Optimized fractional cloudiness determination from five ground‐based remote sensing techniques

Abstract

A 1 year record of fractional cloudiness at 10 min intervals was generated for the Cabauw Experimental Site for Atmospheric Research (CESAR) (51°58′N, 4°55′E) using an integrated assessment of five different observational methods. The five methods are based on active as well as passive systems and use either a hemispheric or column remote sensing technique. The 1 year instrumental cloudiness data were compared against a 30 year climatology of Observer data in the vicinity of CESAR (1971–2000). In the intermediate 2–6 octa range, most instruments, but especially the column methods, report lower frequency of occurrence of cloudiness than the absolute minimum values from the 30 year Observer climatology. At night, the Observer records fewer clouds in the 1–2 octa range than during the day, while the instrumental techniques registered more clouds. During daytime the Observer also records much more 7 octa cloudiness than the instruments. A reference algorithm was designed to derive a continuous and optimized record of fractional cloudiness. Output from individual instruments were weighted according to the cloud base height reported at the observation time; the larger the height, the lower the weight. The algorithm was able to provide fractional cloudiness observations every 10 min for 99.92% of the total period of 12 months (15 May 2008 to 14 May 2009).

Impact of aerosols and clouds on decadal trends in all-sky solar radiation over the Netherlands (1966–2015)

“…Although the half-hour averaging of the cloud observations to some extent compensates for the absence of instantaneous hemispheric information, the two types of observation represent different methods of estimating cloud cover so that the conditional sampling of the radiation is significantly affected. For example, the digital nature of the ceilometer observation results in many more observations in the c = 0 (cloudless) and the c = 8 (overcast) cloud cover selection bin than obtained from the Human Observer (Boers et al, 2010). As a result, the selectively sampled radiation data in both okta bins will be contaminated by data recorded under fractionally cloudy conditions.…”

“…This has led some investigators to group data into regions and rely either on cloud data from stations in the immediate surroundings or from satellites (or both) to supplement their radiation records (Norris and Wild, 2007). Even though good results on trends in clear-sky radiation can be obtained at sites where direct and solar radiation are recorded at the same time, such as Baseline Surface Radiation Network stations (Long and Ackermann, 2000;Long R. Boers et al: Impact of aerosols and clouds on decadal trends et al, Wild et al, 2005;Gan et al, 2009), most often an investigator will have to rely on single global radiation data records that are specific to the region of interest (such as Manara et al, 2016) or on data stored in the Global Energy Balance Archive (GEBA) archive. GEBA data can be used to good effect because of the fact that many stations have submitted data, but the peculiarities of the radiative signals typical to individual localities are invariably lost in the abundance of data.…”

Characterisation of <i>J</i>(O<sup>1</sup>D) at Cape Grim 2000–2005

“…However, the fit did not improve significantly, as measured by the adjusted R 2 (increases of ∼ 0.0005), implying that this is not a useful approach. This could be due to the insensitivity of long-wave radiation measures to higher-level clouds (Schade et al, 2009;Boers et al, 2010). However, cloud bases are often low at Cape Grim (800-1000 m), as observed by lidar measurements, (Young, 2007), and so LDR should be a reasonable measure.…”

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

“…The results show that the differences between algorithm and imaging are lower than between algorithm and human cloud estimations. Boers et al (2010) conducted ground-based measurements with five different methods that were either performed by passive or active remote sensing instruments. These measurements were compared to a 30-year climatology of human observations.…”