Mediterranean Storms
(Proceedings of the 4th EGS Plinius Conference held at Mallorca, Spain, October 2002)
 2003 by Universitat de les Illes Balears (Spain)
SATELLITE OBSERVATIONS OF HEAVY RAIN PRODUCING STORMS
OVER THE MEDITERRANEAN AREA
S. Pinori 1,2, C. Adamo 1,2, S. Di Michele1, S. Dietrich 1, A. Mugnai 1, A. Tassa 1, G.J. Tripoli 3, E.A. Smith 4
(1) Istituto di Scienze dell'Atmosfera e del Clima, CNR, Roma, Italy
e-mail: [email protected]
(2) Università di Ferrara, Dipartimento di Fisica, Ferrara, Italy
(3) Department of Atmospheric and Oceanic Sciences, University of Wisconsin, Madison, Wisconsin, USA
(4) NASA/ Goddard Space Flight Center, Greenbelt, Maryland, USA
ABSTRACT
Severe weather events occurring in the western Mediterranean area can be studied by using a series of satellites
carrying instruments useful to observe the dynamics and the microphysical structure of precipitation systems during
their evolution. At present, the principal satellites observing the south Mediterranean basin to analyze the cloud
properties and estimate precipitation are: the METEOSAT satellite carrying on board the Visible and Infrared Spin
Scan Radiometer (VISSR); the satellites of the U.S. Defense Meteorological Satellite Program (DMSP) series carrying
on board the Special Sensor Microwave/Imager (SSM/I) radiometer; the Tropical Rainfall Measuring Mission (TRMM)
carrying on board the Precipitation Radar (PR), the TRMM Microwave Imager (TMI) and the Lightning Imaging
Sensor (LIS); and the EOS-TERRA satellite with the infrared MODerate–Resolution Imaging Spectroradiometer
(MODIS) In this paper, the different and synergic potential of these satellites for studying precipitating systems is
analyzed by focusing on the November 2001 flood that occurred in the south-western Mediterranean area. In addition,
we will discuss in some detail the potential of new-generation satellites and instruments that are about to become
operational or are planned to be launched in the next future.
1
INTRODUCTION
Satellite meteorology is rapidly changing since the launch in orbit of new sensors on satellite platforms will shortly
provide more and more advanced information on the cloud systems and their physical properties. Satellite data for
precipitation studies are crucial over the sea and in regions where rain monitoring from the ground is more difficult (for
example on mountainous and coastal regions). The uneven distribution of rain gauges and weather radars and the
relative lack of rainfall measurements over the oceans have significantly limited the use of data over extended regions
such as the Mediterranean basin. The new sensors not only add new channels to the traditional ones but also they
improve the both the spatial and the temporal sampling both in the visible/near infrared than in the microwave spectral
range.
In this paper the disastrous flooding event occurred on 9-10 November 2001 in Algiers is analyzed from a satellite
point of view to underline the capability of the satellite based observation to describe the cloud systems properties and
evolution, even in areas where ground measurements are difficult or impossible (such as over the sea and in country
where the rain monitoring is not very evolved).
2
THE ALGERIAN EVENT
An initial baroclinic instability associated with a large-scale tropopause fold over western Europe was at the origin
of the strong rains that affected the west and center of the Algerian country. The meteorological situation on 9 and 10
November 2001 has been characterized by an infiltration of stratospheric air on Spain and then on the Gibraltar Gulf
producing heavy rain over the Algerian coasts and the Balearic Islands. The 9 November, the surface condition was
dominated by a high pressure area centred south-west of Ireland and a low one centred on the Alpine zone. A cold front
is also present on the south-western Europe. At the same time, the 500 hPa situation showed a really different situation
with an area of low pressure extending from Scandinavian area to Gibraltar, where are strong winds with velocity until
150 Kts (at 300 hPa).
On the 10 November, the upper level situation was the same, characterized by the occlusion of the low pressure area
on the Spain and Northern Africa area, while the surface situation changed dramatically: a lowering of the pressure field
occurred over the Algerian coasts also at the lower level of the atmosphere (850 and 700 hPa) originating a depression
that evolved form the south-west Algeria towards north. This depression then evolved in a cyclone moving towards the
Balearic Islands while growing slightly, maintaining a perturbed flow on the west and centres of the Algerian country.
