Role of aerosols on the Mediterranean solar

radiation

Elina TragouInstitute of Oceanography, National Centre for Marine Research, Athens, Greece

Alex LascaratosDepartment of Applied Physics, University of Athens, Athens, Greece

Received 12 December 2001; revised 22 March 2002; accepted 2 April 2002; published 11 February 2003.

[1] The shortwave radiation constitutes a major driving force for the oceanic thermohalinecirculation and must be accurately parameterized. An extensive comparison between themonthly mean solar radiation measured at numerous meteorological stations along theMediterranean coast and the estimated solar radiation from a widely used bulk formulaleads to two important conclusions: first, there is systematic overestimation of theshortwave radiation from the bulk formula at the station sites of about 25 W m�2

(averaged over a 30 year period), and second, there is significant interannual variability inthe observed solar radiation that cannot be explained either by the variability in theavailable data for the cloudiness or by the absorption due to water vapor content in theatmospheric column. As the overestimation occurs during summer cloud-free months, weassume that cloud attenuation is adequately parameterized and the discrepancy betweenthe observations and the estimates comes from the formula for clear-sky insolation. Acorrection to the clear-sky insolation formula is attempted on the basis of recent satellitedata on aerosol optical thickness index. Results from our analysis indicate that aerosolsmay provide an explanation for both the observed weaker shortwave radiation and itsinterannual variations. This implies that a different parameterization scheme must besought for the aerosol attenuation in the shortwave radiation formula. INDEX TERMS: 4504

Oceanography: Physical: Air/sea interactions (0312); 3359 Meteorology and Atmospheric Dynamics:

Radiative processes; 4215 Oceanography: General: Climate and interannual variability (3309); 3360

Meteorology and Atmospheric Dynamics: Remote sensing; KEYWORDS: Mediterranean solar radiation, bulk

formula correction, aerosol attenuation

Citation: Tragou, E., and A. Lascaratos, Role of aerosols on the Mediterranean solar radiation, J. Geophys. Res., 108(C2), 3025,

doi:10.1029/2001JC001258, 2003.

1. Introduction

[2] The amount of shortwave radiation reaching theocean surface is important in physical oceanography as itis a major boundary forcing for the simulations of theoceanic circulation and a crucial parameter for the estimatesof the water mass formation rates. Despite its significance, itremains a very poorly recorded quantity; solar radiationover the ocean is usually estimated either from empiricalparameterizations using the commonly recorded cloudinessand the noon solar elevation [e.g., Budyko, 1974; Reed,1977], or from radiative transfer models using satelliterecords [e.g., Bishop et al., 1997; Gupta et al., 1999].Because of the duration and the simplicity of the formerthey are still widely used in oceanography and climatemodeling, although it is well known that such parameter-izations often introduce systematic errors [e.g., Dobson andSmith, 1988].

[3] In semienclosed basins such as the Mediterranean Seathere is a simple and efficient method to test the validity ofdata sets and bulk parameterizations for the surface heatfluxes. This involves the comparison of the surface heatbudget with the known advective heat flux through theStrait of Gibraltar. Indeed, the restricted connection of theMediterranean with the open ocean allows for a relativelyaccurate estimate of the oceanic advective heat flux. Thelatest estimate is �5 ± 1 W m�2 [Macdonald et al., 1994].Several researchers [e.g., Bunker et al., 1982; Garrett et al.,1993; Gilman and Garrett, 1994] have examined in the pastthe validity of the estimates for the air-sea fluxes in theMediterranean Sea using this constraint as a guide. Signifi-cant differences between the estimated surface fluxes andthese implied by the exchange through the strait are system-atically found. In previous studies, the differences wereexplained by the overestimation of the shortwave radiationand/or the underestimation of the longwave, latent andsensible heat fluxes.[4] In particular, the basin average, long-term mean total

shortwave radiation in the Mediterranean was first estimated

JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 108, NO. C2, 3025, doi:10.1029/2001JC001258, 2003

Copyright 2003 by the American Geophysical Union.0148-0227/03/2001JC001258$09.00

7 - 1

by Bunker et al. [1982]. They used the Budyko [1963]formula including the albedo from Payne [1972] and ships’observations for the years 1941–1972 from the U.S.National Climate Center. The estimated solar radiation byBunker et al. [1982] was 202 W m�2, although a reductionof about 5 W m�2 (or 2.5%) due to aerosols was consideredplausible.[5] A later estimate by Garrett et al. [1993] using the

Comprehensive Ocean Atmosphere Data Set (COADS)[Woodruff et al., 1987] for the 45 year period 1945–1989and the formula of Seckel and Beaudry [1973] with Reed’s[1977] cloud formula, originally gave Qs = 202 W m�2,same as the Bunker et al. estimate. Garrett et al. suggestedthat this is an overestimation, possibly due to the neglectedeffect of aerosols, and drew attention on the uncertainty inthe interannual changes of Qs estimated from atmosphericradiation formulas based on cloud reduction alone. In theextreme hypothetical case where all of the differencebetween the known and the estimated heat budget comesfrom the overestimation of the insolation, a reduction of 36W m�2 (or 18%, i.e., Qs = 166 W m�2) must be applied tobalance the budget.[6] In an attempt to objectively quantify this reduction

