Cloud screening and quality control algorithm for star photometer data: assessment with lidar measurements and with all-sky images

Abstract

Abstract. This paper presents the development and set up of a cloud screening and data quality control algorithm for a star photometer based on CCD camera as detector. These algorithms are necessary for passive remote sensing techniques to retrieve the columnar aerosol optical depth, δ Ae (λ), and precipitable water vapor content, W , at nighttime. This cloud screening procedure consists of calculating moving averages of δ Ae (λ) and W under different time-windows combined with a procedure for detecting outliers. Additionally, to avoid undesirable δ Ae (λ) and W fluctuations caused by the atmospheric turbulence, the data are averaged on 30 min. The algorithm is applied to the star photometer deployed in the city of Granada (37.16 • N, 3.60 • W, 680 m a.s.l.; South-East of Spain) for the measurements acquired between March 2007 and September 2009. The algorithm is evaluated with correlative measurements registered by a lidar system and also with all-sky images obtained at the sunset and sunrise of the previous and following days. Promising results are obtained detecting cloud-affected data. Additionally, the cloud screening algorithm has been evaluated under different aerosol conditions including Saharan dust intrusion, biomass burning and pollution events.

Synchronous polar winter starphotometry and lidar measurements at a High Arctic station

“…Smirnov et al (2000) describe an algorithm based on temporal AOD variations used in the AERONET global sun photometry network. Similarly, Pérez-Ramírez et al (2012) apply temporal cloud screening procedures (such as a moving average test) to star photometry data sets. While this latter algorithm provides a consistent method to remove cloud-contaminated points, the approach and the necessary thresholds should be adapted based on the data set (D. Pérez-Ramírez, personal communication, 2012).…”

“…We expect, for example, that Arctic aerosol phenomena will be weaker in magnitude than those at mid-latitudes. The filters employed in this work are described in Table 3 and partially mimic the methodology proposed by Smirnov et al (2000) and Pérez-Ramírez et al (2012). For the range condition, we have eliminated all negative AOD values as well as AODs higher than 0.35 at 500 nm.…”

“…The moving slope threshold of 0.001 min −1 is considerably less than the 0.006 min −1 threshold employed for the pair-wise time derivative: this is meant to make up for the loss of high frequency sensitivity brought about by the regression over an hour. Additionally, 1 hour optical depth difference filtering is used by Pérez-Ramírez et al (2012) to avoid the inclusion of any outliers (while we depend on an AERONET type of (nightly) outlier filter defined in Table 3). The application of the outliers filters listed above, amounted to our cloud-screening test adapted to polar winter star photometry: one presumes that outliers are very likely to be clouds because of the high frequency variations associated with the latter.…”

“…The application of the outliers filters listed above, amounted to our cloud-screening test adapted to polar winter star photometry: one presumes that outliers are very likely to be clouds because of the high frequency variations associated with the latter. We have adjusted the threshold from 3σ of Smirnov et al (2000) and Pérez-Ramírez et al (2012) down to 2.5σ given the observed variations in AOD.…”

The new sun-sky-lunar Cimel CE318-T multiband photometer – a comprehensive performance evaluation

“…Nighttime AOD information was obtained using the EX-CALIBUR star photometer (Astronómica S. L.) which belongs to IISTA-CEAMA. More details of this system can be found in Pérez-Ramírez et al (2008b, 2012a. This instrument acquires direct star irradiances at 380, 436, 500, 670, 880, 940 and 1020 nm using a Schmid-Cassegrain telescope and a CCD camera as a detector device.…”