do not allow us
to assign a~single event in space and time as the moment when the
waste was disposed to the sea. There is ambiguity in the position as
a~result of unresolved physics, imperfect meteorological model and 
initial conditions used to
force the ocean counterpart, missing dynamics in the ocean model
introduced with a~lack of wavecurrent interaction, spatial model
resolutions in the narrow channels etc.  However, the results do show
that there is a~possibility and is the most probably that the heavy
rain on 9 and 10 November~2010 washed the waste into the sea (or first
to a~river that carried it to the sea by that date).  The computations
further show that not all the numerical drifters initiated in southeast
Adriatic inevitably ended on the coastline of southeast 
Croatia. Surface sea currents enhanced by the wind forcing can carry
the waste back to the shore, or to the closer coastline of Montenegro.
Otherwise, different meteo-ocean conditions can push the waste off
shore, and EAC can carry the waste to central or even north Adriatic,
or in some cases back to the southern regions of the Adriatic Sea.
However, none of the trajectories initiated in our experiments crossed
the Adriatic Sea and approached to Italy, which is probably due to an
absence of intensive bura events during the studied period.

\conclusions

The oceanographic and meteorological conditions that lead to a~severe
deposition of waste material on the southeastern Adriatic Sea coast
on 21 November~2010, are studied using ALADIN -- metorological and
ROMS -- ocean numerical models along with available measurements. 
We tried to answer what, where and when was the cause for the event.
The initial points presented in this study were limited to the area
of southeast Adriatic since the labels indicated that some of the items
possibly originate from Albania and the local current system is northwest.

Based on the meteorological simulations and satellite derived
precipitation reveal several intensive rainfall events that could have
initiated flash floods in Albania and presumably flush the waste
material to the rivers and later to the Adriatic Sea. Moreover,
measured and NWP model rainfall data shows that the rain was more
intensive over the Albania in the event from 8 till 10 November~2010
(E3) than in the other intensive rainfall events that occurred in the
studied area during the 4 weeks before the reported waste
accumulation.

Measured wind speed during the episode E3 was strong to severe from
southern direction, however slightly underestimated by the operational
ALADIN model forecast at several land locations. Improvements in the
atmospheric model resolution could resolve those issues as noted in
\cite{signell}. 
Since strong wind influences the surface currents that advect the drifters, 
this could have an impact on the computed trajectories of virtual drifters.
 It is interesting to note that based on the ASCAT
estimated wind data, for the 16 November~2010, ALADIN model wind speed
was larger than the measured one (Fig~\ref{wind16}).  
During the last studied period
(E4), wind from observations as well from the model, was from
southeast with weaker magnitudes than in the E3 period. In the E4
case the strongest wind was found over the open sea, in the
southeast region of Mljet Island as well south of Dubrovnik
(Fig~\ref{wind16}). This
event is a~typical jugo wind episode which further enhanced the sea
surface current system -- responsible for transport of the waste
material to the Croatian waters and finally to the shore on 21
November~2010.

\begin{acknowledgements}
  The authors would like to thank the Meteorological and Hydrological
  Service of Montenegro for providing the meteorological and water
  level measurements as well AREG -- INGV, Bologna, Italy.
  The authors are grateful to NASA
  for providing valuable satellite derived products throught the
  GIOVANNI web interface as well as TRMM, OMI and MODIS scientists and
  developers. The NOAA NESDIS CoastWatch and NOAA SWFSC ERD are
  acknowledged for providing Metop ASCAT wind data. Dr. I.~Janekovi\'{c}
  was sponsored through project HRZZ-5928 of Croatian Science Foundation.
  Martina Tudor thanks the Croatian Ministry of Science
  for support through the grant 004-1193086-3036.
\end{acknowledgements}

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 \begin{figure}
 \includegraphics[width=120mm]{os-2014-81-discussions-f01.pdf}
 \caption{The South Adriatic region with locations of meteorological
   stations used in this study. The shaded background represents terrain height and sea depth in kilometres according to the scale on the right side. The "Bojana" mark shows position of the point where Bojana river enters the Adriatic Sea (this is also the border between Montenegro and Albania on the coast).}
 \label{terrain}
 \end{figure}

 \begin{figure}
 \includegraphics[width=120mm]{os-2014-81-discussions-f02.pdf}
 \caption{Measured accumulated 24\,hourly precipitation on rain
   gauges in southeastern Croatia (smaller symbols) and Montenegro
   (larger symbols) during November 2010.  Precipitation shown for a certain day is measured at 6 UTC accumulated from the previous 24 hours. Only stations close to Adriatic coast are shown.}
 \label{raingauge}
 \end{figure}