Gilman and Garrett [1994] concluded that the correct usageof the Reed cloud formula resulted in a decrease of 13 Wm�2 (or 6.5%). A further decrease of about 6 W m�2 (or3%) was estimated due to a seasonally varying transmissioncoefficient, introduced to match measurements of a groundstation in Cyprus. Overall, their corrections to the solarradiation formula resulted in a value of Qs = 183 W m�2.[7] The overestimation of the solar radiation from the

bulk formula in the Mediterranean was also demonstratedby Schiano [1996], who was the first to compare theformula with direct marine measurements of Qs duringseveral cruises in the Western Mediterranean in the period1989–1994. She showed that Reed’s cloud formula per-formed reasonably well under cloudy conditions, but underclear sky there was a systematic overestimation of theestimated insolation. She also pointed out the role of thevarying water vapor density in attenuating the solarradiation.[8] Finally, for completeness, we mention that Castellari

et al. [1998] reestimated the solar radiation using the sameformula and data set as Garrett et al. [1993] and Gilmanand Garrett [1994] but for the 9 year period 1980–1988and found a value of 202 W m�2. The above results aresummarized in Table 1.

[9] Because of its geographic location and the regionalweather systems, the atmosphere over the Mediterraneanhas high aerosol concentration [Gilman and Garrett, 1994],both mineral and anthropogenic, as well as high water vapor

content [Schiano, 1996]. Both aerosols and water vaporattenuate the incoming shortwave radiation, but are para-meterized with constant coefficients in the clear-sky inso-lation formula, which possibly overestimates the insolation.[10] In general, previous studies agree that Qs is over-

estimated by the bulk formula in the Mediterranean, becauseof the neglect of the seasonal and spatial variability ofaerosols and water vapor density. The attempts by Gilmanand Garrett [1994] to quantify the bias were based on a fitto the observations from only one station in the easternMediterranean for a 4 year period. Schiano’s [1996] veryinteresting comparisons with direct ship observations werein the Western Mediterranean and only for 88 days over aperiod of 6 years. A systematic long-term basin-scalecomparison of the formula estimates with the observationsis lacking in the Mediterranean. Taking into account thepeculiarities of the Mediterranean area a correction to theformula for radiation is needed.[11] In this paper we present the results of a comparison

between direct measurements of solar radiation at severalcoastal meteorological stations during a 30 year period andthe estimates from a commonly used bulk formula. Toaccount for the spatial and temporal variability of aerosolsover the whole Mediterranean we have used satellite datafor the aerosol optical thickness calibrated at the groundstations and attempted to correct the Qs estimates. In section2, we present bulk formula estimates of solar radiation inthe Mediterranean, and in section 3, the ground records. Thesatellite data are described in section 4 and used in thecorrective method which is introduced in section 5. Ourresults are discussed in section 6.

2. Bulk Formula Estimates

[12] The downward component of the insolation Qs at thesea surface is often calculated from the formula

Qs ¼ QCS 1� cnnþ 0:0019hð Þ: ð1Þ

This is a combination of the cloud reduction formula fromReed [1977], originally derived for daily cloud cover values,and the formula for the clear-sky irradiance QCS from List[1958]. The latter comes from the sum of direct andscattered solar radiation. This is

QCS ¼ 1

2Q0 Tr1=cos z þ 1� Að Þ

� �; ð2Þ

where Q0 is the incident radiation at the top of theatmosphere (Q0= S0 cos z, with S0 = 1370 W m�2 the solarconstant, and z the solar zenith angle), Tr is the transmissionfactor for a clear atmosphere (held constant at 0.7), and A =0.09 is the absorption factor due to ozone and water vapor.In equation (1) the reduction due to clouds is parameterized,following Reed [1977], by the product of the cloud fractionn and a coefficient cn = 0.62; h is the noon solar altitude indegrees.[13] Reed [1977] pointed out that when the cloud cover

fraction is smaller than 3 octas the calculated solar radiationQs from equation (1) is larger than the clear-sky irradianceQCS and an overestimation of Qs is introduced. Therefore,the cloud reduction formula should be truncated at 1; this is

Table 1. Previous Estimates of the Mediterranean Solar Radiation

Reference Formula Period Qs, W m�2

Bunker et al. [1982] Budyko 1941–1972 202Garrett et al. [1993] Seckel & Beaudry

and Reed1945–1989 202

Gilman andGarrett [1994]

Seckel & Beaudryand Reed

1945–1989 183

Castellari et al. [1998] List and Reed 1980–1988 202

7 - 2 TRAGOU AND LASCARATOS: AEROSOL ROLE ON MEDITERRANEAN SOLAR RADIATION

applied in our estimates. Moreover, it should be noted thatReed’s cloud reduction formula was derived for estimatingdaily means of solar radiation, therefore the effects of usingmonthly instead of daily values are ignored in our estimates.These effects are discussed later in the paper. Reed’s formulawas also derived using another formula for the clear-skyradiation by Seckel and Beaudry [1973], though Schiano[1996] showed that this formula for the clear-sky radiation isnot appropriate for the Mediterranean Sea conditions.[14] We note here that previous estimates of Qs men-

tioned in Table 1 are computed using the formula

Qs ¼ QCS 1� cnnþ 0:0019hð Þ 1� að Þ; ð3Þ

which gives the total shortwave radiation budget at seasurface, i.e., the downward minus the upward component.In our analysis we will use estimates from equation (1)because they are comparable to the observed globalradiation.[15] An up-to-date estimate of the solar radiation from the

