Sky Imaging System Research Articles

Development of a sky imager for cloud cover assessment

Based on a CCD camera, we have developed an in-house sky imager system for the purpose of cloud cover estimation and characterization. The system captures a multispectral image every 5 min, and the analysis is done with a method based on an optimized neural network classification procedure and a genetic algorithm. The method discriminates between clear sky and two cloud classes: opaque and thin clouds. It also divides the image into sectors and finds the percentage of clouds in those different regions. We have validated the classification algorithm on two levels: image level, using the cloud observations included in the METAR register performed at the closest meteorological station, and pixel level, determining whether the final classification is correct.

Observations of plasma depletions in 557.7‐nm images over Kavalur

An all sky imaging system was operated at Kavalur (12.56°N, 78.8°E, 4.6°N, geomagnetic), India, during February‐April 2002. We observed plasma depletions in the 557.7‐ and 630.0‐nm images in several nights and found that in most of the nights, the 557.7‐nm depletions appeared around midnight and became very pronounced in the postmidnight hours. A simulation is carried out to understand the important physical parameters that influence the emissions of the 557.7 nm in various local times and solar activities, and that result in the appearance of depletions in the integrated images.

Cloud mapping with ground‐based photogrammetric cameras

Ground‐based digital imager systems in the visible and near infrared region of the solar spectrum have the potential to nicely complement existing instruments and observation networks of National Weather Services with very accurate, high spatial and temporal resolution, 2D and 3D macroscopic cloud data such as cloud amount, cloud‐base height and 3D cloud‐base motion. This paper discusses two current approaches to ground‐based cloud sensing: the prototype instrument used at ETH/MeteoSwiss within Cloudmap and Cloudmap2 for stereoscopy tests, and a Daylight Visible/NIR Whole Sky Imager (WSI) system developed and fielded by the Scripps Institution of Oceanography (SIO). The article includes descriptions of the radiometric and geometric calibration methods. Cloud amount, cloud‐base height and cloud‐base motion results from two ETH/MeteoSwiss measurement campaigns and from the operational WSI use at the German Weather Service (DWD) are shown. Finally, a case study with coincident satellite and ground data illustrates that ground‐based digital imager systems are an interesting technique to validate satellite‐based cloud‐top heights and cloud‐top motion winds of vertically thin clouds.

Estimation of fractional sky cover from broadband shortwave radiometer measurements

We outline a methodology for estimating fractional sky cover for an effective 160° field of view from an analysis of surface measurements of downwelling total and diffuse shortwave (SW) irradiance. The data are screened for optically thicker overcast cases, after which an empirically derived formulation is used to estimate the fractional sky cover for the remaining data. The retrieved fractional sky cover time series is then evaluated to mitigate times of anomalous behavior caused by the thick overcast screening. The resultant sky cover estimates show a high degree of repeatability given nominally well maintained and operated radiometer systems and the use of the Long and Ackerman (2000) methodology for estimating the clear‐sky total and diffuse SW. Thus the resultant fractional sky‐cover estimates appear to be fairly independent of the particular climate regime and model of radiometers used, at least for the climate regimes we have tested so far. The sky‐cover estimates agree to better than 10% root mean square sky cover amount with sky imager retrievals and human observations, which is as good as the agreement between sky imaging systems and observers themselves. As such, this methodology becomes a powerful tool for satellite and model validations and climatological analyses including the study of trends in cloud amount. Analysis shows that the technique also produces realistic frequency distributions, showing that the continental midlatitude regimes included in the study are typified by clear‐sky occurring about 1/3 of the time, overcast about 1/3 of the time, and partly cloudy skies to varying extent occurring the remaining 1/3 of the time. By contrast, the tropical western Pacific oceanic regime during the Nauru99 field experiment exhibits far more frequent occurrence of partly cloudy skies, with sky cover amounts of 20% to 50% occurring about half the time.

Retrieving Cloud Characteristics from Ground-Based Daytime Color All-Sky Images

Abstract A discussion is presented of daytime sky imaging and techniques that may be applied to the analysis of full-color sky images to infer cloud macrophysical properties. Descriptions of two different types of sky-imaging systems developed by the authors are presented, one of which has been developed into a commercially available instrument. Retrievals of fractional sky cover from automated processing methods are compared to human retrievals, both from direct observations and visual analyses of sky images. Although some uncertainty exists in fractional sky cover retrievals from sky images, this uncertainty is no greater than that attached to human observations for the commercially available sky-imager retrievals. Thus, the application of automatic digital image processing techniques on sky images is a useful method to complement, or even replace, traditional human observations of sky cover and, potentially, cloud type. Additionally, the possibilities for inferring other cloud parameters such as cloud brokenness and solar obstruction further enhance the usefulness of sky imagers.

Cloud mapping from the ground: use of photogrammetric methods

from the existing observation networks, except at a very few Abstract places around the world which are equipped with high-cost in- Within the European Union (EU) project Cloudmap, a ground- struments (see Table 1). At most climate stations of the national based sky imager system consisting of two commercial digital networks, cloud macroscopic properties—mainly cloud cover, CCD cameras with wide-angle lenses has been established which cloud depth, and cloud-base height—are still visually ob- can theoretically be used to derive various macroscopic cloud served. Mainly at airports, ceilometers are in operational use to parameters (cloud-base height, cloud-base wind, cloud measure the cloud-base height automatically and continuously amount). In this paper, we present the method to calculate a in additionto thevisual observations. It is well recognized today digital surface model (DSM )o fthe cloud base. It includes both thatthe infrequent,spatially notequallydistributed, subjective, the precise determination of the interior and exterior orienta- and too sparse point observations of clouds do not meet the re- tion of the cameras as well as the automatic derivation of the quirements of numerical weather prediction (NWP )a nd global cloud-base heights using modern photogrammetric algorithms. climate models (GCM). The data from our ground-based stereo The presented measurements were taken during the imager system will in this context nicely complement existing Mesoscale Alpine Programme (MAP )i nS witzerland in October instruments and observations with very accurate, spatial, and 1999. The results from our own matching software and from frequent 3Dcloud-base heights which should be suitable for the commercial photogrammetric systems were validated with meteorological models. semi-automatically measured points and compared with visual Within the EU project Cloudmap2 (Cloudmap2, 2001), we observations, lidar, and radiosonde data from the MAP are currently workingon gaining knowledge aboutthe require- Composite Observing Network and satellite-based cloud-top ments oftemporal frequency,spatial coverage,and accuracyfor heights from ERS2-ATSR2 .T he potential of the system to provide cloud products to be usedas assimilation data into NWP or spe- very accurate areal cloud-base height data was shown. This is cific cloud models. These new requirements are certainly important for the objectives of the EU project Cloudmap2, where higher than the operational user requirements defined in the it is planned to assimilate various cloud parameters, including World Meteorological Organization (WMO) Guide to Meteoro- cloud-base height, into cloud and high-resolution numerical logical Instrumentation and Methods of Observation, which weather prediction (NWP) models.

First airglow all sky images at 23° S

Abstract An all sky imaging system using a cooled CCD camera with 4 interference filters mounted has been constructed and was installed at Cachoeira Paulista airglow observatory (22.7°S, 45.0° W) in February 1996. The airglow OI 557.7 nm, 630.0 nm, hydroxyl Meinel bands in the near infrared region (