One year of AERONET sun-photometric measurements at Lampedusa site: monthly averaged AOT comparison with satellite data and Saharan dust events detection

ÓPTICA PURA Y APLICADA – Vol. 37, núm. 3 - 2004

One year of AERONET sun-photometric measurements at Lampedusa site:

monthly averaged AOT comparison with satellite data and Saharan dust

events detection

Corradini S.

(1)

, Carboni E.

(2)

, Guerrieri L.

(1)

, Lombroso L.

(1)

, Pugnaghi S.

(1)

,

Santangelo R.

(1)

(1) Department of Materials and Environment – Geophysical Observatory

University of Modena and Reggio Emilia (Italy)

(2) Institute of Atmospheric Sciences and Climate (ISAC-CNR)

REFERENCES AND WEB LINKS.

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ABSTRACT:

In this work, one year (May 2003 – May 2004) of measurements collected by

AERONET #172 sun-photometer in Lampedusa has been analyzed. The

computed monthly averaged Aerosol Optical Depth (AOD) has been compared

with MODIS AOD retrieval.

Using both AOD and Angström coefficients, Saharan dust events has been

detected and meteorologically analyzed by means the NOAA HSPLIT 4

lagrangian trajectories and synoptic analysis.

[6] Kaufman, Y. J. et al. “The Smoke, Clouds and Radiation experiment in Brazil (SCAR-B)." Journal of Geophysical Research, Vol. 103, pp. 31783-31808, 1998.

[7] Ramanathan, V. et al. “The Indian Ocean Experiment: an integrated analysis of the climate forcing and e_ects of the great Indo-Asian haze", Journal of Geophysical Research, Vol. 106, pp. 28371-28398, 2001 [8] Gobbi G. P., Barnaba F., Giorgi R., Santacasa A., “Altitude-resolved properties of a Saharan dust event over the Mediterranean", Atmospheric Environment, Vol. 34, pp. 5119-5127, 2000.

[9] Holben B.N., T.F.Eck, I.Slutsker, D.Tanre, J.P.Buis, A.Setzer, E.Vermote, J.A.Reagan, Y.Kaufman, T.Nakajima, F.Lavenu, I.Jankowiak, and A.Smirnov, “AERONET - A federated instrument network and data archive for aerosol characterization”, Rem. Sens. Environ., 66, 1-16, 1998.

[10] Charlson R.J. and Heintzenberg J., "Aerosol forcing of climate". John Wiley & Sons, 1995. [11] MOVAS,Level-3 atmosphere monthly global product (MOD08_M3)].

http://modisatmos.gsfc.nasa.gov/tools_web.html

[12] Angström A., “The parameters of atmospheric turbidity”, Tellus, Vol. 16, pp. 64-75, 1964

[13] Moulin C., Lambert C. E., Dayan U., Masson V., Ramonet M., Bosquet P., Legrand M., Balkanski Y. J., Guelle W., Marticorena B., Bergametti G., Dulac F., “Satellite climatology of African dust transport in the Mediterranean atmosphere”, Journal of Geophysical Research, Vol. 103, No. D11, pp. 13137-13144, 1998 [14] Eck T. F. , Holben B. N., Reid J. S. , Dubovik O. , Smirnov A., O’Neill N. T. , Slutsker I. , Kinne S., “Wavelength dependence of the optical depth of biomass burning, urban, and desert dust aerosols”, Journal of Geophysical Research, Vol. 104, pp. 31333-31350, 1999

[15] Draxler, R.R. and Rolph, G.D., 2003. HYSPLIT (HYbrid Single-Particle Lagrangian Integrated Trajectory) Model access via NOAA ARL READY Website (http://www.arl.noaa.gov/ready/hysplit4.html). NOAA Air Resources Laboratory, Silver Spring, MD.

[16] Forecasting the dust storm. METED forecaster library web page http://meted.ucar.edu/mesoprim/dust/

1.- Introduction.

