Download Air mass change along trajectories in the western Mediterranean

Mediterranean Storms
(Proceedings of the 4th EGS Plinius Conference held at Mallorca, Spain, October 2002)
 2003 by Universitat de les Illes Balears (Spain)
AIR MASS CHANGE ALONG TRAJECTORIES IN THE WESTERN MEDITERRANEAN BASIN IN THE
TORRENTIAL RAINS EVENTS IN THE VALENCIA REGION
M.J. Estrela 1, F. Pastor1 & M. Millán1
(1) Fundación Centro de Estudios Ambientales del Mediterráneo (CEAM), Valencia, Spain
ABSTRACT
One of the most outstanding characteristics of the Valencia Region's climate is the torrential nature of its
precipitations. The authors analyze the genesis of the torrential rains within the Back-Door Front concept, which
includes the integration of several factors. According to this theoretical framework, the easterly advection of cold air,
caused by a strong high located over central Europe pushing a continental cold air-mass along its southern edge from
Eastern Europe through the warmer Mediterranean, is the most important factor in the genesis of these precipitations.
A key factor in the Back-Door cold Front is the relatively high temperatures of the Mediterranean sea. In this work we
analyze the exchanges between the cold air mass and the sea from: SST derived from NOAA satellites, air mass
trajectories and sounding data across the Western Mediterranean basin (cross section). The results of the analysis from
cross and vertical sections show the movement of continental cold air over the Mediterranean basin, provoking the
increase in air-mass temperatures from East to West, the progressive increase in surface humidity and the increase in
the thickness of the unstable surface layer. Most of the analyzed events show the conditions of a typical Back-door-cold
Front event.
1
INTRODUCTION
From a geographical and climatic point of view, the Valencian Region is a typical Mediterranean area: coastal plains
are of limited extension while interiors are predominantly mountainous displaying a complex orography. One of the
most outstanding characteristics of the Valencian Region climate is the torrential nature of its precipitation. It is
frequently affected by heavy rainfall episodes resulting in floods that cause material and, sometimes, human losses. The
probable daily maximum for the 10-year return period exceeds 100 mm throughout the practical totality of the region.
This torrential characteristic is especially pronounced in the central coastal sector, where the daily 100 mm shows a
return period of only two years and the absolute maximum registered exceeds 800 mm in 24 hours (Peñarrocha et al.,
2002).
As an alternative to usual accounts, these torrential precipitations can be explained within the Back-Door-Front
concept, which includes the integration of several factors that take part in the development of these storms. The BackDoor Front appears when a strong high is located over central Europe pushing a continental cold air-mass along its
southern edge from Eastern Europe through the Mediterranean. A key factor in the Back-Door cold front is the
relatively high temperatures of the Mediterranean Sea. For a long distance, the cold air mass moves over a warmer sea
triggering air-sea heat and moisture exchanges and the air-mass warming. So, the Back-Door-Front consists of (Millán
et al., 1995):
•
A strong high in Central Europe acting as a forcing mechanism, along with the presence of a low pressure
system in North Africa that steers the surface flow towards the Spanish East coast.
•
The development of a potentially unstable mass above the Mediterranean Sea as a result of driving cold
continental air easterly over a warmer sea. The temperature contrast between the air and the sea surface is one of
the most important parameters influencing air-sea interaction processes.
•
Vorticity is a result of the previous processes. The surface anticyclonic flow becomes cyclonic as a result of
surface (convective) drag and, in addition, the temperature contrast determines the strength and depth of the
vertical convection.
•
Once active, the convective system can generate intense rains, favoured by the location of the mountain ranges
on the Spanish East coast.
•
The combination of all these processes, initiated on the surface, can be influenced and strengthened by the
circulation at high levels (cold air pool aloft).
Following this hypothesis one of the key elements in the genesis of torrential rains is the easterly advection of cold
air driven by a high pressure system located in central Europe (Back Door Front), which comes along the warm surface
of the Mediterranean. This advection can be the starting point (genesis) of an unstable air mass, depending on the heat
exchange between the cold air mass and the warmer water (the temperature difference between air and sea determines
the amount of humidity the air mass is able to absorb). Thus, throughout all the process SST is the key element.
