Abstract. The mountainous Canary Islands present obstacles to the trade winds and to the Canary Current flowing equatorward past them. In situ observations of hydrographic properties and surface winds south of Gran Canaria, together with advanced very high resolution radiometer and synthetic aperture radar images during 2 weeks in summer 1995 are analyzed. A cyclonic eddy shed from the west of the island drifted southwestward at 5 cm s -•, while the southeast coast was approached by an upwelling filament originating off NW Africa. A wind lee region bounded by intense horizontal shear lines had a weak return islandward wind in its center. The lee formed a triangular, diurnally varying, warm water pool with two sea surface temperature maxima separated by lower temperatures below the return wind. Shallow temperature stratification occurred behind the island in contrast to the uniform surface mixed layer in exposed regions. Upwelling and downwelling of 10 -20 rn d -• were indicated on the cyclonic and anticyclonic sides of the lee region. In the SAR images, lines of strong current shear along a temperature front between the cyclonic eddy and the upwelling filament were identifiable. However, the radar images were dominated by atmospheric phenomena, including mountain •ee wave packets, windrows, and wind shear lines. Estimation of the wind field from the SAR backscatter intensity revealed complex structure and intensification on the edges of the warm lee.

Abstract. We report on measurements of atmospheric transmission (ATr) and aerosol optical depth (AODr) made at three wavelengths (368, 500, and 778 nm) with a spectroradiometer placed on Tenerife (28.5øN, 16.3øW), Canary Islands. Using the National Oceanic and Atmospheric Administration (NOAA) advanced very high resolution radiometer (AVHRR) channel 1, we also measured the aerosol optical depth (AODs) and albedo over a region of the North Atlantic Ocean extending from 15ø-35øN to 12ø-25øW. We observe large changes in ATr and AODr when dust outbreaks pass over this region. Using all these data, we derive the asymmetry factor (g), the single-scattering albedo (co), and the local mean AODr and we compute 'the direct radiative forcing AF attributable to mineral

No abstract

A method is presented for determining the optical thickness, eVective droplet radius and temperature of oceanic stratocumulus clouds from NOAA-AVHRR infrared channels. The satellite data used in the present study correspond to night-time images in which large-scale stratiform clouds overlay the ocean. The procedure is based on the inversion of an atmospheric radiative transfer model that makes use of the discrete ordinates method called DISORT. A detailed study is presented which shows that cloud parameter retrieval is ambiguous, resulting in several possible solutions, because some pairs of eVective particle radius and optical thickness produce the same brightness temperatures. The discretization of these two parameters, based on the cloud layer radiative behaviour, is proposed to avoid these multiple solutions. In addition, the model inversion presents some diYculties that have been solved using a genetic algorithm. The current retrieval scheme is applied to satellite data to compare results with local in situ measurements and a good agreement is obtained in those pixels near the sample site.

A method for retrieval of the droplet radius and temscopic, including water content, droplet size, and its phase. In turn, the sizes and number of droplets are reperature of oceanic stratocumulus is presented. It is lated with the cloud condensation nuclei (CCN) concenbased on night imagery obtained from the infrared chantration present during the cloud formation (Twomey, nels of NOAA-AVHRR and an atmospheric radiative 1977; Twomey et al., 1984). In recent years, data from transfer model that makes use of the discrete ordinate multispectral radiometers have been used to extract method DISORT. It uses the observed satellite brightness these parameters, especially data provided by the Adtemperature differences (BTD) between channels 4 and 5 vanced Very High Resolution Radiometer (AVHRR) on to obtain the cloud temperature and between channels 3 board National Oceanic and Atmospheric Administration and 4 to extract the effective radius of the cloud droplets. (NOAA) operational satellites. We also studied the peculiarities of the method, taking Numerous studies have addressed getting the painto account the behavior of the single scattering paramerameters that characterize clouds of very different naters, deduced from Mie theory, with droplet size. Results ture, from fog (Wetzel et al., 1996) to cirrus (Ou et al., obtained are compared with in situ data collected at the 1993), including midlevel and low clouds, such as strato-Canary Islands (Spain) during summer 1996. ©Elsevier cumulus, and even overlapping clouds at different Science Inc., 2000 heights (Baum et al., 1994). Relationships between the different satellite channels are used in both the visible and the infrared regions, to obtain the microscopic and