Laboratory and Field Evaluation of the NubiScope

The Royal Netherlands Meteorological Institute (KNMI) evaluated the usefullness of the NubiScope, a scanning pyrometer, for cloud observations. The NubiScope is a passive remote sensing instrument which consists of a pyrometer mounted on a pan and tilt unit. The pyrometer measures the brightness temperatures in the atmospheric thermal infrared window (8-14 μm).. The NubiScope operates fully automated and performs a scan of the entire hemisphere every 10 minutes including 2 surface temperature measurements. The observed temperatures are processed to derive the obscuration type (fog, precipitation, clouds) and cloud characteristics (cloud cover, layering and altitude). During a one year field experiment at the Cabauw Experimental Site for Atmospheric Research [CESAR] the stability and the sensitivity of the pyrometer to contamination has been monitored by repeated laboratory measurements against a black body radiator. The field measurements of temperature have also been analyzed and compared with other measurements.

The NubiScope cloud product has been evaluated by observers at Rotterdam airport (at 25 km distance) using a near real-time access to the 10-minute NubiScope results, the automated ceilometer cloud product and total sky imager data. The evaluation showed that the observed differences between NubiScope and ceilometer could mostly be attributed to the better spatial representativeness of the NubiScope. Furthermore, the sensitivity of the NubiScope for high clouds is often better. Over a one year period the total cloud cover of NubiScope and ceilometer gave 44 % of the time identical results and 80 % and 87 % the differences are within ±1 and ±2 okta, respectively. The averaged difference in total cloud cover is 0.07 okta and mean absolute deviation is 1.03 okta. These differences between NubiScope and ceilometer are similar to the differences between an observer and a ceilometer. Scanning enables the NubiScope to detect isolated clouds in clear sky situations or gaps in overcast situations. This reduces the number of occurrences of 0 and 8 okta for the NubiScope as compared to a ceilometer and is in better agreement with human observed distributions. As a result of this investigation the Climate department of KNMI decided to keep the NubiScope permanently at CESAR. The Weather department confirmed the added value of the NubiScope for cloud cover observations, but for applications such as aviation the cloud height information is crucial. The combination of NubiScope cloud cover information with accurate height information requires further research.