Therefore, the objective of this work is to check the role of the SST (heat and moisture exchanges) in the
cyclogenetic processes from: 1) SST derived from NOAA satellites, 2) Sounding data across the Western
Mediterranean basin (cross section), and 3) Air mass trajectories. These data permit the characterization of the Easterly
advection from soundings analyzed jointly in a cross section, and they define the different air masses that take part and
show how they are changed over the surface (Estrela, et al., 2002). These kinds of changes are very difficult to analyse
from synoptic charts.
The selected episodes show two types of synoptic situations recognized as related to the precipitations, that is to say,
omega block (event of October 2000) and situations of weak circulation at high levels (September 1989).
2
SST AND TRAJECTORIES
SST NOAA satellite images for the days before and after the event have been used to detect changes in SST.
Isobaric trajectories ending in the city of Valencia have also been calculated. In this work we try to find out the source
of water vapour in such a strong cyclogenesis.
2.1
September 1989 event
The heaviest rains were registered on 5 September. In most of the Western Mediterranean basin SST values were
around 28-29 ºC. The image for the post-event days shows an important and generalized SST drop throughout the entire
basin, of about 3ºC. The most important decrease, greater than 5 ºC, took place in areas where the highest temperatures
were observed before the event (Figure 1).
The trajectories associated to days with precipitation show that the air mass arriving at the Spanish East coast comes
from the warmest areas in the Mediterranean Sea. In the post-event trajectories the air mass travels over areas where the
temperature has decreased and where processes of water vapour transference have finished (Figure 1).
2.2
October 2000 event
The most intense rains were registered on 22, 23 and 24 October. The image for the days before the event shows the
Algerian basin, Tyrrhenian Sea and Tunisian Sea as the warmest areas with values around 23-24ºC. The image for the
following days shows a noticeable decrease of SST in the Tunisian (3ºC) and Tyrrhenian (1-2ºC) seas (Figure 2).
The trajectories analysis shows that the air mass arriving at the Spanish East coast comes from the area where the
satellite images show the highest temperatures (Tyrrhenian Sea and Tunisian basin). The area between the two
trajectories, geostrophic (a) and non-geostrophic (b), clearly represents the flow nearest to the surface, coinciding with
the greatest decrease in temperature (Figure 2).
3
CROSS AND VERTICAL SECTIONS
Because the air masses that provoke torrential rains usually have long trajectories, the analysis of meteorological
conditions from vertical profiles are completed with cross sections. The data used for the cross sections are the
soundings from Ajaccio (Corsica) and Trapani (Sicily), Baleares and Murcia.
3.1
September 1989 event
The rainfall triggering mechanism is the arrival of a polar air mass from Eastern Europe that flows over the
Mediterranean towards its western basin. The September 4th vertical cross section shows high moisture levels over the
whole Mediterranean basin and a cold air flow at the mid levels over Corsica and the Balearic Islands. The September
5th vertical cross section shows the movement of the warmest sector of the perturbation and the strengthening of the
cold air flow at mid levels over Murcia. In the vertical cross section, the temperature increase that the air-mass has
experienced while advancing towards the Iberian Peninsula (Figure 3) can be seen.
This situation shows the typical conditions of a cold air mass travelling over the Mediterranean basin with a
temperature and moisture content increase along its path over the sea while progressively gaining instability.
3.2
October 2000 event
The October 22nd vertical cross section at 23h UTC shows typical conditions in an easterly advection. There is a
gradual increase in the absolute and relative humidity levels arriving at the Iberian Peninsula. There are also very high
moisture values in a very thick layer at mid levels. In the eastern sector of the advection relative moisture levels are
lower at mid and upper levels because of the presence of a high pressure ridge over the central Mediterranean Sea. Over
the Balearic Islands there are very high absolute humidity values and a temperature increase (Figure 4).