The radiative forcing induced by direct and indirect aerosol effects is one of the largest uncertainties still open in climate studies [1]. The retrieval of global distribution of aerosol optical properties, its variation and seasonal trends, represents one of the principal goal of atmospheric research. Many space-borne instrument are used to retrieve the aerosol optical properties (see for example: Advanced Very High Resolution Radiometer (AVHRR) [2], Total Ozone Mapping Spectrometer (TOMS) [3], MODerate resolution Imaging Spectroradiometer (MODIS) [4], Multi-angle Imaging SpectroRadiometer (MISR) [5]). Ground-based measurements are strongly request to improve and validate the satellite aerosol retrieval. Recently many comprehensive regional experiments in clean and polluted environments has been done (see for example [6], [7] and [8]) and AERONET network has been created. The AErosol RObotic NETwork (AERONET, [9]) is a federation of ground-based sun-photometers; the network goal is to provide globally distributed observations of aerosol optical properties in geographically different aerosol regimes.

The AERONET sun-photometer #172 is located in the area of Military Base LORAN-C, close to the west highest point of Lampedusa (150 m asl). Lampedusa (Lat. 35.5 North, Long. 12.6 East) is a small Italian island (a sort of triangle with a base of 4 km and 11 km long) located in the middle of the

east from African coast (Tunisia) and 200 km south from Sicily. Thanks to its location, far from anthropogenic sources of pollution, the instrument is particularly adapted to detect natural maritime aerosols and the frequent dust events coming from the Sahara desert. The desert dust particles represent a large fraction (of order 30-50%) of the naturally occurring tropospheric aerosol, seconds only to maritime ones [10].

In this work one year (from May 2003 to May 2004) of sun-photometric measurements collected at Lampedusa has been analysed and compared with satellite measurements made by MODIS (Section (2)). In Section (3) the AOD values and the Angström coefficients has been used to identify the most meaningful dust events and in Section (4) a complete meteorological analysis of the events has been performed.

2.- AERONET – MODIS monthly

comparison.

The AERONET #172 sun-photometer monthly averaged AOD (from May 2003 to May 2004) has been computed and compared with MODIS data [11]. The AOD comparison has been done at 550 nm that represents the MODIS reference wavelength for aerosol products. The 550 nm AERONET AOD has been computed, for each day, from the Angström formula:

where

τ(λ)

is the AOD at wavelength

λ

, and

α

and

β

the Angström coefficients. The latter coefficients has been obtained from linear regression fit using the logarithm of the four sun-photometer wavelengths (440, 670, 870, 1020 nm) versus the logarithm of the mean daily AOD (

τ

d) obtained

from AERONET data set.

The monthly average 550 nm AOD (

τ

m) has been

computed using a weighted mean relationship taking into account the number of daily measurement:

where

n

d is the number of measurements for each

day (

n

d ≥ 3). The monthly standard deviation has

been computed considering a Gaussian distribution. The Figure (1) shows the comparison between the monthly AOD retrieved by the sun-photometer (with standard deviation) and MODIS. In the Figure can be seen the good agreement between MODIS and sun-photometer retrieval from April 2003 to February 2004. The meaningful difference in May 2003 is due to the lack of AERONET measurements (the sun-photometer has been installed the 18 May 2003).

The monthly averaged trend also reproduce either the METEOSAT climatological behaviour, as seen by Moulin et al [13], then the monthly absolute AOD values. Between May to September the maximum AOD values occurs.

3.- Desert dust identification

Desert dust is dominated by coarse mode particles, composed of airborne desert soil material. For such particles the relationship between the AOD natural logarithm versus the wavelength natural logarithm results nearly linear [14].

The desert dust events has been identified by the Angström coefficients α and β, computed as shown in the previous Section.

These events are characterized by high AOD (i.e. high

β

Angström coefficient) values and low AOD spectral slope (i.e. low

α

Angström coefficient). Figure (2) shows the daily averaged Angström coefficients for all the sun-photometer measurements.