formula is possible by extracting the cloudiness values overthe Mediterranean Sea from the revised COADS by da Silvaet al. [1994a, 1994b, 1994c, 1994d, 1994e] at the Depart-ment of Geoscience of the University of Wisconsin-Mil-waukee (henceforth referred to as UWM/COADS). Thisdata set covers the 49 year period from January 1945 toDecember 1993 and comes from the original COADSrelease objectively analyzed on a 1� � 1� global grid. Thedata set includes corrections to the wind speed, cloud cover,and Present Weather observations, as well as global esti-mates for the four components of the heat budget evaluatedusing standard bulk formulas.[16] We have calculated Qs using equation (1) and the

monthly cloud cover fraction from UWM/COADS. For theMediterranean Sea the Qs average is 218 W m�2. This valueincludes a small decrease (of about 3 W m�2) due to thecorrect application of the Reed cloud formula, i.e., truncatedfor small cloudiness values in order to obtain solar radiationalways less than the clear-sky insolation. However, monthlyrather than daily cloud values have been used in our

estimates, which, according to Gilman and Garrett[1994], can lead to an overestimation of about 4%. Thespatial distribution of the 49 year mean of this new estimateis presented in Figure 1 which shows, as expected, a north-south gradient in Qs.[17] Results from formula (1) have been compared in the

past against ground truth observations to test the perform-ance of this bulk formula in the Mediterranean conditions.For example, Schiano [1996] used short-term data fromship’s records during various cruises, while Gilman andGarrett [1994] used a few years of data at a single site inCyprus. Still, there are no long-term, large-scale compar-isons that would give a statistically robust result. The resultsof these previous comparisons will be discussed later in thepaper. At this point, to check the validity of the bulkformula (equation (1)) we compare our estimates with anextensive data set of ground truth observations for Qs

presented in the following section.

3. Ground Truth Observations

[18] Observations of surface solar irradiance are availablefrom the World Radiation Data Centre (WRDC) at numer-ous meteorological stations along the Mediterranean coast.These include monthly mean measurements of Qs under anycondition (both cloudy and clear sky) from January 1964 toDecember 1994 at the locations shown in Figure 2. These23 stations have been selected from a larger group of 83stations, so that the selected stations are evenly distributedover the whole Mediterranean in order to reduce the bias ofa nonuniform distribution; there are several stations avail-able along the northern coast of the Mediterranean, but veryfew along the North African coast. The specific stationswere also selected for their duration of observations andtheir elevation above sea level (which is less than 135 m andonly in one case at 191 m).[19] Details about the data available at each station are

presented in Table 2. The table includes information aboutthe station location, elevation above sea level, the period ofobservations, the number of monthly data available at each

Figure 1. Long-term mean net shortwave radiation Qs estimated from equation (1) with the cloudcorrection and cloudiness from UWM/COADS.

TRAGOU AND LASCARATOS: AEROSOL ROLE ON MEDITERRANEAN SOLAR RADIATION 7 - 3

station, the mean Qs and the standard deviation, as well asthe mean Qs of contemporaneous estimated values (usingformula (1) and cloudiness data from UWM/COADS) andtheir difference. We note that for all of the estimates of Qs

we have allowed for a truncation to Reed’s formula forsmall cloudiness values. We compared the WRDC obser-vations with bulk formula estimates at the grid pointsoverlapping with or nearest to the coastal stations. Thedifference of the estimates from the measurements is pos-itive at all but one station, indicating that there is over-estimation in Qs from the bulk formula. The meandifference is about 25 W m�2.[20] All station matches (around 5300 data pairs) are

shown in the scatter plot of Figure 3. We found that around3900 data pairs of monthly mean values show overestima-tion of Qs, mostly for high Qs values, of as much as 100–150 W m�2. There are also around 1400 data pairs, whichshow a slight underestimation, mostly for low Qs values.[21] The time series of measured and estimated values of

Qs are shown in Figure 4 for four indicative stations. Theoverestimation of the solar radiation estimated with formula(1) is evident at all four stations.[22] We have also estimated the mean time series from all

the selected 23 stations shown in Figure 5a. The solid linecorresponds to the mean values of all the available meas-ured values for each month, and the dashed line correspondsto the mean value of the estimated Qs (included only if thecontemporaneous measured value is available) for eachmonth, at grid cells next to the stations which have data.It should be noted that the insolation from formula (1) isfound to be systematically larger than Qs observed at theWRDC stations throughout the low-pass filtered time series.This indicates that some attenuating processes are notproperly parameterized in the bulk formula. Moreover, itis obvious that there is strong interannual variability in theobserved Qs compared to the estimated insolation. Theseasonal signal shows that the summer values are over-estimated whereas there is a small underestimation in the

winter solar radiation. We also note that data are not alwaysavailable for all 23 stations during the 30 year period underexamination. The number of observations for each month ispresented in Figure 5d, which shows that there is shortageof data during the last pentad.[23] The fact that Figure 5a shows similar results as the

randomly chosen stations in Figure 4 is a strong indicationthat, statistically, the reason for the discrepancies betweenthe estimates and the records is the bulk formula, not thepossible errors in the records. We also note that the meansolar radiation from the 23 stations is very similar to themean solar radiation obtained from all the 83 stations (notshown here).[24] The 30 year mean of the estimated Qs at all stations