 \begin{figure}
 \includegraphics[width=120mm]{os-2014-81-discussions-f03.pdf}
 \caption{ERA Interim 850\,\unit{hPa} wind (colour of the vectors shows wind speed in m/s as on the colour bar below), geopotential (blue isolines) and
   temperature (red isolines) with measured aerosol optical thickness at
   12:00\,UTC 7 November~2010 from MODIS (circles) and OMI (squares).
    AOT is shown in colour according to the colour bar on the right.}
 \label{erai850}
 \end{figure}

 \begin{figure}
 \includegraphics[width=70mm]{os-2014-81-discussions-f04.pdf}

 \caption{Measured accumulated 24\,hourly precipitation on rain
   gauges in Croatia, Montenegro and Macedonia (circles), TRMM
   rainfall data (squares) and 8\,\unit{km} ALADIN forecast data
   (shaded background), the precipitation is accumulated for the period from 06:00\,UTC on 8 until 06 UTC on 9 (top) and from 06 UTC on 9 until 06 UTC on 10 (bottom) November~2010.  The corresponding rain-gauge data is also shown in Fig.~\ref{raingauge} as measured on 9 and 10 November~2010.}
 \label{rain1}
 \end{figure}

 \begin{figure}
 \includegraphics[width=120mm]{os-2014-81-discussions-f05.png}
 \caption{ERA Interim 10\,\unit{m} wind (colour of the vectors shows wind speed in m/s as on the colour bar below), mean sea level pressure
   (blue isolines) and cloudiness (shades of gray) for 12 UTC on 16 November~2010.}
 \label{eraiuvnp}
 \end{figure}

 \begin{figure}
 \includegraphics[width=120mm]{os-2014-81-discussions-f06.pdf}
 \caption{Measured wind speed (dark blue, scale on the left) and mean sea level pressure
   (red) reduced by 980\,\unit{hPa} (to fit the scale on the left) and wind direction (light blue, in degrees from north clockwise, scale on the right)
   for November~2010. Sv. Jure is marked as Biokovo in Fig.~\ref{terrain}.}
 \label{wind}
 \end{figure}

 \begin{figure}
 \includegraphics[width=120mm]{os-2014-81-discussions-f07.pdf}
 \caption{Forecast 10\,\unit{m} wind in 8 (top) and
   2\,\unit{km} (bottom) resolution and measured wind speed and
   direction (arrows) from MetOp ASCAT data (above the sea surface),
   SYNOP and automatic stations for 12:00\,UTC 16 (left) and 17
   (right) November~2010. Colour of the vectors shows wind speed in m/s as on the colour bars. Model data are shown as thin vectors on a denser grid in the background, measured wind is shown as thicker vectors on the location of measurement.}
 \label{wind16}
 \end{figure}

 \begin{figure}
 \includegraphics[width=120mm]{os-2014-81-discussions-f08.pdf}
 \caption{Measured water levels of rivers in Montenegro and Skadar
   lake in October and November~2010 and climatological river run-off
   of Albanian rivers (embedded figure). Moraca and Crnojevica rivers are tributaries to the Skadar lake, Bojana river takes the outflow from the Skadar lake and flows into the Adriatic sea at the position marked as Bojana in Fig~\ref{terrain}.}
 \label{rivers}
 \end{figure}

 \begin{figure}
 \includegraphics[width=60mm]{os-2014-81-discussions-f09.pdf}
 \caption{Surface currents (vectors) and sea surface temperature
   (shaded background) from ROMS for 00:00\,UTC on 11 \textbf{(a)}, 15
   \textbf{(b)} and 19 \textbf{(c)} November~2010.}
 \label{roms}
 \end{figure}

 \begin{figure}
 \includegraphics[width=120mm]{os-2014-81-discussions-f10.pdf}
 \caption{Trajectories of drifters released at 12:00\,UTC 19
   \textbf{(a)} and 12:00\,UTC 26 October \textbf{(b)}, 00:00
   \textbf{(c)} and 12:00 \textbf{(d)} UTC 8, 00:00 \textbf{(e)} and 12:00
   \textbf{(f)} UTC 9, 00:00 \textbf{(g)} and 12:00 \textbf{(h)} UTC 10 and
   00:00\,UTC 11 \textbf{(i)} November~2010. Trajectories initiated off different parts of a rectangle in southeast Adriatic are plotted in different colours. }
 \label{trajs}
 \end{figure}

\end{document}

\endinput
