F. Herrera

Radiative properties of aerosols in Saharan dust outbreaks using ground‐based and satellite data: Applications to radiative forcing

We report on measurements of atmospheric transmission (AT T ) and aerosol optical depth (AOD T ) made at three wavelengths (368, 500, and 778 nm) with a spectroradiometer placed on Tenerife (28.5°N, 16.3°W), 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 (AOD S ) and albedo over a region of the North Atlantic Ocean extending from 15°-35°N to 12°-25°W. We observe large changes in AT T and AOD T when dust outbreaks pass over this region. Using all these data, we derive the asymmetry factor (g), the single-scattering albedo (ω), and the local mean AOD T and we compute the direct radiative forcing ΔF attributable to mineral dust. The local radiative forcing obtained is over the ocean ΔF = -9.7 W/m 2 and for the land ΔF=-4.5 W/m 2 with an error of±25%. Extending these results to global-scale averages, we obtain values of ΔF of -1.22 W/m 2 over the ocean and -0.57 W/m over land. The forcings attributable to dust are comparable in magnitude to those reported in the literature for anthropogenic sulphate and for biomass burning aerosols.

Chemical and microphysical properties of marine stratiform cloud in the North Atlantic

The chemical and microphysical properties of marine stratiform cloud were measured at a ridgetop elevation of 992 m above mean sea level (AMSL) on Tenerife in the Canary Islands in the eastern North Atlantic during the summers of 1995 and 1996. The results show an inverse relationship between hourly-averaged cloud droplet diameter and droplet number concentration, which ranged from 116 to 1355 cm−3. Strong relationships were observed between droplet number and equivalent clear air concentrations of non-sea-salt sulfate, nitrate, and elemental carbon in the droplets. Droplet sizes inferred from radiances measured by satellite for clouds offshore and upwind agreed with droplet sizes derived for clouds over the mountain sampling site, and also with those measured in cloud 4–5 hours later. Estimated cloud short-wave radiative forcing was enhanced by 8% in radiative model studies of polluted versus clean clouds with droplet concentrations of 786 and 127 cm−3 and droplet effective radii of 6 and 10 μm, respectively.

Stratocumulus properties retrieval method from NOAA-AVHRR data based on the discretization of cloud parameters

A method is presented for determining the optical thickness, effective 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 effective 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 difficulties 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.

Retrieval of Marine Stratus Cloud Droplet Size from NOAA-AVHRR Nighttime Imagery

Abstract A method for retrieval of the droplet radius and temperature of oceanic stratocumulus is presented. It is based on night imagery obtained from the infrared channels of NOAA–AVHRR and an atmospheric radiative transfer model that makes use of the discrete ordinate method DISORT. It uses the observed satellite brightness temperature differences (BTD) between channels 4 and 5 to obtain the cloud temperature and between channels 3 and 4 to extract the effective radius of the cloud droplets. We also studied the peculiarities of the method, taking into account the behavior of the single scattering parameters, deduced from Mie theory, with droplet size. Results obtained are compared with in situ data collected at the Canary Islands (Spain) during summer 1996.

Portable data acquisition system for EKG measurements in marine environments

In the present paper, an electronic prototype designed to acquire electrocardiographic signals from marine mammals has been developed. The system consists of a portable device that allows the on-line acquisition of EKG signals through a parallel port interface that is connected to a laptop computer. The EKG waveform, the voltage level, the bandwidth and the most relevant information from this type of signals is stored and analysed under real-time conditions by means of specifically implemented software. The whole system has been successfully tested to obtain the EKG from captive dolphins. The characteristics of the equipment presented here, with its low cost, size and energy requirements accomplish a portable system suitable for the acquisition of this type of signals in conductive media such as the seawater.

Efficiency of a global algorithm for retrieving SST from satellite data in a subtropical region

Abstract A global split-window algorithm for the retrieval of sea surface temperature from AVHRR has been developed using a radiative transfer code. In order to analyze the performance of the equation obtained which is applied to the Canary Islands zone, we have simulated a set of satellite measurements. The purpose was to classify the errors determined by the global algorithm, depending on the atmospheric situation at each moment. In the final results we give an explanation of the inefficient performance of global algorithm in comparison with a regional algorithm.

First intercomparison between the aerosols optical depth data obtained by the NOAA satellite sensors and the Optronic OL752 spectroradiometer in the Canary Islands

Abstract The aerosol optical depth data obtained by two methods are compared. The first method uses solar radiance measurements with the spectroradiometer Optronic OL752 installed inside the Santa Cruz de Tenerife Harbour (28.48 N, 16.24 W). The second one takes the solar radiation scattered by the atmosphere using the Advanced Very High Resolution Radiometer (AVHRR) radiometer channels on board the NOAA meteorological satellites. A 91 per cent correlation between the two procedures was obtained for channel 1 (630nm).

Removing the thermal component of the NOAA-AVHRR Channel 3 in cloudy conditions

In this paper, an operational method for the extraction of the solar component from the AVHRR-Channel 3 radiance, is addressed. This is based on the implementation of a simple radiative transfer model that explains the radiative behavior of each pixel in the image. The outputs of the model are expressed in terms of brightness temperature differences between Channels 3 and 4 (BTD34). As a first step, the model behavior is proved for night images and then it is inverted for daylight ones, supplying the two contributions from the total radiance detected by the sensor.

Stratocumulus parameter retrieval using MODIS nighttime imagery: a theoretical approach

This work is a preliminary study of the viability of retrieving macro physical and micro physical cloud parameters from nighttime radiances provided by MODIS sensor, onboard Terra spacecraft. It is based on the analysis of the sensitivity of every MODIS IR band to each of the parameters that describe the different layers composing the earth-cloud-atmosphere system. IN order to make this analysis, an atmospheric radiative transfer model that makes use of the discrete ordinates method DISORT is employed. Multiple simulations are performed for a great variety of clouds and atmospheric conditions, taking into account the main absorbers in each band. As a first result, the more adequate bands for our purpose are select and, using these channels, the proposed method extracts the parameters characterizing the different layers through a numerical inversion of the radiative model based on an evolutionary method for solving optimization problems called scatter search. In addition, a sensitivity analysis is carried out in order to estimate the impact on the retrieved values of the uncertainties in model inputs and assumptions.

Aerosol phase function modeling in Saharan dust invasions using the ratio AVHRR/NOAA Ch1/Ch2

Abstract Some authors have pointed out that the mineral dust could have an important effect in the atmosphere radiative properties in oceanic regions where this component is the principal aerosol constituent (mainly in regions close to arid zones). The radiative characterization of this component is a global scale problem, so it is necessary to use satellites techniques. One of the most useful parameter to study the dynamic and radiative properties of the mineral dust is the aerosol optical depth (AOD). To minimize the possible error sources, in order to give this parameter, it is necessary to work with a realistic aerosol phase function. We propose a Henyey-Greenstein type phase function calculated only by the ratio AVHRR/NOAA Ch1/Ch2, whose parameters have been optimized to situations of mineral dust invasions. This proposed phase function has been obtained by radiometric measurements of ground-based instruments.