is 210 W m�2, whereas the observed Qs is 185 W m�2,smaller by about 25 W m�2. This discrepancy can beattributed to systematic errors due to erroneous parameter-izations for the various attenuating factors (such as aerosols,water vapor, and other atmospheric molecules) in the clear-sky formula (2), and/or to systematic errors in the cloudcover observations and the cloud reduction formula.[25] For the cloud reduction in particular, Schiano [1996]

showed that Reed’s [1977] formula performs reasonablywell in the Mediterranean, though it is possible that there areerrors in the available data for the cloud cover. Never-theless, Reed’s formula applies only for cloudiness greaterthan approximately 3 octas (equivalent to 0.38 cloudfraction [Reed, 1977]). The basin-averaged 49 year meancloudiness from the UWM/COADS over the whole Medi-terranean is 0.42, close to the lower limit of 0.38; the easternMediterranean which occupies the greatest part of the basinhas a mean cloudiness of 0.40, and the Western Mediterra-nean has 0.47. Moreover, Figures 4 and 5a clearly show thatthe greatest differences between the observed and theestimated solar radiation exist during summer months whencloud cover is very small in the Mediterranean, below thelimit of 3 octas. Therefore we will assume that any differ-ence between the observations and the estimates comes

Figure 2. Locations of the selected 23 World Radiation Data Centre stations along the Mediterraneancoast.

7 - 4 TRAGOU AND LASCARATOS: AEROSOL ROLE ON MEDITERRANEAN SOLAR RADIATION

from the clear-sky formula only. Further discussion on thisassumption will be provided in section 6.[26] Gilman and Garrett [1994] first showed that for the

Mediterranean insolation there is a systematic error in theparameterization of atmospheric attenuation in the clear-skyradiation formula. The transmission coefficient Tr, which isused to parameterize extinction due to absorption andscattering by aerosols and other atmospheric constituents,is assumed to be uniform and constant. However, concen-trations of natural and anthropogenic aerosols may varyboth in space and time so that considering aerosol effects asconstant may lead to systematic errors in the Qs estimate forcertain areas of the world. The Mediterranean Sea appearsto be in such a region with high aerosol load. This is evidentfrom the global distributions of the aerosol optical thicknessindex detected by polar orbiting satellites (e.g., AVHRR

Pathfinder) over the oceans, first presented by Husar et al.[1997], who also showed that there is significant seasonalvariability of the aerosol concentration in the atmosphere.This currently available aerosol optical thickness indexcannot be simply related to the transmission coefficient Trof clear-sky insolation, because the data do not provideinformation about aerosol properties (i.e., size, shape, andcomposition), and lack the required calibration [Lacis andMishchenko, 1995].[27] To quantify the attenuation due to aerosols in the Red

Sea, Tragou et al. [1999] suggested a simple calibrationscheme of satellite measurements of the aerosol extinctionindex depending on the availability of ground truth obser-vations. Here we will follow a similar approach in theMediterranean where there is a relatively large number ofground truth measurements of Qs spanning several years

Table 2. Comparison of Measured Solar Radiation From the WRDC and the Estimated Values Using the Bulk Formula (1) and