4
CONCLUSION
Analyzing a large number of torrential rain events, one of the most interesting conclusions concerns the relations
between the two types of synoptic conditions and the cross sections.
Regarding to the omega block type, there are two points we must emphasize. First, the air temperature distribution
seems to indicate that these events are associated with the arrival of easterly colder winds that enter in contact with the
previously humid and warm air mass already located offshore of the Valencian region. There is also a temperature drop
along the East-West direction showing a polar air mass associated with an Atlantic low pressure moving to the
Southeast of the Iberian Peninsula forming the so called Alboran Sea low.
The second type of synoptic situation in which intense rain events occur is related to a weak zonal circulation at
upper levels. This type of situations usually appears during late summer and early autumn. During these situations a
strong increase in humidity can be seen in the whole Mediterranean basin; in the most intense precipitations, advection
of humidity, mainly between Ajaccio and the Balearic Islands, is detected. Although it is a situation with a weak zonal
circulation index, the temperature distribution does not show the typical behaviour of a continental polar air mass
advection, as we could detect in other synoptic situations. Finally, we should say that most of these events show fairly
stable conditions at upper levels, contrary to what could be expected from a cold air mass arriving at our region; from
the soundings we can appreciate that the frontal part of the air mass presents high humidity levels along the vertical,
while the soundings at the eastern part of the advection area show a colder and drier air mass at high levels.
One of the most typical events is that of September 1989. The process that produced the strongest storms was the
arrival of an arctic-polar air mass, which came from Eastern Europe into the Western Mediterranean basin. The vertical
profiles show the increasing gradient of temperature at surface levels and mainly at medium levels. From the soundings
previous to the intense precipitations we can appreciate the heat and humidity transfer process between the cold
incoming air mass and the warmer sea; as a consequence we have a very humid column at medium and high levels.
Finally, most of the analysed cases show the conditions of a typical Back-Door Front event. Vertical cross sections
show the temperature increase in the East-West direction and the progressive recharge of absolute humidity at surface
levels. Also seen is the increase in the thickness of the unstable surface layer.
Acknowledgements. This research was supported by the Ministerio de Ciencia y Tecnología, contract number
REN2001-1086/HID (Ministerio de Ciencia y Tecnología). CEAM is supported by the Generalitat Valenciana and
BANCAIXA.
REFERENCES
Estrela, M., Peñarrocha, D., Pastor, F., and M. Millán, (2000): Torrential events on the Spanish Mediterranean coast
(Valencia Region). Spatial precipitation patterns and their relation to synoptic circulation. In Mediterranean Storms.
Proceedings of the EGS Plinius Conference, Maratea, Italy, 97-106.
Estrela, M., Millán, M., Peñarrocha, D., and Pastor, F, (2002): De la Gota Fría al Frente de Retroceso. Las
precipitaciones intensas en la Comunidad Valenciana. Colección interciencias 17.Centro Francisco Tomás y
Valiente, UNED Alzira-Valencia, 260 pp.
Millán, M. M., M. J. Estrela, and V. Caselles, (1995): Torrential precipitations on the Spanish east coast: The role of the
Mediterranean sea-surface temperature. Atmospheric Research, 36, 1-16.
Pastor, F., Estrela, M. J., Peñarrocha, D., Millán M., (2000): Torrential rains on the Spanish Mediterranean coast:
modeling the effects of the sea-surface temperature. Journal of Applied Meteorology. 40, 1180-1195.
Peñarrocha, D. Estrela, M. J., Millán, M., (2002): Classification of daily rainfall patterns in a Mediterranean area with
extreme intensity levels: the Valencia Region. International Journal of Climatology, 22, 677-695.
Figure 1 and 2. Maps of SST corresponding to the days previous to and after the events, and three day back trajectories for the
surface airmass arriving at the Spanish east coast on days during and after the torrential rain events.
Figure 3 and 4. Cross and vertical sections corresponding to the September 1989 and October 2000 events
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