Comparing satellite- to ground-based automated and manual cloud coverage observations – a case study

In this case study we compare cloud fractional cover measured by radiometers on polar satellites (AVHRR) and on one geostationary satellite (SEVIRI) to ground-based manual (SYNOP) and automated observations by a cloud camera (Hemispherical Sky Imager, HSI). These observations took place in Hannover, Germany, and in Lauder, New Zealand, over time frames of 3 and 2 months, respectively. Daily mean comparisons between satellite derivations and the ground-based HSI found the deviation to be 6 14% for AVHRR and 8 16% for SEVIRI, which can be considered satisfactory. AVHRR’s instantaneous differences are smaller (2 22 %) than instantaneous SEVIRI cloud fraction estimates (8 29 %) when compared to HSI due to resolution and scenery effect issues. All spaceborne observations show a very good skill in detecting completely overcast skies (cloud cover 6 oktas) with probabilities between 92 and 94% and false alarm rates between 21 and 29% for AVHRR and SEVIRI in Hannover, Germany. In the case of a clear sky (cloud cover lower than 3 oktas) we find good skill with detection probabilities between 72 and 76 %. We find poor skill, however, whenever broken clouds occur (probability of detection is 32% for AVHRR and 12% for SEVIRI in Hannover, Germany). In order to better understand these discrepancies we analyze the influence of algorithm features on the satellite-based data. We find that the differences between SEVIRI and HSI cloud fractional cover (CFC) decrease (from a bias of 8 to almost 0 %) with decreasing number of spatially averaged pixels and decreasing index which determines the cloud coverage in each “cloud-contaminated” pixel of the binary map. We conclude that window size and index need to be adjusted in order to improve instantaneous SEVIRI and AVHRR estimates. Due to its automated operation and its spatial, temporal and spectral resolution, we recommend as well that more automated ground-based instruments in the form of cloud cameras should be installed as they cover larger areas of the sky than other automated ground-based instruments. These cameras could be an essential supplement to SYNOP observation as they cover the same spectral wavelengths as the human eye.DF

Remote sensing of lunar aureole with a sky camera: Adding information in the nocturnal retrieval of aerosol properties with GRASP code

The use of sky cameras for nocturnal aerosol characterization is discussed in this study. Two sky cameras are configured to take High Dynamic Range (HDR) images at Granada and Valladolid (Spain). Some properties of the cameras, like effective wavelengths, sky coordinates of each pixel and pixel sensitivity, are characterized. After that, normalized camera radiances at lunar almucantar points (up to 20° in azimuth from the Moon) are obtained at three effective wavelengths from the HDR images. These normalized radiances are compared in different case studies to simulations fed with AERONET aerosol information, giving satisfactory results. The obtained uncertainty of normalized camera radiances is around 10% at 533 nm and 608 nm and 14% for 469 nm. Normalized camera radiances and six spectral aerosol optical depth values (obtained from lunar photometry) are used as input in GRASP code (Generalized Retrieval of Aerosol and Surface Properties) to retrieve aerosol properties for a dust episode over Valladolid. The retrieved aerosol properties (refractive indices, fraction of spherical particles and size distribution parameters) are in agreement with the nearest diurnal AERONET products. The calculated GRASP retrieval at night time shows an increase in coarse mode concentration along the night, while fine mode properties remained constant.This work was supported by the Andalusia Regional Government (project P12-RNM-2409) and by the “Consejería de Educación, Junta de Castilla y León” (project VA100U14).Spanish Ministry of Economy and Competitiveness and FEDER funds under the projects CGL2013-45410-R, CMT2015-66742-R, CGL2016-81092-R.“Juan de la Cierva-Formación” program (FJCI-2014-22052).European Union's Horizon 2020 research and innovation programme through project ACTRIS-2 (grant agreement No 654109)

Derivation of sky luminance and spectral sky radiance from images taken with a CCD camera

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Comparison of solar spectrum from measurements and that from radiative transfer model under overcast sky condition

Abstract The solar spectrum under overcast sky condition is of importance for determining the optical properties of cloud by using a radiative transfer model. In this study, solar spectrum under overcast sky condition was measured by employing a spectroradiometer (Instrument System, model SP-320) at Nakhon Pathom station (13.82 ˚N, 100.04 ˚E), Thailand. The wavelength range of the measurements is 220-2400 nm, with the resolution of 1 nm in the wavelength range: 220-1700 nm and the resolution of 20 nm in the wavelength range: 1700-2400 nm. The overcast sky condition is determined by using the images of the sky taken by an automatic sky camera. Fifteen datasets of the solar spectrum were compared with those calculated by a radiative transfer model called “LIBRADTRAN”. The input data of the radiative transfer model were obtained from both measurements and literature. As part of the input data, aerosol optical properties were obtained from an AERONET sunphotometer, total ozone column from OMI/AURA satellite and cloud base height from a ceilometer. It was found that the solar spectrum from the measurements and that from the calculation are in reasonable agreement, with the discrepancy in terms of root mean square error (RMSE) of 0.043 W·m−2·nm−1. This comparison indicates that this radiative transfer model is accurate enough for use in determining the optical properties of clouds.</jats:p

An estimation of net radiation from global solar radiation in the main regions of Thailand

Abstract Net radiation can be used for different purposes, especially for studying the energy or radiation balance, which can be further analysed to investigate a global warming. In order to utilize these applications, it is necessary to know the amount of net radiation in that area. This can be done by installing a net radiometer for measuring the net radiation. However, there are few monitoring stations of net radiation compared to a global solar radiation in Thailand. Therefore, this research aims to analyse a statistical characteristic of the measured net radiation and to develop a model for estimating the net radiation from the global solar radiation at four solar monitoring stations in the main regions of Thailand, namely Chiang Mai, Ubon Ratchathani, Songkhla and Nakhon Pathom during the year of 2017 to 2021. The results showed that the most net radiation of these stations had a range of 8 to 14 MJ/m2. The relationship between daily and monthly average daily net radiation and global solar radiation was found to be a linear. After that, the developed model was validated by comparing the estimated and measured net radiation. The discrepancy between the calculated net radiation and that obtained from the measurements was presented in terms of root mean square difference (RMSD) and mean bias difference (MBD) ranged from 6.98% to 16.03% and -1.82% to 7.09%, respectively.</jats:p

Cross-boundary aerosols: a case study of aerosol problem in Bangkok during January-February, 2019

Abstract The event of high concentration of aerosols in Bangkok and its suburb during January-February, 2019 was investigated using ground and satellite-based data. The Hybrid Single Particle Lagrangian Integrated Trajectory Model (HYSPLIT) was also used for the investigation. The results from the investigation implied that the presence of high concentration of aerosols in Bangkok and its suburb during that period was likely due to the cross-boundary aerosols produced from biomass-burning in Cambodia.</jats:p