Cloudiness From UWM/COADS

Index Number LocationStationCode

Elevation,m

Period ofObservations

Number ofMonthlyData

Mean ± StdObservations

Mean ± StdUWM/COADS Difference

1 Dar el Beida (Alger)36�430N 3�150E

603900 25 1970–1975 62 194.6 ± 75.7 215.2 ± 97.9 20.6

2 Mersa Matruh (Egypt)31�200N 27�130E

623060 25 1981–1993 144 229.9 ± 79.6 227.1 ± 98.2 �2.8

3 Ajaccio (France)41�550N 8�480E

77610 6 1970–1993 271 175.5 ± 83.2 193.6 ± 101.8 18.0

4 Nice (France)43�390N 7�120E

76900 4 1967–1993 288 167.3 ± 79.6 184.3 ± 100.4 17.0

5 Perpignan (France)42�440N 2�520E

77470 43 1980–1993 154 162.2 ± 73.0 189.2 ± 99.2 27.0

6 Andravida (Greece)37�550N 21�170E

166820 17 1978–1993 134 176.6 ± 92.1 217.4 ± 108.4 40.8

7 Athens Obs.(Greece)37�580N 23�430E

167140 107 1964–1986 264 181.9 ± 80.7 217.7 ± 107.3 35.8

8 Tymbakion (Greece)35�000N 24�450E

167590 7 1977–1993 133 159.6 ± 73.3 231.1 ± 100.5 71.5

9 Bet Dagan (Israel)32�000N 34�490E

401790 30 1964–1978 164 224.5 ± 76.8 243.5 ± 93.4 19.1

10 Alghero (Italy)40�380N 8�170E

165200 23 1964–1989 298 189.1 ± 86.9 196.6 ± 101.8 7.5

11 Amendola (Italy)41�320N 15�430E

162610 57 1964–1993 356 182.2 ± 82.8 200.5 ± 104.7 18.3

12 Brindisi (Italy)40�390N 17�570E

163200 15 1964–1993 357 172.8 ± 86.1 204.8 ± 106.3 32.0

13 Genova/Sestri (Italy)44�250N 8�510E

161200 2 1964–1989 304 149.9 ± 74.8 186.4 ± 100.5 36.5

14 Messina (Italy)38�120N 15�330E

164200 59 1964–1993 348 184.8 ± 81.8 210.8 ± 104.5 26.0

15 Napoli (Italy)40�510N 14�180E

162890 88 1964–1989 293 173.1 ± 82.9 202.9 ± 104.4 29.8

16 Pantelleria Is. (Italy)36�490N 11�580E

164700 191 1964–1993 301 184.4 ± 79.3 219.2 ± 101.1 34.9

17 Roma/Ciampino (Italy)41�480N 12�330E

162390 129 1964–1993 294 179.8 ± 81.5 201.2 ± 102.7 21.4

18 Beyrouth Aprt (Lebanon)33�490N 35�290E

401000 29 1964–1981 157 192.3 ± 75.0 226.4 ± 96.6 34.1

19 Qrendi, Malta Is. (Malta)35�500N 14�260E

165960 135 1964–1978 105 210.2 ± 81.8 220.5 ± 101.6 10.3

20 Murcia (Spain)38�000N �1�100E

84300 61 1981–1993 139 199.0 ± 74.9 210.9 ± 94.1 11.9

21 Palma de Mallorca (Spain)39�330N 2�370E

83010 6 1981–1993 127 182.1 ± 77.4 198.3 ± 97.0 16.2

22 Sidi Bouzid (Tunisia)36�520N 10�210E

607175 127 1968–1993 305 199.2 ± 78.7 210.4 ± 101.0 11.2

23 Bar (Yugoslavia)42�060N 19�060E

134610 4 1964–1991 306 178.3 ± 85.4 199.5 ± 106.2 21.2

TRAGOU AND LASCARATOS: AEROSOL ROLE ON MEDITERRANEAN SOLAR RADIATION 7 - 5

from coastal meteorological stations provided by theWRDC.

4. Aerosols Over the Mediterranean Sea

[28] Sea Satellite data for the index of optical thicknessare available online from the U.S. NOAA (National Oceanicand Atmospheric Administration). These include two datasets for the aerosol optical thickness index ts

A derived overthe ocean from NOAA/AVHRR: The Pathfinder data prod-uct for the monthly mean aerosol optical thickness providedon a 1� � 1� resolution for the period from July 1981 toSeptember 1994 [Stowe et al., 1997]. However, this versionof AVHRR aerosol data, despite covering a relatively longperiod of 14 years, covers only 45% of the Mediterraneanarea where UWM/COADS data are available. A bettercoverage is provided by another product of the AVHRRthe Aerosol optical thickness (AOT) 100 km field. This dataset includes weekly values of AOT computed in near realtime, for the period November 1998 to April 2001. The twoproducts are the same in terms of the method used tocompute optical thickness from the AVHRR channel 1reflectance. However, the way in which the two productsare gridded is different with the AOT 100 km field sampledat higher densities near the coastal areas. The spatialdistribution of the mean AOT from the two products is

Figure 3. Monthly mean values of coastal solar radiationobservations from the WRDC versus contemporaneous bulkformula estimates using the UWM/COADS cloudiness.

Figure 4. Solar radiation time series at four WRDC stations. The dashed line corresponds to theestimated Qs from the List-Reed formula and cloudiness from the UWM/COADS. The solid linecorresponds to the observed Qs from the WRDC records. The thin lines are the monthly values, whiletheir thick counterparts are the low-pass filtered time series showing the interannual variations; the thickline scale is on the right axis of the diagram. The filter is a 23-point triangular filter that removes theseasonal cycle.

7 - 6 TRAGOU AND LASCARATOS: AEROSOL ROLE ON MEDITERRANEAN SOLAR RADIATION

shown in Figure 6, whereas the time series of the basinaverage of the two products is shown in Figure 7. We notethat the basin average value of the AOT 100vkm fieldproduct is lower than the Pathfinder product as the formerincludes values at areas of lower aerosol load as shown inFigure 6.[29] Figure 7 shows that there is a distinct seasonal and

interannual signal which implies that a constant value forthe transmission coefficient could be a source of error in theclear-sky insolation estimate. It is also noted that there is aconspicuous peak during 1992, also present in time seriesfrom other areas in the world, which is probably related tothe eruption of Mount Pinatubo in June 1991. High aerosolload in the atmosphere over the Mediterranean may havecaused stronger attenuation to the insolation during thoseyears and may be associated with the so-called Mediterra-nean transient [Roether et al., 1996; Lascaratos et al.,1999]. However, such a signal of reduced Qs does notappear in the observations, perhaps due to the scarcity ofdata during that period, so further conclusions are notpossible at this stage.[30] In our study the availability of aerosol data near

the coastal stations is essential, therefore we have chosento proceed with the AOT 100 km field, although thepossible interannual variability is missing from this dataset. We also note that this data set covers a time periodwhere no solar radiation data (either estimates or obser-vations) are available. To overcome this problem we have

averaged the monthly mean values and used a climato-logical spatially varying seasonal cycle, so that only theseasonal, not the interannual variability of the aerosols isincluded in our analysis. In the next section we will

Figure 5. Average time series of the observed solar radiation at 23 WRDC stations (solid line) and thecontemporaneous values of the estimated solar radiation (dashed line) from (a) original estimates (withoutcorrections), (b) corrected estimates, and (c) estimates using Schiano’s corrections. The line style is sameas in Figure 4. (d) Number of observations available at each month of the time series.

Figure 6. Spatial distribution of the mean aerosol opticalthickness index from (top) the Pathfinder AOT product and(bottom) the AOT 100 km field.

