David Fuertes

GRASP: a versatile algorithm for characterizing the atmosphere

GRASP (Generalized Retrieval of Aerosol and Surface Properties) is the first unified algorithm to be developed for characterizing atmospheric properties gathered from a variety of remote sensing observations (an introductory video is available elsewhere1). GRASP is based on a recent algorithm2 created to improve aerosol retrieval from the French Space Agency’s PARASOL3 imager over bright surfaces like deserts where high surface reflectance dwarfs the signal from aerosols. Moreover, GRASP relies on the heritage of retrieval advances4–7 implemented for AERONET,8 a worldwide network of over 200 radiometer sites that generate the data used to validate nearly all satellite observations of atmospheric aerosols. The AERONET retrievals derive detailed aerosol properties,6 including absorption, providing information of vital importance for reducing uncertainty in assessments of climate change. GRASP is based on several generalization principles with the idea of developing a scientifically rigorous, versatile, practically efficient, transparent, and accessible algorithm. There are two main independent modules. The first, numerical inversion, includes general mathematical operations not related to the particular physical nature of the inverted data (in this case, remote sensing observations). The second module, the forward model, was developed to simulate various atmospheric remote sensing observations. Numerical inversion is implemented as a statistically optimized fitting of observations following the multi-term least squares method (LSM) strategy, which combines9 the advantages of a variety of approaches and provides transparency and flexibility in developing algorithms that invert passive and/or active observations and derive several groups of Figure 1. Diagram illustrating the principle of combined synergetic processing of complementary observations using a multi-pixel2 retrieval approach. CALIPSO is a joint lidar mission of NASA and the French Space Agency, which also manages the PARASOL imager. AERONET is a worldwide network of radiometer sites.

Assessment of Sun photometer Langley calibration at the high-elevation sites Mauna Loa and Izaña

The aim of this paper is to analyze the suitability of the high-mountain stations Mauna Loa and Izaña for Langley plot calibration of Sun photometers. Thus the aerosol optical depth (AOD) characteristics and seasonality, as well as the cloudiness, have been investigated in order to provide a robust estimation of the calibration accuracy, as well as the number of days that are suitable for Langley calibrations. The data used for the investigations belong to AERONET and GAW-PFR networks, which maintain reference Sun photometers at these stations with long measurement records: 22 years at Mauna Loa 5 and 15 years at Izaña. In terms of clear sky and stable aerosol conditions, Mauna Loa (3397m a.s.l.) exhibits on average of 377 Langleys (243 morning and 134 afternoon) per year suitable for Langley plot calibration, whereas Izaña (2373m a.s.l.) shows 343 Langleys (187 morning and 155 afternoon) per year. The background AOD(500nm) values, on days that are favorable for Langley calibrations, are in the range 0.01-0.02 throughout the year, with well defined seasonality that exhibits a spring maximum at both stations plus a slight summer increase at Izaña. The statistical analysis of the long-term determination of 10 extraterrestrial signals yields to a calibration uncertainty of ~0.2-0.5%, being this uncertainty smaller in the near infrared and larger in the ultraviolet wavelengths. This is due to atmospheric variability that cannot be reduced based only on quality criteria of individual Langely plots.

Characterizing aerosol optical depth measurements and forecasts of Saharan dust events at Camagüey, Cuba, during July 2009

Se comparan las mediciones del espesor optico de aerosoles (AOD) de polvo del Sahara realizadas en Camaguey, Cuba, durante Julio del 2009 con valores pronosticados de AOD por el modelo SKIRON y con mediciones de AOD realizadas por los satelites MODIS. Las diferencias entre las medias diarias del AOD pronosticado por SKIRON y medido por MODIS (tanto Terra como Aqua) son menores que las diferencias entre los maximos diarios. Los resultados demuestran la capacidad de desarrollar un sistema de alerta y seguimiento de los eventos de polvo del Sahara a traves del Atlantico.