Also, this infiltration of cold air towards Gibraltar contributed to the advection of hot Saharan air towards the coasts
then towards the Western Mediterranean basin.
3
SATELLITE OBSERVATIONS
We investigate the Algerian event by using infrared (IR) and passive microwave MW (MW) data. They provide
complementary information about the cloud characteristic. At IR wavelengths clouds are opaque bodies: the radiance
measured by the Meteosat are emitted only from the top of the cloud. In this way it is possible to observe the cloud
systems evolution but not the precipitation. Form the WV band it is possible to study the different water content in the
atmosphere and then the air masses with different humidity that allow the formation of the clouds systems. On the
contrary, the MW data is able to see inside and above the cloud and then it is possible to derive microphysical
properties and precipitation estimate. Combined techniques IR-MW allow us to exploit the IR temporal sampling and
the MW capability to obtain inner cloud features, and to retrieve reliable precipitation estimate (Pinori et al., 2001).
Furthermore, we describe also the potentiality of other two instrument on board the TRMM satellite to study the
precipitating systems: the Precipitation Radar (PR) and the Lightning Imaging System (LIS).
3.1
IR Measurements: METEOSAT and MODIS
The infrared (10.5-12.5 µm) and water vapor (5.7 - 7.1 µm) data over the Mediterranean basin are provided from the
VISSR, flying on board the European Space Agency’s METEOSAT geostationary satellite. The ground resolution for
the IR and WV channels is approximately 5x7.5 km2 over northern Italy, with an image period of 30 minutes.
a)
b)
Figure 1. METEOSAT IR channel: a) 9th November at 12:00 UTC, b) on 10th November at 12:00 UTC.
From the IR series of images of the event (in Figure 1 we show only two examples) it is possible to see how the
frontal situation occurring during the first hours of the 9th November is suddenly modified from the injection of cold and
dry air coming from higher latitudes (the black stream crossing all Europe in Figure 2). Consequence of the presence of
this cold air mass was the rapid drop of the surface pressure, creating a vortex over the Algerian coast and the formation
of a cyclone during the 10th November. During the night between 10th and 11th November this cyclone moved from the
Algerian coasts towards the Balearic Islands.
a)
b)
Figure 2. METEOSAT WV channel: a) 10th November at 12:00 UTC, b) 11th November at 00:00 UTC.
Another instrument working in the IR spectral range and at our latitudes is MODIS. It flows on board the Terra
polar-orbiting spacecraft and measures radiances in 36 spectral bands in the VIS-IR spectral range with a resolution
varying from 250 m to 1 km. In Figure 3 are shown some products of the MODIS: the cloud mask (to discern cloudy
from clear pixels), the cloud top pressure and the cloud top temperature (Ackerman et al., 1998; Platnick et al., 2002).
Comparing Figure 3 with Figure 1a (the two images are taken with only one hour of difference) it is possible to see
the cyclone structure more in detail. The center of the cyclone is located just over Algiers. From the cloud top
temperature panel we can see that the cloud structure over the south-eastern Mediterranean basin shows an intermediate
temperature (between 230 and 260 K), characteristic of diffuse stratified precipitation. A convective area is present over
the central Italy, where the cloud top temperature are the lowest (200-210 K). The cloud top pressure confirm the
presence of the convective cells over Italy, where are present the highest clouds (400 hPa). Other small high clouds are
present in front of the Algerian coast, probably due to dissipating convective cells. The remaining part of the cloud
system is between 500 and 600 hPa, associated to the stratiform part of the system.
Clear
Prob
clear
Prob
cloudy
Cloudy
Figure 3. MODIS products for the 10th November 2001 at 10:45. From left to right: cloud mask, cloud top temperature (K) and
cloud top pressure (hPa).