Analysing Figure (2) different meaningful events can be noted either in July, September and October 2003 than in February and March 2004. Three cases has been considered as example and for each, to confirm the aerosol dust Saharan origin, a complete meteorological analysis has been done: 17 and 24 July 2003 events, detected also from AERONET #254 sun-photometer located in Nicolosi (Mt. Etna Volcano) (see Figure (3)) and 22 February 2004 event that create a very interesting phenomena of red snow in the north of Italy.

Figure. 1.- Monthly averaged AOD from Lampedusa AERONET sun-photometer and MODIS retrieval (1x1

degree box centred in Lampedusa). The error bars represent the AERONET monthly standard deviation.

∑

∑

⋅

=

d d

d d d

m

n

)

n

(τ

τ

(2)

Figure. 2.- Daily averaged Angström coefficient for one year of sun-photometer measurements in Lampedusa. The

red and blue lines represents respectively the α and β Angström coefficients.

Figure. 3.- Daily averaged Angström coefficients computed from Lampedusa sun-photometer #172 and

Nicolosi (Etna) sun-photometer #254 for July 2003. The two desertic dust events of 17 and 24 July has been

4.- Meteorological analysis

4.a.- 17/07/03 and 24/07/03: Summer northernliest dust events

Both on 17 and 24 July 2003, the NOAA HSPLYT 4 backward lagrangian trajectories [15] for Lampedusa and Nicolosi (see Figure (4)), confirm the aerosol Saharan dust origin and shows that it was driven by an subtropical anticyclonic situation. SW wind blow on Africa whereas on our sites the main flow was from NW: in this case no Sahara dust was observed on northern Italy. We suppose that the main dust lifting process was due to the strong convecting motion over a large area due to the strong heating (more than 40°C at ground and 25°C at 850 hPa was reported by sounding stations over North Africa). This situation usually induce low size dust particle lifting due to low wind speed.

4.a.- 22/02/04: a winter prefrontal dust event

The Saharan dust case on 22 February 2004 carried a very big amount of dust that reach the Alps and Apennines with a massive dust deposition during a snowfall. Yellow-red sky on different Italian town has been observed. NOAA picture on 21/02/04 (not shown in this paper) highlights that dust cloud follow the jet stream, in agreement with the 250 hPa wind speed (see Figure (5)). The lagrangian trajectory (see Figure (6)) shown a well definite south-west path both in the lower level and in the higher for Lampedusa. The Modena new

level (300 m and 500 m, red and green lines) and a south-west path at 1500 m. This is a typical winter prefrontal case [16]: the dust source was ahead the red area (max wind >100 knots on Sahara region, see Figure (5)) and the strong wind can lift also bigger dust particle.

Conclusions.

In this work one year of sun-photometric measurements collected at Lampedusa has been analysed. The comparison between sun-photometer and MODIS monthly averaged aerosol optical depth shown a good agreement. The monthly averaged trend also reproduce the METEOSAT climatological behaviour [Moulin et al, l999] and values.

Both AOD and Angström coefficients has been used to identify the main Saharan dust aerosol transport; three meaningful events has been considered as example: 17 and 24 July 2003 and 22 February 2004. The back-trajectories analysis has been used to confirm the aerosol desert dust origin. The weather analysis shows a typical summer northernliest on 17 and 24 Jul 2003 due to a dynamic anticyclone over North Africa and Mediterranean: this situation drive a convective lifting of dust due to strong heating at surface. On 21-22 Feb 2004 the situation was a prefrontal case Figure. 4.- NOAA HSPLYT 4 backward lagrangian

trajectories for Lampedusa (top and bottom left plates) and Nicolosi (top and bottom right plates). Top plates: 17 July 2003 trajectories; bottom plates:

24 July 2003 trajectories.

Figure. 6.- NOAA HSPLYT 4 backward lagrangian trajectories for Lampedusa (left plate) and Modena

Appennines (right plate) for 22 February 2004 Figure 5.- NOAA ARL reanalysis [16], FNL

and the dust was mainly originated ahead of the jet stream due to the unstable air flow.

Acknowledgments.

We would like to thanks: the LORAN-C base for the basic logistic and technical support,