TRAGOU AND LASCARATOS: AEROSOL ROLE ON MEDITERRANEAN SOLAR RADIATION 7 - 7

attempt to quantify the aerosol effect on the solar radia-tion of the Mediterranean.

5. Correction to Qs

[31] In the Qs formula the effect of aerosols is assumedconstant globally. In order to bring the calculated surfacesolar irradiance (denoted Qs) in agreement with surfaceobservations from WRDC (denoted QsG

), we define thetransmission anomaly coefficient Tr* as

Tr* ¼ QsG

Qs

: ð4Þ

This transmission anomaly coefficient is defined bycomparing ground measurements QsG

with the insolationestimated from equation (1) for the same period of time, at

the 1� � 1� grid point containing or nearest to the locationsof the observations, using cloudiness data from UWM/COADS. The resulting transmission anomaly coefficient atall stations is shown in Figure 8a. We note that there is aremarkable interannual and seasonal variability with Tr*less than one during summer and slightly greater than oneduring winter (Figure 8b). This implies that the insolationhas been overestimated by the bulk formula during summerand slightly underestimated during winter.[32] The transmission factor Tr* calculated above can be

used to calibrate satellite data for the optical thickness indexin order to obtain the spatial and temporal variability of Tr*everywhere in theMediterranean, not only at the station sites.[33] We assume that the satellite data for the index ts

A

define a transmission coefficient TrsA = exp(�ts

A/cos z) dueto aerosol scattering; aerosol absorption appears in a differ-ent term. In order to estimate the clear-sky radiation at thesea surface we introduce a coefficient fc for the attenuationof Q0 due to absorption and scattering by atmosphericmolecules and absorption due to aerosols, so that clear-sky insolation at the sea surface is given by

QCSsat ¼ Q0 fcTrAs : ð5Þ

[34] We assume that the transmission anomaly coefficientTr* is only due to errors in the estimation of QCS and thereare no errors in the cloudiness or the parameterizationscheme for the cloudiness reduction. Then, from equations(1), (2) and (4) the clear-sky insolation estimated from theformula at the coastal stations and corrected by Tr* is givenby

QCSG ¼ Q0 f Tr*; ð6Þ

Figure 7. Time series of the basin-averaged aerosoloptical thickness index in the Mediterranean Sea. The thinline corresponds to the mean value from the PathfinderAOT product, whereas the thick line corresponds to theAOT 100 km field.

Figure 8. (a) Time series of the transmission anomaly coefficient equation from equation (4) estimatedat all WRDC coastal stations in the Mediterranean Sea and (b) its mean seasonal cycle. (c) Calibrationfactor for the satellite transmission coefficient index estimated from the comparison of the insolation fromsatellite data with the ground truth records and (d) its mean seasonal signal. Dashed lines are the errorbounds for Tr* and fc.

7 - 8 TRAGOU AND LASCARATOS: AEROSOL ROLE ON MEDITERRANEAN SOLAR RADIATION

where f = 12(Tr1/cos z + (1 � A)). Then, from equations (5)

and (6), the relation between the clear-sky radiationestimated from satellite data and the clear-sky radiationfrom the corrected formula to the ground stations is

fcTrAs ¼ f Tr*: ð7Þ

[35] To calibrate the clear-sky insolation from the satellitedata to the measured clear-sky insolation, the coefficient fcmust be adjusted so that

fc ¼ fTr*

TrAs: ð8Þ

At the coastal stations the calibration factor fc is evaluatedusing the average monthly values of Tr*, the monthlyaerosol transmission coefficient Trs

A (at the grid pointsnearest to the coastal stations), and the monthly values ofthe parameter f. Because of the short duration of Trs

A wehave used the monthly mean values of the 2.5 years ofavailable data, i.e., one seasonal cycle. The time series of fcestimated at all ground stations and its seasonal cycle isshown in Figures 8c and 8d. We note that the calibration

factor fc includes all the corrections required to match QCS

from satellite data to the measured quantities at groundstations. These include the attenuation of Q0 due toabsorption and scattering by atmospheric molecules,aerosol absorption, as well as errors in the estimation ofTrs

A.[36] The estimated fc at all 23 stations has been averaged

and the temporally varying calibration factor fc has beenused, along with the factor f and the satellite data for Trs

A, toestimate the transmission anomaly coefficient Tr* every-where in the Mediterranean Sea (Figure 9). The spatialdistribution of Tr* shows that Qs has been overestimated inareas of high aerosol load. For example, the solar radiationin the Ionian Sea and the Northern Aegean has been over-estimated.[37] Figure 10 shows the basin average time series of the

estimated Tr* and its seasonal cycle. Besides the smallinterannual variability in Tr* there is slight underestimationduring winter months (Tr* > 1), while there is overestima-tion during summer.[38] The surface solar radiation obtained with equation

(1) is multiplied with this spatially and seasonally varyingtransmission coefficient Tr*. The resulting corrected QsC

=QsTr* is presented in the upper panel of Figure 11. We note

Figure 9. Long-term mean of the transmission anomaly coefficient Tr* estimated from equation (7)using data for the satellite transmission coefficient Trs

A.

Figure 10. Basin average of the transmission anomaly coefficient Tr* estimated from equation (7) usingdata for the satellite transmission coefficient Trs

A and its seasonal cycle.