3.2
MW Measurements: DMSP and TRMM
The SSM/I radiometer on board the USAF Defense Meteorological Satellite Program (DMSP) measures MW
radiation over a 1400 km swath width at four different frequencies: 19.4, 22.2 (only vertical), 37.0 and 85.5 GHz in dual
polarization at each frequency (both vertical and horizontal) (Wilheit, 1986; Spencer et al., 1989). The effective ground
resolution varies with the frequencies from 16x14 km2 (at the highest frequency) to 70x45 km2 (at the lowest).
a)
b)
Figure 4. Observed TBs at 85 GHz-V and corresponding IR images: a) TMI on 10th November 2001 at 2:05 UTC and METEOSAT
at 00:00 UTC; b) SSM/I on 10th November 2001 at 19:55 UTC and METEOSAT at 18:00 UTC.
The TMI is a five-frequency (10.7, 19.4, 21.3, 37.0, and 85.5 GHz) dual-polarized (vertical and horizontal)
conically-scanning microwave radiometer derived from the SSM/I instrument but with the addition of two 10.7 GHz
channel and on board the TRMM satellite (Kummerow et al., 1998).
The analyzed Brightness Temperatures (TBs) show some convective cells that represent the most intense part of the
storm (Figure 4), embedded in a stratiform envelope clearly visible in the IR images. The blue and cyan areas (in the
MW images), usually correspond to the cold parts of the cloud, where are present large ice particles (such as graupel),
and associated to the most raining regions. We can see that the most intense rain areas are located always along the
Algerian coast, where was recorded the most intense rain rates.
3.3
Other Measurements: Radar and Lightning from Space
On board the TRMM satellite are present also other two instrument devoted to the investigation of precipitation and
cloud microphysics: PR and LIS. PR is the first space-borne radar dedicated to the study of precipitating systems, able
to examine not only the horizontal spatial distribution of the precipitation fields, but also the vertical properties of the
precipitating systems. LIS is an optical sensor that detects lightning by looking for small changes in light intensity and
is capable of recognizing the presence of lightning also during the daylight hours.
Convective
Stratiform
Figure 5. A classification scheme of the different rain regimes in the left image (on the left). Event-rate computed from LIS data (on
the right). The event-rate reaches maximum values of 20 events/min.
Adamo et al. (2001) have recently shown that the use of concurrent data from the Precipitation Radar (PR) and
Lightning Image Sensor (LIS) instruments, onboard the TRMM satellite, give unique information about the connection
between the electrification and convection for the discrimination of convective and stratiform regimes (Figure 5). In
particular, properties of the cloud system observed can be obtained by using a spatial technique that combines PR
vertical profiles data and LIS information to discriminate stratiform and convective regimes. The average convective
profile is associated with larger values of reflective than the stratiform, in this way we can easily distinguish the two
different regimes.
4
FUTURE PERSPECTIVE FOR THE MEDITERRANEAN AREA
The launch of two new satellite during the past months will rapidly change the method to study the cloud systems
properties. The former is the Meteosat Second Generation (MSG), a new series of geostationary meteorological
satellites (Schmetz et al., 2002), that carries a twelve channels imager, called SEVIRI (Spinning Enhanced Visible and
Infrared Imager); the latter is the Advanced Microwave Scanning Radiometer for EOS (AMSR-E), a passive radiometer
system with twelve channels. Both present an enhanced spatial resolution compared with the instrument data used until
now and will improve the ability of sensing cloud structure and precipitation forming processes in the IR and MW
wavelengths. The challenges deriving from the exploitation of the combination of the channels are unprecedented and
the science community is at work to use the available data and be ready for the new ones.
Acknowledgments
This study has been funded within the frame of EURAINSAT -- a shared-cost project (contract EVG1-2000-00030)
co-funded by the Research DG of the European Commission within the RTD activities of a generic nature of the
Environment and Sustainable Development sub-program (5th Framework Program) and the Italian National Group for
Prevention from Hydro-geological Hazards (GNDCI).
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