Fig. 1.
The study area was located between Dakar, Cap Blanc and the Cape Verde Islands covered by the white-dashed box between 16° W and 25° W and 15° N and 20° N. The map shows the ship track (black line from point ‘Start’ to point ‘End’; black points correspond to the position of the ship: e.g. 5 Feb = 5 February 2008) of the first cruise from 5 to 19 February 2008 and the location of the permanent station in Mindelo on the island São Vicente of the second field campaign from 19 to 28 May 2009. The statistics of dust storms of 2008 to 2009 were determined from daily dust aerosol data, which were area-averaged for the white-dashed box. The location of the AERONET station and the METAR station GVAC on the Cape Verde Island Sal is included.
Fig. 2.
The MODIS aerosol components AODdust, AODma and AODan at 550 nm of cruise 2008 (panel A) and of field campaign 2009 (panel B) are shown. The dust storms are clearly seen between 16 and 19 February 2008 as well as between end of 23 May and 27 May 2009. The total aerosol optical depth of AERONET station (AODAERONET) at 550 nm is given for comparison.
Fig. 3.
The daily downward irradiances Ed at 500 nm measured during the two field campaigns are shown. 14 daily cycles were collected along the ship track (cf. with Fig. 1) in year 2008 (panel A). 10 daily cycles of downward irradiances were obtained at the permanent land station in Mindelo (cf. with Fig. 1) in the year 2009 (panel B). The downward irradiances were measured with a time resolution of 30 s. The observed daily irradiance cycles varied due to different atmospheric conditions. Clouds and dust events were indicated. The dust storms occurred between 16 and 19 February 2008 as well as between end of 23 and 27 May 2009. Outliers from the mean daily cycles represented the impact of clouds and dust on irradiance.
Fig. 4.
Modelled downward irradiances (Gregg and Carder, 1990) are compared with observations on 14 February 2008 in dependence of time (panel A) and wavelength (panel B). The ratio between modelled irradiances and measured irradiances are shown for different wavelengths in Fig. 4C. The 14 February 2008 was the clearest day during both field campaigns. The observed irradiances matched very well the modelled ones because the spectral model was optimised for cloudless and dustless maritime atmospheres. The maximum differences were 5% for solar zenith angles (SZA) < 55 deg and higher than 10% for SZA > 65 deg. Correction terms (eq. 3) modelled on the basis of meteorological data set (see Sections 2.3.3 and 2.4.4) for different days are given in Fig. 4D (SZA = 40 deg). These terms were applied for the irradiance normalisation.
Fig. 5.
The maximum relative irradiance deviations (eq. 2) due to case I – and case II – clouds as well as Saharan dust are shown. They were determined by scanning the whole data set of campaigns 2008 and 2009 with restriction to zenith angles smaller than 60 deg because of the higher deviations of measured to modelled irradiances near the horizon. The date and time of observation, the maximum relative deviation of photosynthetically available radiation (eq. 7) as well as the available dust components of aerosol optical depths of MODIS are shown on the right (see Section 2.4.2).
Table 1. Results of relative deviations of irradiance () and photosynthetically available radiation () are summarised. The maximum measured decrease (case I) and increase (case II) by clouds as well as the maximum decrease by dust are presented. (1) is the maximum measured decrease for the observed dust storms of both field campaigns, (2) is the decrease in dependence of dust component of aerosol optical depth (AOD) derived by a linear fit and (3) is the maximum decrease in the years 2008 and 2009 determined by the linear fit in combination with the maximum observed dust component of AOD in these years
Fig. 6.
The relative deviations of photosynthetically available radiation (eq. 7) of cloudy and dusty days are presented as a function of zenith angle. The deviations disappeared if the case of a clear day was assumed (dashed line). The outliers from the mean curves represented the impact of different cloud types.
Fig. 7.
The relative deviations of photosynthetically available radiation (eq. 7) are shown in dependence of dust component of the aerosol optical depth (AODdust). Points were classified to time-matched points if the time difference between field and MODIS observation was lower than 60 min. There are only few points in the figure because the AOD data of MODIS were limited by satellite swath and clouds.
Fig. 8.
The daily reductions of photosynthetically available radiation by atmospheric dust are shown for the years 2008 and 2009. They were calculated on the basis of the linear fit (Section 3.4) and the dust components of aerosol optical depths (AODs) (cf. Section 2.4.2). The highest observed dust components of AODs of 2.64 on 17 July 2008 and of 2.59 on 22 September 2009 were marked. The corresponding highest reductions of photosynthetically available radiation were about 32% and 31%, respectively. The dashed line relates to the mean value of reduction. The shaded region corresponds to the standard deviation σ in the time period 2008 to 2009.