TRAGOU AND LASCARATOS: AEROSOL ROLE ON MEDITERRANEAN SOLAR RADIATION 7 - 9

that, although the north-south gradient of the solar radiationremains similar to that of the originally estimated Qs, thespatial distribution is markedly different with smaller scalefeatures in the long-term mean Qs. In particular, it is worthmentioning that the shortwave radiation is significantlyreduced in the Libyan basin. The effect of the correctionis more clearly seen in the lower panel of Figure 11, whichshows the 30 year mean difference between the original andthe corrected Qs over the whole Mediterranean. The correc-tion is greater in the Ionian Sea where the concentration ofaerosols is large and near urban areas such as the NorthAegean and the North Adriatic Seas.[39] Overall, the 30 year mean (from January 1964 to

December 1993) of the basin-averaged corrected estimate isfound to be QsTr* = 184 W m�2 smaller by 33 W m�2

compared to the original estimate of 217 W m�2 for thesame period.

6. Comparison to Corrected Qs

[40] A validation of our correction to the estimated Qs

comes from the comparison with the observed values at thecoastal stations. Admittedly, the comparison is performedbetween two data sets that are not independent and theimprovement is inevitable. However, we carry out thiscomparison because the averaging of the calibration factorfc at the 23 stations, and the use of one seasonal cycle of thespatially varying aerosol extinction Trs

A introduce errors inour method. These errors are quantified with the followingcomparison and offer a possible validation of our method.

Figure 11. (top) Long-term mean (30 year period) of thecorrected shortwave radiation given from QsTr*. (bottom)Long-term mean difference between the original andcorrected Qs.

Figure 12. Solar radiation time series at 4 WRDC stations same as in Figure 4. The line style is same asin Figure 4.

7 - 10 TRAGOU AND LASCARATOS: AEROSOL ROLE ON MEDITERRANEAN SOLAR RADIATION

[41] In Figure 12 we present the observed time series atfour stations (same as those in Figure 4) compared to thecorrected values. The improvement is clearly seen in theseasonal signal. The filtered time series, although improvedcompared to the original estimates, do not follow exactlythe details of the recorded time series.[42] A more thorough comparison of the corrected esti-

mates with the observations is possible from the statistics ofeach station. In Figure 13 we show the differences of thecorrected mean Qs estimates from the mean observed Qs ateach station. The three panels correspond to the differencesof the mean summer values (Figure 13(top)), annual meanvalues (Figure 13(middle)), and the winter (Figure 13(bot-tom)). The root mean square of the differences between theoriginal estimate and the observations at all 23 stations arepresented in Table 3.

[43] The improvement in the estimates is evident in theannual mean values, but the summer months values aresignificantly ameliorated compared to the original estimates,whereas there is a negligible deterioration in the wintervalues. Figure 13 shows that the systematic positive bias inthe summer and the annual mean values are significantlyreduced, although the corrected summer values show asmall negative bias.

[44] Finally, we present the results from our analysis andcorrections to Qs (which have been attributed to aerosolattenuation), and compare them to the original mean obser-vations (Figure 5b). Both the long-term mean and theseasonal and interannual variations are reasonably wellrepresented in the corrected data.[45] As mentioned in section 4, our corrective method

is based on the assumption that all the difference betweenthe observed and the estimated solar radiation comesfrom erroneous parameterization in the clear-sky formula.This assumption is strengthened by the fact that most ofthe reduction introduced by our method occurs duringcloud-free months, i.e., when the cloud reduction formulais redundant. For example, in Figure 14 a decade (1980–1990) of original and corrected estimates are plottedalong with the contemporaneous cloudiness fraction atstation 164200 (Messina, Italy). Figure 14 clearly showsthat the reduction is applied during summer, cloud-freemonths.[46] Corrections to Qs have been suggested by other

researchers in the past. Gilman and Garrett [1994], forexample, found that daily (instead of monthly) data for thecloudiness in the Mediterranean may cause a reduction ofup to 4% (or about 8 W m�2) to the solar radiation. Thiseffect is overlooked in our study, because of the monthlymean data used here. However, this reduction is too small toaccount for the discrepancy between the estimates and theobservations. They also showed that Qs may be furtherdecreased by 6 W m�2 due to aerosol attenuation. Thisappears to be a reasonable result within error bounds, buttheir estimate was tested against ground truth observationsfrom a single station in the middle of Cyprus, away from thecoast.[47] In a later study Schiano [1996] analyzed daily Qs

data recorded during several cruises in the western Medi-terranean and suggested that the clear-sky radiation must becorrected for both the aerosol attenuation and the watervapor content in the atmospheric column. For the aerosoldepletion she introduced a weaker transmission coefficientof 0.66, but temporally and spatially uniform, while for theabsorption due to water vapor she suggested a step functionof the absorption coefficient A, depending on the watervapor density; A = 0.15 when rv � 12 gm�3.[48] The application of Schiano’s [1996] corrections to

the bulk formula using cloudiness and water vapor datafrom the UWM/COADS resulted in a small reduction of 10W m�2 to the 30 year mean Qs to 207 W m�2, but still highcompared to the 185 W m�2 estimated from the WRDCobservations. A comparison of the time series estimatedusing Schiano’s corrections with the ground truth datashows that these corrections cannot capture either the

Figure 13. Difference between the mean value of theoriginal estimates and the observations (solid squares) ateach ground station and difference between the mean valueof the corrected estimates and the observations (shadedcircles), for (top) summer (July), (middle) annual mean, and(bottom) winter (January).

Table 3. Results of the Comparison Between the Observed and

the Estimated Mean Solar Radiation at All Ground Stations, Before

and After the Correctiona

OriginalRMS, W m�2

CorrectedRMS, W m�2

Summer (July) 66.9 25.0Annual mean 28.5 15.5Winter (January) 10.5 10.7

aSee text for details.

TRAGOU AND LASCARATOS: AEROSOL ROLE ON MEDITERRANEAN SOLAR RADIATION 7 - 11

seasonal or the interannual signal in the observations(Figure 5c).

7. Conclusions

[49] There are at least two reasons to focus our attentionon the solar heat flux of the Mediterranean Sea: first, becausethe surface forcing is a major component of the dynamic andthermodynamic forcing of the Mediterranean basin, and,second, because the examination of solar heat flux estimatesin this basin allows discussion for the validity of parameter-ization schemes and data sets used for global estimates.[50] The comparison of the estimated solar heat flux with

ground truth observations for the Qs at several coastalmeteorological stations showed that the former is systemati-cally overestimated by 25 W m�2 for a period of 30 years,and the bulk formula cannot capture either the interannualvariability of the observations, or the seasonal signal. Duringsummer the formula overestimates Qs whereas during winterQs is slightly underestimated. As the cloud reduction formulabecomes inapplicable for cloud fraction less than 3 octas(which occurs at summer, cloud-free months) the most likelyreason for the large differences during summer appears to bethe formula for the clear-sky radiation that does not includethe spatial and temporal variations of the attenuation due toaerosols and, possibly, to water vapor absorption.[51] As the available data for the aerosol extinction index

are not readily related to the transmission coefficient Tr, wehave attempted a correction to the Qs from a calibration ofthe satellite data. This correction resulted in a reduction tothe shortwave radiative flux of about 33 W m�2 for a periodof 30 years (from January 1964 to December 1993). More-over, the corrected mean time series at all ground stationsfollows closely the seasonal and the interannual variationsof the observed data, and perhaps gives some credibility toour analysis. The RMS of the difference between the meanestimated and measured Qs values at each station is reducedfor the corrected estimates. The remaining differences areprobably due to the averaging of the calibration factor fc atall 23 stations and then applying it to the whole Mediterra-nean, and the usage of one seasonal cycle for the spatiallyvarying aerosol extinction Trs

A. We also note that thesatellite data are provided only for 2.5 years and for aperiod that does not overlap with the estimates and obser-vations of Qs. This possibly introduces some error in ourcorrection.

[52] Problems in our analysis may as well come from thefact that we have used coastal instead of marine stations.This may give rise to biases due to different cloud amountespecially if a station is close to a steep land topography.Also, some stations are close to urban sites, where thepresence of anthropogenic aerosols is stronger. We have nomeans to account for these effects in this study. However,the fact that the statistics of all stations, including islandstations, show the same trend gives some credibility to ouranalysis.[53] In this analysis we have used monthly mean instead

of daily mean cloudiness values for the solar radiationestimates. Daily mean values would have been more appro-priate as the cloud reduction formula was originally derivedfor daily mean values. As Gilman and Garrett [1994]pointed out, usage of daily mean instead of monthly valuesmay cause a reduction of about 4% (or 8 W m�2) in thelong-term mean insolation. Therefore, part of the discrep-ancy between the observations and our estimates may beexplained by this effect. Still, the difference between theestimates and observations is larger (25 W m�2), indicatingthat the formula is missing some processes.[54] The present analysis has shown that a new param-

eterization scheme for the clear-sky insolation must befound to include the time and space dependence of thetransmission coefficient Tr (at least for areas of highaerosol load). Since the Reed cloud formula includes partof the aerosol reduction, this implies that in order toincorporate some regularly measured quantity in the for-mula that represents aerosol and possibly water vaporattenuation, a major revision in the formula for the solarradiation is needed. The corrective method applied in thiswork is a small step toward the improvement of theformula, though it depends on the availability of oceanicin situ measurements of solar radiation for long periods,which are very localized and rather scarce in the worldocean. A systematic calibration of the satellite data tooceanic observations in other areas of the world oceanmay provide a useful data set for the correction of the solarradiation bulk formula.

[55] Acknowledgments. The WRDC data were downloaded from theWorld Radiation Data Centre homepage (http://wrdc-mgo.nrel.gov). Wethank John Sapper and Larry Stowe of NOAA/NESDIS for makingavailable the satellite data set and for explaining details about the data.Chris Garrett, Simon Josey, and two anonymous reviewers are thanked forcomments on an earlier draft of the paper.

Figure 14. Original (dashed line) and corrected (thin solid line) estimates of the incoming shortwaveradiation at station 164200 (Messina, Italy) between January 1980 and December 1989. The thick solid lineis the contemporaneous cloudiness fraction from UWM/COADS; the cloudiness scale is on the right axis.

7 - 12 TRAGOU AND LASCARATOS: AEROSOL ROLE ON MEDITERRANEAN SOLAR RADIATION

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�����������������������A. Lascaratos, Department of Applied Physics, University of Athens,

University Campus, Bldg PHYS-V, Athens 15784, Greece. (alasc@oc.phys.uoa.gr)E. Tragou, Institute of Oceanography, National Centre for Marine

Research, P. O. Box 172, Anavyssos 19013, Greece. (tragou@.ncmr.gr)

TRAGOU AND LASCARATOS: AEROSOL ROLE ON MEDITERRANEAN SOLAR RADIATION 7 - 13